CSPP51036 Java Programming Instructor: Andrew Siegel Syllabus, discussion

CSPP51036 Java Programming Instructor: Andrew Siegel Syllabus, discussion

CSPP51036 Java Programming Instructor: Andrew Siegel Syllabus, discussion group, etc. Everything is posted on course website http://masters.cs.uchicago.edu/~asiegel/courses/cspp51036 Please pay particular attention to grading policy/homework submission

First homework already posted. Please be sure to sign up for discussion group asap! Grading Bi-weekly homework assignment 5 total 3 day automatic extension for 10% Serious emergency = not graded Submit electronically, receive confirmation. 24 hours to deal with any problems.

Typically, functionality only will be graded. Wrong answer = no credit. Design will be graded only when problems specify design requirements. Final In-class, conceptual, purposeful. Who is this class for? There is no assumption that you know any Java or other OO programming language. If you do, you may find the first few weeks

boring. You must have programming experience. We will not spend very much time on syntax, procedural constructs, operators, etc. It is assumed that you can pick this up. Getting Help TAs The class will be split into a number of subgroups, each of which is assigned to one of the TAs.

The TAs will organize office hours/review sessions according to the needs of their group. Quickest way to answer small questions is with email to course list. Everyone is encouraged to answer will be factored into grade. Very good way to learn. Try not to spend too much class time discussing homeworks tas will handle this. Course Format

All lecture Will follow book closely! Feel free to ask questions any time Probably wont call on you unsolicited Probably will make fun of you

Attendance is optional but recommended Might use overhead projector, might not. Will post notes each week in any case Software/hardware requirements Command-line tools best when first learning. Suns Java Software Development Kit (SDK) Already installed on Linux cluster May choose to install on your own machine free and easy, download manageable (book CD has copy). See beginning of chapter 2 for installation instructions. Some IDE (Forte, Visual Age, Visual Caf, etc.)

not recommended. May use but are responsible for exporting in a manner understandable by jdk. Your favorite text editor emacs is recommended. Textpad is pretty good for Java. History of Java First version released in 1995 Four major versions released since then

1.02 (1996) JDBC, Distributed Objects 1.1 (1997) New Event Model 1.2 (1998) Swing 1.3 (2000) Cleanup 1.4 (2002) Programming with Java What is/isnt Java?

Read chapter 1 of Core Java Discussion is very balanced Basically, compared to C Java is a relatively high-level language with many built-in features for portably doing useful things such as:

Multithreading Writing distributed programs Writing GUI clients Error handling Extending Web servers Embedding programs in Web browsers Connecting to commercial databases Compiling/running first java program

Create source code file (call it for example MyFirstProgram.java). To compile: prompt >> javac MyFirstProgram.java This produces byte code file named MyFirstProgram.class To run: prompt >> java MyFirstProgram

Observations .class file is not machine code. It is intermediate form called Java Byte code. Can run on any platform as long as platform has a Java Virtual Machine (JVM). The second step on previous slide invokes the JVM to interpret the byte code on the given platform. In theory, byte code can be moved to another platform and be run there without recompiling this is the magic of applets.

Leave off the .class part when invoking the JVM. Observations This is an old-fashioned command-line program. Java also supports GUI applications and web-browser hosted programs called applets. After the first couple of weeks we will use graphical rather than scripting front-ends. Writing first program

To keep things simple our first few programs will all be written as just a single main program. In java, the file in which your program resides must contain a .java extension (e.g. MyFirstProgram.java). Writing first program Then, the program must be wrapped in a class definition which is the same as the file

basename (MyFirstProgram). Careful, Java is case-sensitive. class MyFirstProgram { } Finally, main is defined similar to C, but with a few more modifiers: public static void main(String[] args){ } These are all required. No shortcuts. Writing first program

Just as in C, main(..) is the principle entry point into the program. When you say java Program Java looks in Program for a procedure named main. This is where the program starts. To print to stdout in java use: System.out.println( ); MyFirstProgram. Basic Programming Constructs

What you should learn on your own Breakdown of a java program Strategy to start is write evertything in a single main program and very quickly review the basics of java syntax (very little time here). Then we break into procedures. Then class/packages. Single-threaded program

For a single thread of execution, each line of code is executed sequentially (as in C). Each statement is terminated with a semicolon. Unlike C, declarations can occur anywhere within a program. Basic operators, control statements almost

exactly like C. A few minor differences. Best to just look at some examples. Java Data Types Sizes fully specified by Java standard. Java is a very strongly typed language Integer types int (4 bytes signed) short (2 bytes signed) long (8 bytes signed) use suffix L (eg 1000000000L) byte (1 byte signed) Floating-point types

float (4 bytes) use suffix F (eg 1.28F) double( 8 bytes) Additional Data Types char Two-byte unicode Assignment with e.g. char c = h; boolean true or false e.g. boolean x = true;

if (x){}; Operators/Control Flow Almost exactly like regular ANSI C. +, *, -, /, %, ++, --, +=, etc. ==, !=, >, < , etc. if statements, for loops, while loops, do loops, switch statements, etc. continue, break, return, System.exit(0). Read pp 54 in Core Java. No need to spend class time going over these.

Scoping Braces are used as in C to denote begin/end of blocks Be careful of the following: int j = 0; if ( j <1 ){ int k = 2; }

k = 3; //Error! k inaccessible here Declarations do not propogate upwards. Adding datatypes -- classes Java has handful of built-in datatypes just discussed (int, float, etc.) Just like in C, user typically creates own homemade datatypes to work with particular application (ie structs and enums). In Java these are called classes.

Many class definitions come as a standard part of the Java distribution. Most common Example is String class. Strings Java provides a class definition for a type called String Since the String class is part of the java.lang package, no special imports are required to use it (like a header file in C). Just like regular datatypes (and like C), variables of

type String are declared as: String s1; String s2, s3; //etc. Note that String is uppercase. This is the Java convention for classnames. Strings Initializing a String is painless s1 = This is some java String; Note that double quotes are required. Memory is allocated dynamically. Think of above method as shortcut for more

standard way (assuming s1 has been declared): s1 = new String(This is some java String); new operator required to create memory for new String object. String methods Given a String object we can then access any public String method or instance variable (field). Best to think of analogy with C. Given a variable of some struct type, we can access any of the

structs members. If one of these members is a pointer to a function, we can essentially call a function using the struct. (x.doit(x,)) In Java, this idea is taken quite a bit further, but the above analogy is a good start. String Examples Best to see by way of example: String s = new String(Hello); Char c = s.charAt(3); System.out.println(c); Method charAt called on String object s

taking single integer parameter. How might this look in a procedural language with structures? (homework) String class documentation Incredibly important! Each standard java class definition is fully documented online You must become facile at reading/interpreting these documents. This is how everything is done in Java. A little hard at first but then very simple. Makes learning

new functionality much easier (if its well written). Make a link to http://java.sun.com/j2se/1.4/docs/api/index.html Much of homework will come from interpreting this page. 1d Arrays Arrays in Java are dynamic; they are allocated with the new operator. Creating a (1d) array is a two-step process:

int[] x; //declare x to be an array of ints //x has the value of null right now x = new int[100]; //allocate 100 ints worth At this point the values of x are all zeroes. Assignment is then just like C. 1d Arrays Note that in Java, unlike C, the compiler will not let you overrun the bounds of an array. Give it a try.

Note that you must use the new operator to size the array. It cannot be done statically as in C. Until this is done, the array reference has a value of null. The array comes with a field called length which stores the number of elements in the array. Try printing x.length in the previous example. This is a simple but nice convenience. Misc tricks to get work done Parsings Strings

Recall that the args array is an array of Strings. Thus, to accept key input as integers, float, etc. we must convert. To convert to int, use the Integer.parseInt(String) function. Ex. String s1 = 32; int j = Integer.parseInt(s1); // j now holds 32 Converting to double is analogous: Ex. String s1 = 32; double j = Double.parseDouble(s1); // j now holds 32

Parsing Strings Note that the conversion methods are just regular procedural funtions. That is, no object was created in order to call the method. The entity before the . is _not_ an object. More on this later. What if the String is unparseable? (e.g. andrew rather than 32). Study the Integer and Double classes and look for a description of the errors that are thrown by the various methods. Does anything stand out? See

example Looper.java. More on this later. Reading Keyboard input at runtime How do we prompt for keyboard input at runtime? Requires knowing a little weird i/o stuff. For now, lets use a simple class that I wrote with a few simple methods. Class is called ParserUtils.java. Please retrieve from Java Library directory on

course site. ParserUtils.java Class definition contains two methods: String getKeyInput() String[] getTokens(String) First method blocks program until user enters some data (followed by enter). What user types in then stored in String return. Second method takes String and breaks into tokens based on white space, stores in String array return.

To call these methods, use class name followed by . before method name. See Parser.java Reading text file This also requires some ugly i/o operations well cover later. For now, well use a simple class TextManipTools.java (see course site). This has single method String[] readFileByLines(String)

This method reads the filename specified by String into an array of Strings, each element of which holds one line of text. See DumpFile.java Intro to Java Classes Java Classes Placeholder for beginning notes on making simple java classes

Interfaces, Inhteritance, Polymorphism What is an Java interface? Like a class but only contains abstract methods and final variables example: interface FooInterface{ void foo1(); int foo2(double x);

} abstract interface FooInterface{ public abstract void foo1(); public abstract int foo2(double x); } Both are correct! the abstract and public keywords are implied, so the shorthand is recommended.

Interfaces, cont. Unlike a class, an interface cannot be instantiated! Rather, an interface is implemented by some other class: class FooClass implements FooInterface{ .... } This means one thing only: FooClass must contain

versions of both foo1 and foo2 that actually do something useful. We say that FooClass must provide implementations for all of the methods in FooInterface. Interfaces, cont. When a class implements an interface, think of the class as entering a contract to provide meat for all methods in the interface. The compiler will check that this contract is adhered to.

Otherwise, the class implementing the interface can contain anything else that a regular class contains. Again: do not try to instantiate an interface this is meaningless! More interface rules A class may implement any number of interfaces as: class FooClass implements A, B, C{

... } An interface may also contain public static final variables. Usually, these qualifiers are left off. We just say: interface MyConstants{ double PI = 3.141592; double E = 1.7182818; }

can be acessed as either MyConstants.PI or just PI by any class that implements the interface Certification-type questions What happens when multiple interface implementation results in name conflicts? if the methods have different signatures, they are considered overloaded and there is no problem

if the methods have the same signature and the same return type, they are considered to be the same method. if they have the same signature and different return types, a compilation error will result. If they have same signature/return type but throw different exceptions, they are considered to be same, and resulting throws list is union of original two Marker Interfaces Some interfaces contain no methods or constants at all (ie they are empty).

These are called marker interfaces. Examples are Cloneable and Serializable in java library. We will understand these better once we understand subtype-supertype relationships. Subtyping with Interfaces Understanding mechanics of interfaces is only about 1/3 of the story. Next, we have to understand how interfaces allow for polymorphism.

Finally, we study probably the hardest part how to best use polymorphism to really write superior code. Rules of subtyping If class C implements interface I, then C is a subtype of I. What does this imply? We can do things like: C aC; aC = new C(); I aC;

aC = new C(); This is the regular stuff Can do this also! In the latter case, we often say that the runtime type of aC is C, but the static or declared type is I. In the former case, both types are C Substitutability of Types Rule: A value of a subtype can appear wherever a

value of its supertype is expected. In other words, a value of a sybtype can always substitute for a value of a supertype. To rephrase for objects: An instance of a subclass can appear wherever an instance of a superclass is expected. Note: superclass here refers to interface at this point. We will make more general soon. Generic examples of subtyping class Circle implements Drawable, Scalable{ ...

Circle aCircle; Drawable aDrawable; Scalable aScalable; aCircle = new Circle(); //ok aCircle = new Drawable(); //BAD! aDrawable = new Circle();//ok aScalable = new Circle(); //ok 1,3,4 are ok because a Circle object is created and it is assigned to a declared type of either Circle or one of its supertypes. More examples

Drawable aDrawable; Circle aCircle; aCircle = new Circle();//fine aDrawable = aCircle;//fine aDrawable is type superclass aDrawable = new Circle(); aCircle = aDrawable; //NO; cannot assign more general to //more specific without an explicit //cast aCircle = (Circle) aDrawable; //this is ok explicit cast! Widening and Narrowing The conversion of a subtype to one of its supertypes

is called widening. The conversioning of a supertype to one of its subtypes is called narrowing. Widening happens automatically during an assignment. Nothing special is required. This is also typically called upcasting. Narrowing requires a proper explicit cast, otherwise the compiler will complain. This is an example of Javas very strong typing. It is your best friend. Narrowing is also known as downcasting. ClassCasts and instanceof

The java compiler will allow any cast. If the cast is illegal, a runtime exception will be thrown (ClassCastException). There are two ways to safeguard against this. 1. with a try-catch block (later) 2. Using the instanceof operator as: if (aDrawable instanceof Circle){ aCircle = (Circle) aDrawable

} instanceof tells the actual type of an object rather than its declared type. Why on earth do this? How could this ever be used to your advantage? Why not simlply type all Circles as Circle, etc. In other words, why ever do: Drawable aCircle = new Circle(); vs. Circle aCircle = new Circle();

Much easier to understand if we use upcasting in method calls. Using Upcasting in method calls Imagine there is a class Renderer that has a method Render that can operate on any object that knows how to morph any two objects that can draw themselves. e.g.; class Renderer{ ... public void morph(Drawable o1, Drawable o2){ // calls made to o1.draw(), o2.draw() here\

} You can call morph as e.g.: Renderer rn = new Renderer(); Circle c = new Circle(); Rectangle r = new Rectangle(); rn.morph(c,r); //c,r are automatically upcast to Drawables Comparable interface Another good example is Javas Comparable interface, which contains the compareTo method. One of the Arrays.sort methods operates on

any array of Objects that implement the Comparable interface. Thus, specific objects to be sorted are implicitly upcast when passed to the sort method. When is downcasting needed? Once an object is upcast, you cannot call methods that do not exist in the supertype. For example, if Rectangle objects have a method called rotate(), that method cannot be called from

within morph unless an explicit downcast is performed. This is an example of Javas strong typing. The compiler cannot be sure that the actual object passed in has a rotate method, so it forces you to say so explicitly. Extending interfaces An interface may also extend another interface. In that case, the extender is known as the superinterface and the extendee is the

subinterface. Example: interface FooInterface{ int foo1(); } interface FooInterface2 extends FooInterface{ int foo2(); } FooInerface2 contains both methods foo1 and foo2, and anything that implements FooInterface2 must implement both of these.

Part III: How interfaces are used This is difficult because there is no single, simple answer to the question. Like any semantic feature in a programming language, there are no hard and fast rules about in what situations it should best be exploited. However, there are many guidelines and general strategies (or else the feature would have never bee included). Well cover a few ways in which interfaces are typically used.

Some interface uses 1. 2. 3. 4. 5. To simply clarify the the functionality associated with a particular abstraction. To abstract functionality in a method to make it

more general, such as pointers to functions are used in C. sort is a good example. To implement callbacks, such as in GUI programming To write more general, implementation depending code that is easy to extend and maintain. To simulate global constants. Clarifying functionality Often you just want to write a code to do something. It will never do anything else.

You are not concerned about extensibility. Even in such a case, it can be nice to organize your program using interfaces. It makes the code easy to read and the intent of the author very clear. Callbacks Callbacks are a general programming technique where a method call another method which then calls the calling method (typically to inform the caller when some

action has taken place). Timer and Swing ActionListeners are good examples. To write more general implementation A method that operates on a variable of type interface automatically works for any sub-type of that interface. This is much more general than writing your program to operate only on a particular

subtype. See Shape example. Abstracting functionality A method can often be made more general by customizing its work based on the implementation of some other function that it calls. sort(...) is a good example. A sort() function can sort any list of items as long as you tell it how to compare any two items. Numerical methods for solvering differential

equations often depend on taking discrete derivatives: you can make such a routine general by specifying the derivate technique independently. Ineritance Basic inheritance Interfaces can be thought of as a special case of a more general class relationship called inheritance. When a class C2 inherits from or extends another

class C1, C1 is called the superclass of C2, and C2 the subclass of C1. This means that all of the public and protected members of the superclass are available to the subclass. This includes implementation and instance variables! Any class that is not explicitly declared as final can be extended. Inheritance syntax For one class to be a subclass of another, we use the extends keyword:

class GradStudent extends Student{ ...} class Manager extends Employee{...} etc. The superclass requires no special syntax. Subtyping Everything we learned about typing and subtyping holds a fortiori for super and subclasses. Specifically, classes which extend other classes are of both type superclass and type

subclass. A class can only extend a single class (no multiple implementation inheritance). Method overriding Overriding refers to the introduction of an instance method in a subclass that has the same name, signature, and return type of a method in the superclass. Implementation of this method in the subclass replaced the implementation of the

method in the superclass. Overriding example class A{ public void m1(){...} } class B extends A{ public void m1(){...} } For objects of class B, its own unique version of m1 will be called. We say that the method m1 in B overrides the method m1 in its superclass.

What is the point? Great advantage of implementation inheritance is code re-use. By factoring functionality common among many classes to a single superclass, each subclass is much simpler. Furthermore, code changes need to occur in only one place. However, when distributing a library, this can also be anathema to clients when used nonjudiciously breaks encapsulation!

When to use inheritance As with every semantic construct, there are no firm rules. Even general guidelines are not uniformly agreed upon. A non-controversial statement is probably: Be very careful not to overuse. Deep inheritance hierarchies are very hard to keep track of and maintain. We will be exploring these issues constantly throughout the rest of the class.

The rules: Certification fodder The easier (but still sometimes subtle) question is what the exact rules are. Divide rules up into several sections instance variables

constrcutors methods issues with static ivs and methods Rules for instance variables First must understand concept of a package in Java. A package is a collection of related class definition. For example, java.lang, java.util, java.util.regex, etc. Classes that are not placed within a package are

automatically in the default package. Its generally a good idea to explicitily place all of your programs within a package. Using classes from a package Two ways to access classes from a package: use full package name: java.util.ArrayList x = new java.util.ArrayList(); use import statement: import java.util.ArrayList then can use just class name everywhere after import

What if names conflict? compiler warning must use full name to distinguish Can also use wildcard for all classes in package import javva.util.*; Creating packages To create a package, place the package identifier at the top of your source code. e.g. package myjava.lib;

This places the current class/classes in a package called myjava.lib (notice the lowercase convention for package names). Important: this class must be placed in a corresponding directory structure $ROOT/myjava/ lib. Your CLASSPATH must contain $ROOT Class visibility Classes themselves can be public or default (Im avoiding discussion of inner classes here)

public class Foo{ ...} class Foo{ ... } classes that are public are visible outside of their package classes that are default are visible only within their package Every source code file can have at most one public class but infinitely many package-scope classes. Rules for instance variables public: accessible from any class default: accessible from any class within

same package protected: default + any subclass private: accessible only in class where defined. note: this includes any object created from that class More on private ivs If class B extends class A and A has private ivs, those ivs are part of class B.

However, those ivs cannot directly be accessed by class B. This is an important distinction they are part of class B, but cannot be accessed directly by class B!? class B would have to call non-private accessor/mutator methods to access As ivs. Private vs. Protected ivs To give direct access to superclass ivs, make them protected.

This is sometimes a very good idea, and sometimes a very bad idea. Ideas? Horstmann points out that it breaks encapsultion. This is true, but the whole idea of inheritance breaks encapsulation. General rule: when constructing a library, be very careful when making anything non-private, espcially ivs. When programming within a package, requirements less stringent. Rules for constructors Unlike other methods and ivs, constructors

are not inherited When instantiating a class B that is a subclass of some class A: To call As constructor the first line of Bs constructor must be super(...); // assume this is a valid constructor super(...) may be ommitted if you wish to call the superclass default constructor. Rules for methods

Visibility rules are the same for variables. Public methods are often preferred way to access ivs. A method in a superclass may be redefined in a subclass. This is called overriding (see next slide). Regular rules of overloading apply. Method overriding Overriding refers to the introduction of an

instance method in a subclass that has the same name, signature, and return type of a method in the superclass. Implementation of this method in the subclass replaced the implementation of the method in the superclass. Simple examples Best to start abstract. Lets do data-only classes: class A{ private int iv1; public int iv2;

protected int iv3; int iv4; } class B extends A{ Overriding example class A{ public void m1(){...} } class B extends A{ public void m1(){...} }

For objects of class B, its own unique version of m1 will be called. We say that the method m1 in B overrides the method m1 in its superclass. Rules for static methods and fields Static methods and fields cannot be overriden. I dont ever do this. Please consult book if you are interested. It is not typical.

Abstract classes Weve covered two extremes of inheritance: interfaces: all methods are abstract in superclass and superclass (ie interface) serves to define common type non-abstract superclasses: all methods are nonabstract in superclass and subclass actually inheritents implementation. good for code re-use also good for defining common type Abstract classes, cont. Using abstract methods, we can actually program

in between these two models. This is done by creating superclasses that are mixtures of abstract and non-abstract methods plus ivs. The non-abstract methods and ivs are inherited just like with regular classes. The abstract methods are treated just like interface methods they must be implemented. Rules for abstract classes Any method in a class may be given the

abstract keyword. public abstract void foo(...) If one or more methods in a class are abstract, the class itself must be declared abstract abstract class Foo{ ...} Abstract classes may be subclassed, but not instantiated. More on abstract classes Abstract methods must have no meat.

It is not required that a subclass implement every (or any) abstract method in an abstract superclass. However, if all abstract methods are not implemented in the subclass, the subclass must be declared abstract. classes with all abstract methods are almost exactly like interfaces (what are the differences?) Graphic view pure impl inheritance

regular class Code Reuse mixed impl/ interface inheritance abstract class pure interface inheritance

interface polymorphism Example: InputStream class java.io.InputStream is a good case-study in

abstract classes InputStream is abstract because int read() is not implemented. It is up to a subclass of InputStream to implement this method. FileInputStream one such concrete subclass? Question: what is the advantage of this structure? Overview of Swing Widgets, layouts, events What is Swing

All of our clients to this point have been text- based. These are also known as scripting clients. Most commercial software is event driven events are mouse clicks, mouse drags, etc. events take place on components, aka widgets buttons, text areas, comboboxes, etc. Swing is Javas library for creating graphical clients, typically knowns as GUI (Graphical User Interfaces)

GUI vs. Scripting Advantages Easy for non-computer person to use Self-documenting Attractive Disadvantages More complex coding Memory hogs Slower to use for expert user How to Create GUIs in Java

Three steps for simple GUI Determine what components that you would like your form to contain. Examples are buttons, checkboxes, text areas, graphics panels, etc. Determine how you would like these panels to layout on your form. This is the domain of layout managers. Attach event handlers to the desired components in the GUI. Creating a GUI application

Must create a JFrame and call the show() method. This is normally done by creating a class that extends JFrame as: class MyFrame extends JFrame{ MyFrame(){ setSize(300,300); //sets the pixel size } main(){ MyFrame frame = new MyFrame(); frame.show(); }

} Swing Components Swing components, cont. Each component is a Java class with a fairly extensive inheritency hierarchy: Object Component Container JComponent

Window JPanel Frame JFrame Using Swing Components Very simple, just create object from appropriate class examples:

JButton but = new JButton(); JTextField text = new JTextField(); JTextArea text = new JTextArea(); JLabel lab = new JLabel(); Many more classes. Dont need to know

every one to get started. See ch. 9 Hortsmann Adding components Once a component is created, it can be added to a container by calling the containers add method: Container cp = getContentPane(); cp.add(new JButton(cancel)); cp.add(new JButton(go));

How these are laid out is determined by the layout manager. Laying out components Not so difficult but takes a little practice Do not use absolute positioning not very portable, does not resize well, etc. Laying out components Use layout managers basically tells form how to

align components when theyre added. Each Container has a layout manager associated with it. A JPanel is a Container to have different layout managers associated with different parts of a form, tile with JPanels and set the desired layout manager for each JPanel, then add components directly to panels. Layout Managers Java comes with 7 or 8. Most common and

easiest to use are FlowLayout BorderLayout GridLayout Using just these three it is possible to attain fairly precise layout for most simple applications. Setting layout managers Very easy to associate a layout manager

with a component. Simply call the setLayout method on the Container: JPanel p1 = new JPanel(); p1.setLayout(new FlowLayout(FlowLayout.LEFT)); JPanel p2 = new JPanel(); p2.setLayout(new BorderLayout()); As Components are added to the container, the layout manager determines their size and positioning. Event handling

What are events? All components can listen for one or more events. Typical examples are: Mouse movements Mouse clicks Hitting any key

Hitting return key etc. Telling the GUI what to do when a particular event occurs is the role of the event handler. ActionEvent In Java, most components have a special event called an ActionEvent. This is loosely speaking the most common

or canonical event for that component. A good example is a click for a button. To have any component listen for ActionEvents, you must register the component with an ActionListener. e.g. button.addActionListener(new MyAL()); Delegation, cont. This is referred to as the Delegation Model. When you register an ActionListener with a component, you must pass it the class

which will handle the event that is, do the work when the event is triggered. For an ActionEvent, this class must implement the ActionListener interface. This is simple a way of guaranteeing that the actionPerformed method is defined. actionPerformed The actionPerformed method has the following signature:

void actionPerformed(ActionEvent) The object of type ActionEvent passed to the event handler is used to query information about the event. Some common methods are: getSource() object reference to component generating event getActionCommand() some text associated with event (text on button, etc).

actionPerformed, cont. These methods are particularly useful when using one eventhandler for multiple components. Simplest GUI import javax.swing.JFrame; class SimpleGUI extends JFrame{ SimpleGUI(){ setSize(400,400); //set frames size in pixels

setDefaultCloseOperation(EXIT_ON_CLOSE); show(); } public static void main(String[] args){ SimpleGUI gui = new SimpleGUI(); System.out.println(main thread coninues); } } Another Simple GUI import javax.swing.*; class SimpleGUI extends JFrame{

SimpleGUI(){ setSize(400,400); //set frames size in pixels setDefaultCloseOperation(EXIT_ON_CLOSE); JButton but1 = new JButton(Click me); Container cp = getContentPane();//must do this cp.add(but1); show(); } public static void main(String[] args){ SimpleGUI gui = new SimpleGUI(); System.out.println(main thread coninues); }}

Add Layout Manager import javax.swing.*; import java.awt.*; class SimpleGUI extends JFrame{ SimpleGUI(){ setSize(400,400); //set frames size in pixels setDefaultCloseOperation(EXIT_ON_CLOSE); JButton but1 = new JButton(Click me); Container cp = getContentPane();//must do this cp.setLayout(new FlowLayout(FlowLayout.CENTER cp.add(but1);

show(); } public static void main(String[] args){ SimpleGUI gui = new SimpleGUI(); System.out.println(main thread coninues); Add call to event handler import javax.swing.*; import java.awt.*; class SimpleGUI extends JFrame{ SimpleGUI(){ setSize(400,400); //set frames size in pixels

setDefaultCloseOperation(EXIT_ON_CLOSE); JButton but1 = new JButton(Click me); Container cp = getContentPane();//must do this cp.setLayout(new FlowLayout(FlowLayout.CENTER but1.addActionListener(new MyActionListener()); cp.add(but1); show(); } public static void main(String[] args){ SimpleGUI gui = new SimpleGUI(); System.out.println(main thread coninues);

Event Handler Code class MyActionListener implements ActionListener{ public void actionPerformed(ActionEvent ae){ JOptionPane.showMessageDialog(I got clicked, null); } } Add second button/event class SimpleGUI extends JFrame{ SimpleGUI(){ /* .... */

JButton but1 = new JButton(Click me); JButton but2 = new JButton(exit); MyActionListener al = new MyActionListener(); but1.addActionListener(al); but2.addActionListener(al); cp.add(but1); cp.add(but2); show(); } } How to distinguish events Less

good way class MyActionListener implents ActionListener{ public void actionPerformed(ActionEvent ae){ if (ae.getActionCommand().equals(Exit){ System.exit(1); } else if (ae.getActionCommand().equals(Click me){ JOptionPane.showMessageDialog(null, Im clicked); } }

Good way class MyActionListener implents ActionListener{ public void actionPerformed(ActionEvent ae){ if (ae.getSource() == but2){ System.exit(1); } else if (ae.getSource() == but1){ JOptionPane.showMessageDialog(null, Im clicked) } } Question: How are but1, but2 brought into scope to do this?

Question: Why is this better? Putting it all together See LoginForm.java example in class notes Java I/O classes Flexible and somewhat slick, but a bit of a mess Java classes for doing i/o Includes file i/o, memory i/o, socket i/o, inter-

process (pipes), etc. All stored in package java.io Excellent example of OO design Very general and scaleable Unfortunately, also obfuscates simple tasks. How to proceed Understand basic design Create some libraries to do common tasks InputStream/OutputStream Start by studying the java.io.InputStream and java.

io.OutputStream API These are base class for performing all binary i/o Note that these classes are abstract each with a single abstract method abstract int read() abstract void write(int) Concrete subclasses must provide implementation of read/write that can get/put a single byte to/from the relevant source

Concrete subclasses of InputStream/OutputStream Since InputStream/OutputStream are abstract, they cannot be used to create objects (of course, they can be used for typing). A very common non-abstract subclass is FileOutputStream/FileInputStream. These can be used in a simple way to do the most basic byte-based file io

Example with FileInputStream /* class example DataInput1.java */ /* assumes each char is one byte -- dangerous import java.io.FileInputStream; public class DataInput1{ public static void main(String[] args) throws Exception{ String file = args[0]; int input; FileInputStream fin = new FileInputStream(file); while ( (input = fin.read()) != -1){ System.out.print((char) input);

} } } Example with FileOutputStream /* class example DataOutput1.java */ /* assumes each char is a single byte */ import java.io.FileOutputStream; public class DataOutput1{ public static void main(String[] args) throws Exception{ String file = args[0]; String output = "Hello World";

FileOutputStream fout = new FileOutputStream(file); char[] outputAsChars = output.toCharArray(); for (int i = 0; i < outputAsChars.length; ++i) fout.write(outputAsChars[i]); } } Higher-level functionality FileInputStream and FileOuputStream allow you to do pretty much any file i/o at a very low level. However, this is too low-level for Java.

Java provides many more libraries to read/write higher-level constructs: characters Strings native datatypes arrays

arbitrary objects (serialization) Decorator Pattern These capabilities are added using a design called the Decorator Pattern. InputStream/OutputStream instances are passed to a wrapper or decorator class that uses them and adds to their functionality. For example, floating point numbers can be read from a file by chaining together a FileInputStream and another class that

assembles bytes into portable floating point. Purpose of Decorator Best way to think of this is as follows: There are two important issues when constructing an i/o library

Where the i/o is going (file, etc). How the data is represented (String, native type, etc.) Rather than create a class for each combination, Decorator classes allow you to mix and match, augment functionality of base classes.

This is a bit confusing but is very flexible. Decotators can also add other capabilities, such as peek ahead, push back, write line number, etc. Decorator Pattern Java i/o Decorator Classes All Java i/o decorator classes inherit from FilterInputStream and FilterOutputStream Look at the api for these classes and note a few things:

They wrap instances of InputStream/OutputStream respectively. They inherit from InputStream/OutputStream respectively This is an odd inheritence hierarchy but is necessary to ensure that the FilterStreams support the same interface as the underlying class. More on Filter Streams Easiest way to think of the filter streams as

wrapping an underlying class which they augment the functionality of. Consider the respective constructors FilterInputStream(InputStream in); FilterOutputStream(OutputStream out); In each case, the FilterStreams use an underlying presumably simpler inputstream and augment its functionality.

Some FilterStream examples to clarify this Perhaps most common FilterInputStream is DataInputStream. Study the API and be sure you understand the inheritance hierarchy DataInputStream stores an InputStream and uses this to do higher-level i/o readInt, readDouble, etc. DataOutputStream is analogous

Example of DataInputStream /* DataInputStream2 example in course examples */ import java.io.DataOutputStream; import java.io.FileOutputStream; public class DataOutput2{ public static void main(String[] args) throws Exception{ String file = args[0]; double[] data = {1.1,1.2,1.3,1.4,1.5}; DataOutputStream dout = new DataOutputStream (new FileOutputStream(file)); for (int i = 0; i < data.length; ++i){

dout.writeDouble(data[i]); } dout.close();}} Example of DataInputStream /* DataOutput2 example in course examples */ import java.io.DataInputStream; import java.io.FileInputStream; import java.io.EOFException; public class DataInput2{ public static void main(String[] args) throws Exception{

String file = args[0]; DataInputStream din = new DataInputStream(new FileInputStream(file double data; try{ while (true){ data = din.readDouble(); System.out.println(data); } } catch (EOFException eofe){} din.close();}}

Other Decorators Another common set of decorator classes is BufferedInputStream and BufferedOutputStream. These augment the functionality of the underlying stream by providing system buffering for higherperformance i/o They also add support for the mark method. Examples on next slide (notice how these classes can be multiply chained together in various ways. BufferedInputStream Example import java.io.*; /

public class DataInput3{ public static void main(String[] args) throws Exception{ String file = args[0]; DataInputStream din = new DataInputStream (new BufferedInputStream (new FileInputStream(file))); double data; /* need an exception to know when end of file is hit */ try{ while (true){ data = din.readDouble(); System.out.println(data);

} } catch (EOFException eofe){} din.close();}} BufferedOutputStream example import java.io.BufferedOutputStream; import java.io.DataOutputStream; import java.io.FileOutputStream; public class DataOutput3{ public static void main(String[] args) throws Exception{ String file = args[0];

double[] data = {1.1,1.2,1.3,1.4,1.5}; DataOutputStream dout = new DataOutputStream (new BufferedOutputStream (new FileOutputStream(file))); for (int i = 0; i < data.length; ++i){ dout.writeDouble(data[i]); } dout.close();}} Other output streams FileOutputStream is probably the most common. However, note that we could replace

FileOutputStream with another Outputstream in these examples. In that case, the same decorated or undecorated data would be sent to some other device. Good example of this is thread communicatoin, memory i/o, and socket i/o (using Socket class). I strongly encourage you to familiarize yourself with these classes. Character-based i/o Reader and Writer classes

Reader/Writer Java maintains a second class hierarchy for performing higher-level character-based i/o. The two base classes in this case are java.io.Reader java.io.Writer Study the API for these classes. Very similar to InputStream/OutputStream Here Ill show how to do some common i/o

tasks as examples FileWriter Example /* example Writer1.java in course examples */ /* using a simple FileWriter for String-based i/o */ import java.io.FileWriter; public class Writer1{ public static void main(String[] args) throws Exception{ String file = args[0]; String output = "Hello World!"; FileWriter fw = new FileWriter(file);

fw.write(output); fw.close(); } } Reading lines from stdin import java.io.BufferedReader; import java.io.InputStreamReader; public class Reader1{ public static void main(String[] args) throws Exception{ /* convert System.in, which is an InputStream, to a Reader by wrapping in InputStreamReader,

then wrap everything in BufferedReader */ String input; BufferedReader bin = new BufferedReader converts an (new InputStreamReader InputStream (System.in)); to a Reader while ( (input = bin.readLine()) != null){ System.out.println("you typed " + input); }}}

Reading by line from file import java.io.BufferedReader; /*Reader2.java */ import java.io.InputStreamReader; import java.io.FileInputStream; public class Reader2{ public static void main(String[] args) throws Exception{ /* convert a FileInputStream, which is an InputStream, to a Reader by wrapping in InputStreamReader, then wrap everything in BufferedReader and call the readLine method to get a line at a time */

String input; String file = args[0]; BufferedReader bin = new BufferedReader (new InputStreamReader (new FileInputStream(file))); while ( (input = bin.readLine()) != null){ System.out.println(input); } }} Exercise Study the jdk API for GZIPOutputStream

and GZIPInputStream. Write a program that reads and writes gzip files. Serialization Objects can be written to streams also. This process is known as serialization. This is a huge convenience compared with having to marshal and unmarshal ivs. But the issue is even deeper how are methods represented, objects that contain objects as ivs, etc.

Java takes care of all of this with a very nice serialization interface. Serialization classes Relevant classes java.io.ObjectInputStream java.io.ObjectOutputStream Note that these required an underlying Input/OutputStream to do their work. For a class to be serializable, it also must implement the Serializable interface (no methods).

Finally, a class-scope variable can be declared as transient, meaning that it is ignored during serialization. Serialization Example /* simple example of Serialization -- writing an object directly to an OutputStream without having to marshal and unmarshal */ import java.io.*; public class Serialization{ public static void main(String[] args) throws Exception{ String flag = args[0]; String file = args[1]; Currency c = new Currency("US Dollar", "USD, 10, 5);

Currency d; if (flag.equals("-w")){ ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(new File(file))); out.writeObject(c); } else if (flag.equals("-r")){ ObjectInputStream in = new ObjectInputStream(new FileInputStream(new File(file))); System.out.println("Reading serialized object"); d = (Currency) in.readObject(); }}}

Related Topics java.io.File class Very nice. Many methods for portably manipulating files java.io.Socket class Provides Input/OutputStreams for communication across ports of different computers PrintWriter class (e.g. println method) Writing zip files, jar files, etc. java rmi: Remote Method Invocation: DOs on top of serialization

Suggested Readings Eckels detailed section on i/o Patterns in Java, A Catalog of Reusable Design Patterns Illustratred with UML, Mark Grand, Wiley Press. Design Patterns, Elements of Reusable Object-Oriented Software, Gamma et al. Java Exceptions

Intro to Exceptions What are exceptions? Events that occur during the execution of a program that interrupt the normal flow of control. One technique for handling Exceptions is to use return statements in method calls. This is fine, but java provides a much more general and flexible formalism that forces programmers to consider exceptional cases.

Exception Class hierarchy Object must handle may handle too serious to catch Throwable Error Exception many

RuntimeException IndexOutOfBounds NullPointerException Exception Handling Basics Three parts to Exception handling 1. claiming exception 2. throwing exception 3. catching exception

A method has the option of throwing one or more exceptions when specified conditions occur. This exception must be claimed by the method. Another method calling this method must either catch or rethrow the exception. (unless it is a RuntimeException) Claiming Exceptions Method declaration must specify every exception that the method potentially

throws MethodDeclaration throws Exception1, Exception2, ..., ExceptionN Exceptions themselves are concrete subclasses of Throwable and must be defined and locatable in regular way. Throwing Exception To throw an Exception, use the throw keyword followed by an instance of the Exception class

void foo() throws SomeException{ if (whatever) {...} else{ throw new SomeException(...)} Well talk about passing data via the Exception constructor soon. Note that if a method foo has a throw clause within it, that the Exception that is thrown (or one of its superclasses) must be claimed after the signature.

Catching Exceptions The third piece of the picture is catching exceptions. This is what you will do with most commonly, since many of javas library methods are defined to throw one or more runtime exception. Catching exceptions: When a method is called that throws and Exception e.g SomeException, it must be called in a try-catch block: try{ foo(); }

catch(SomeException se){...} Catching Exceptions, cont. Note that if a method throws an Exception that is NOT a RuntimeException, you must do one of two things: try-catch it (often called handling it) rethrow it In the latter case, responsibility then moves

up the calling chain to handle it, and so on all the way up to main. More on try-catch The general form of the try-catch structure is: try{ /* any number of lines of code that call any number of methods with any thrown Exceptions */ } catch(Exception1 e1){ /* do anything you want here

e.g. change value and try again. print error and quit print stacktrace */ catch (Exception2 e2){ /* any number of exceptions can be handled ... */ Example1 import java.io.*; public class Exception1{ public static void main(String[] args){ InputStream f;

try{ f = new FileInputStream("foo.txt"); } catch(FileNotFoundException fnfe){ System.out.println(fnfe.getMessage()); } } } Example2 import java.io.*; public class Exception2{

public static void main(String[] args){ InputStream fin; try{ fin = new FileInputStream("foo.txt"); int input = fin.read(); } catch(FileNotFoundException fnfe){ System.out.println(fnfe.getMessage()); } catch(IOException ioe){ System.out.println(ioe.getMessage()); } }}

import java.io.*; public class Exception2{ public static void main(String[] args){ InputStream fin; try{ fin = new FileInputStream("foo.txt"); int input = fin.read(); } catch(FileNotFoundException fnfe){ System.out.println(fnfe.getMessage()); }

catch(IOException ioe){ System.out.println(ioe.getMessage()); } } } Recommendations Do not use Exceptions to handle normal conditions in the program that can be checked with if statements. For example: to find the end of an array

to check if an object is null See other commented examples in course notes. Creating your own Exceptions You can follow this procedure exactly when creating your own Exception. Create a class that subclasses Exception (or RuntimeException).

You may also add functionality so that a relevant message is stored when the error is thrown, and any other customized functionality you choose. See Exception5.java example Swing Graphics Programming Using Graphics object Use JPanels They have a surface on which you can draw.

They are containers and thus can hold other components How to draw on a Jpanel Class MyPanel extends JPanel{ public void paintComponent(Graphics g){ //use Graphics methods to draw } } Drawing on a JPanel How does this work? Framework calls paintComponent

automatically whenever the application needs to be redrawn. Can force this yourself, but do not call paintComponent directly. Instead, call repaint(), which will cause paintComponent to be called for all components. Graphics object method Framework hand over an object that implements the abstract class Graphics.

Contains functions for drawing shapes to panel. Simplest Example: Class HelloWorld extends Jpanel{ public void paintComponent(Graphics g){ g.drawString(Hello World, 50,50); } } More Graphics Object Methods Many more methods. Consult API as

always. Typical examples: drawOval drawCircle drawImage drawPolyLine

fillArc Graphics2D Object Much more sophisticated rendering capabilites. To access, use same technique and then downcast Graphics object to Graphics2D object (latter is subclass of former). See Graphics2D API for additional methods See DrawTest.java and

Colors Use java.awt.Color class 13 standard colors stored as static variables Color.red, Color.blue, Color.yellow, etc. To specify rbg value, create Color object: new Color(int red, int green, int blue); Use setPaint(Color) method of Graphics object to specify a Color. Also setBackground(Color) method

Useful: brighter() and darker() methods for Color objects Filling shapes Can fill the interior of any closed shape. Use fill() method. See FillTest.java Drawing images Very fun Supports standard GIF images.

Example Toolkit tk = getDefaultToolkit(); Image img = tk.getImage(foo.gif); g.drawImage(img, 0, 0, null); (assuming g is instance of Graphics class) Drawing images Very fun Supports standard GIF images. Example Toolkit tk = getDefaultToolkit();

Image img = tk.getImage(foo.gif); drawImage(img, 0, 0, null); Odds and Ends Timing, system commands, toString and .equals Timing Java code Simplest is to use static method in System class double currentTimeMillis(); Example:

public static void main (String args[]){ double begin = System.CurrentTimeMillis(); doWork(); double end = System.CurrentTimeMillis(); double time = (end begin)/(1000.*60); System.out.println(Total time minutes: + time); } Spawning an OS process Very simple but a little different architecturally Every java program keeps a single copy of a class

Runtime which allows the user to interact with the OS This is obtained with a static method call (Singleton pattern) and returns a Process object: Runtime rt = Runtime.getRuntime(); Process p = rt.exec(ls *.java); InputStream in = p.getInputStream(); //exercise: wrap this PipeInputStream in Reader Overriding equals method A method with signature boolean equals(Object)

exists in the object class. This method by default determines whether two Objects point to the same memory location: SomeObject o1, o2; o1 = new SomeObject(); o2 = o1; if (o1.equals(o2)) // yes, same loc in mem This is not very useful in general. Good idea to override with more useful comparison metric .equals, cont. Very good example is String class. .equals is overriden for String class to

actually do a character by character comparison. Thus, two String which occupy different memory but have the same contents are considered equal. Note that using == always does a memory comparison! Very import!! .equals, cont. There are a few subtle rules that any

overriding of a .equals must follow. Well study these plus other object methods (in particular clone) next class. Very Basic Applets Programs that run within web browsers What are applets? Applets are simply java programs that run in web browsers.

Created a big stir in mid-90s when Netscape agreed to embed jvm in Navigator web browswer. Great promise applications automatically distributed to anyone on any platform! Reality non-uniform browswer support, limitations imposed by security, easier ways to accomplish same thing! What are applets, cont. Still useful in just the right situation fancy, full-fledged client can make some assumptions/have some control over

client technology Also, very good for understanding issues in web client-server programming Otherwise, server-heavy programming with html or client-side scripting wins out. Also, Java WebStart new alternative can launch full applications remotely without need for host browser Applet inheritance tree Object

Component Container Window Panel Frame Applet JFrame

JApplet Use to access Swing Components Some Hello World Applets To see how applets work, we start with a couple of versions of a HelloWorld program One uses the fact that an Applet is a Panel with a graphics object The other uses a button label

Soon, we will add full event-handling capabilities and nice graphics. This is just a start. HelloWorldApplet1 import java.awt.*; import javax.swing.*; public class HelloWorldApplet1 extends JApplet{ public void init(){ getContentPane().add(new JLabel(

Hello World, SwingConstants.CENTER)); } } Note that all Applets are class that extend either Applet or JApplet init() is called when the Applet is loaded HelloWorldApplet2 import java.awt.*; import javax.swing.*; public class HelloWorldApplet1 extends JApplet{

public void painComponent(Graphics g){ g.drawString(Hello World, 50, 50); } } Since Applets are Panels, we can override PaintComponent and get a Graphics Object This will become particularly handy when doing animations Running Applets To run applets, do the following: Compile class file in regular way

Create an html file that includes an applet tag pointing to the applet (width and height are pixel coords) Invoke browser or appletviewer on html file Note that applet tag can include other parameters, some of which of used by browser automatically (e.g. width). Also, Java2 plug-in required for browsers to support

applets. Good to test with appletviewer first. Life Cycle of Applet To write more sophisticated applets, a number of other methods may need to be overwritten.

void init() void start() void stop() void destroy() We discuss the role of each next. init() method The browswer calls the init method when the applet is loaded for the first time. init() behaves much like a constructor.

Typical uses: parsing param values from html files opening streams and sockets creating GUI components opening database connections Important: note that the refresh button doesnt

necessarily reload applet. This is browser-dependent. To guarantee reloading, browser needs to be killed and restarted. start() method called every time page is located this can mean moving off and back onto page, hitting reload, etc. always called after init() when page is first loaded. Typical uses: starting animations

anything else that needs to start anew with every location of applet for simpler things this method can be ignored stop() method called whenever user moves off the page where applet sits always called after start typical uses ending animations stopping other time-consuming system activity

often ignore this for simpler applets destroy() method called once when browser shuts down normally always called after stop() typical uses: close database connections close streams clean up any other resources

often ignored for simpler applets More on applet life-cycle Its very import to be aware that some other software, ie the host browser, is calling these methods at certain times. You the program do not call these methods. This is classic OO framework: dont call us, well call you. You write the class and specialize certain

methods, some other code calls your custorm versions of those methods at specified times. Other applet issues Security Horstmann pg. 499 Pop-up windows Horstmann pg. 500 Applet tags and attributes Horstmann pg. 502 Passing info to applets

Horstmann pg. 506 Applets making socket connections Jar Files Packaging java applications What is a jar file? JAR: Java Archive typical way of distributing java applications/libraries JARs are ZIP files containing java class files

possibly other resource files (images, audio, text, etc.) optional manifest file: describes certain attributes of JAR file. Applets, beans, applications typically all distributed as single of collection of JAR files. Creating JAR Files Either use commandline tool like jar cvf whatever.jar *.class or API

see java.util.jar docs No name restrictions for jar file whatever is legal on a given platform Can jar directories as well as files. In that case, contents are added recursively Syntax similar to tar. See http://java.sun.com/docs/books/tutorial/jar for more details Self-running JAR files jars can get quite complex when using with J2EE.

Manifest files can be used to specify security attributes, versioning, extensions, services, etc. Also, for better performance multi-class applets can be packaged as jar files. Course web page contains links to more detailed info on creating sophisticated JARs Here were present executable JAR files as a simple example. Steps to making executable jar To make an executable jar, take the following steps:

create a .jar of all class files in your application using jar cvf Whatever.jar *.class (packages and nested directories work fine here). create a manifest file with any name (say manifest.mf). manifest.mf must at least contain a line pointing to the main class, e.g.: Main-Class: com/mypackage/MainAppClass finally, add manifest using the update flag u jar uvfm Whatever.jar manifest.mf run as java jar Whatever.jar Gotchas

Manifest file must contain linefeed as last character or else wont be parsed properly. You should always use packages with jar files. be careful to put full pathname to Main-Class. Other issues Launching a jar from an icon System dependent. See Horstmann. Locating Resources See URL class

Sealing JARs Sealed: true attribute in manifest file Signing JARs can give trusted permission to trusted parties. See web link above for more details Overriding Object Methods The Object Class Every java class has Object as its superclass and

thus inherits the Object methods. Object is a non-abstract class Many Object methods, however, have implementations that arent particularly useful in general In most cases it is a good idea to override these methods with more useful versions. In other cases it is required if you want your

objects to correctly work with other class libraries. Some Object class methods Object methods of interest: clone equals hashcode toString finalize Other object methods getClass wait, notify, notifyAll (relevant for threaded

programming) Clone method Recall that the = operator simply copies Object references. e.g., >> Student s1 = new Student(Smith, Jim, 3.13); >> Student s2 = s1; >> s1.setGPA(3.2); >> System.out.println(s2.getGPA()); 3.2

What if we want to actually make a copy of an Object? Most elegant way is to use the clone() method inherited from Object. Student s2 = (Student) s1.clone(); Subtleties of clone() method First, note that the clone method is protected in the Object class.

This means that it is protected for subclasses as well. Hence, it cannot be called from within an Object of another class and package. To use the clone method, you must override in your subclass and upgrade visibility to public. More subtleties of clone Also, any class that uses clone must implement the Cloneable interface.

This is a bit different from other interfaces that weve seen. There are no methods; rather, it is used just as a marker of your intent. The method that needs to be implemented is inherited from Object. More clone() issues Finally, clone throws a CloneNotSupportedException. This is thrown if your class is not marked

Cloneable. This is all a little odd but you must handle this in subclass. Steps for cloning To reiterate, if you would like objects of class C to support cloning, do the following: implement the Cloneable interface override the clone method with public access privileges call super.clone()

Handle CloneNotSupported Exception. This will get you default cloning, but more subtleties still lurk. Shallow Copies We havent yet said what the default clone()

method does. By default, clone makes a shallow copy of all ivs in a class. Shallow copy means that all native datatype ivs are copied in regular way, but ivs that are objects are not recursed upon that is, references are copied. This is not what you typically want. Must override clone explicitly clone object ivs!

Immutable Objects A special class of Objects are called immutable because their state cannot be changed once set. Common examples are String, Integer, etc. Immutable object simplify programming in certain instances, such as when writing thread safe code.

They also simplify cloning, since an object that cannot be changed doesnt really need to be deepcopied. See ShallowCopy2.java in course examples Deep Copies For deep copies that recurse through the object ivs, you have to do some more work. super.clone() is first called to clone the first level of ivs. Returned cloned objects object fields are

then accessed one by one and clone method is called for each. See DeepClone.java example Additional clone() properties Note that the following are typical, but not strictly required: x.clone() != x; x.clone().getClass() == x.getClass(); x.clone().equals(x);

Finally, though no one really cares, Object does not support clone(); toString() method The Object method String toString(); is intended to return a readable textual

representation of the object upon which it is called. This is great for debugging! Best way to think of this is using a print statement. If we execute: System.out.println(someObject); we would like to see some meaningful info about someObject, such as values of ivs, etc. default toString() By default toString() prints total garbage that no

one is interested in getClass().getName() + '@' + Integer.toHexString(hashCode()) By convention, print simple formatted list of field names and values (or some important subset). The intent is not to overformat. Typically used for debugging. Always override toString()!

equals() method Recall that boolean == method compares when applied to object compares references. That is, two object are the same if the point to the same memory. Since java does not support operator overloading, you cannot change this operator. However, the equals method of the Object class gives you a chance to more meaningful compare objects of a given class.

equals method, cont By default, equals(Object o) does exactly what the == operator does compare object references. To override, simply override method with version that does more meaningful test, ie compares ivs and returns true if equal, false otherwise. See Equals.java example in course notes.

equals subtleties As with any method that you override, to do so properly you must obey contracts that go beyond interface matching. With equals, the extra conditions that must be met are discussed on the next slide: equals contract It is reflexive: for any reference value x, x.equals(x) should

return true. It is symmetric: for any reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true. It is transitive: for any reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true. It is consistent: for any reference values x and y, multiple

invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the object is modified. For any non-null reference value x, x.equals(null) should return false. hashcode() method Java provides all objects with the ability to generate a hash code. By default, the hashing algorithm is typically based on an integer representation

of the java address. This method is supported for use with java.util.Hashtable Will discuss Hashtable in detail during Collections discussion. Rules for overriding hashcode Whenever invoked on the same object more than once, the hashCode method must return the same integer, provided no information used in equals comparisons on the object is modified. If two objects are equal according to the equals(Object) method,

then calling the hashCode method on each of the two objects must produce the same integer result. It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables. finalize() method Called as final step when Object is no longer used,

just before garbage collection Object version does nothing Since java has automatic garbage collection, finalize() does not need to be overridden reclaim memory. Can be used to reclaim other resources close streams, database connections, threads. However, it is strongly recommended not to rely on

this for scarce resources. Be explicit and create own dispose method. A little more Swing MVC Pattern More graphics Graphics See course examples applets/BrownianMotionApplet.java shows how to do a simple animation with some basic drawing in the

start() method graphics/Mandelbrot.java shows how to create an image from raw data and draw to an applet by overriding paintComponent. graphics/Shapes.java A great example of how to architect a simple GUI that seems more complex when poorly designed. graphics/Painter.java Classic example of how to paint on a Panel

graphics/Bounce.java Classic example of why one needs threads. Miscellaneous Some how-to snippets See the following codes in the course examples GetProperties.java

How to query various system properties ProcessTest.java An example spawning an OS executable NumberFormat.java An example formatting a double TimeTest.java An example using simple System timers. What is the Collections

framework? Collections framework provides two things: implementations of common high-level data structures: e.g. Maps, Sets, Lists, etc. An organized class hierarchy with rules/formality for adding new implementations The latter point is the sense in which Collections are a framework. Note the difference between providing a framework + implementation and just implementation.

Some other differences: code reuse clarity unit testing? History Pre Java SDK1.2, Java provided a handful of data structures: Hashtable Vector

Bitset These were for the most part good and easy to use, but they were not organized into a more general framework. SDK1.2 added the larger skeleton which organizes a much more general set of data structures. Legacy datastructures retrofitted to new model. General comments about data structures

Containers for storing data. Different data structures provide different abstractions for getting/setting elements of data. linked lists hastables vectors

arrays Same data structures can even be implemented in different ways for performance/memory: queue over linked list queue over arrays More on data structures Everyone should take a basic class in building data

structures I recommend the book Mastering Algorthims with C by Kyle Loudon In Java, one does not usually build data structures, but rather uses the provided one Using Javas data structures requires a little understanding of the Collections framework Adding your own requires a deeper understanding.

Learning to use data structures Dual purposes for us to study Collections: Be able to choose, properly use built-in data structures. Another study in OO class design Thus, we start by study the Collections class design. Then, we provide many examples of how to use the built-in types in real programming. Collections-related Interface

hierarchy Collection List Set Map Iterator SortedMap

ListIterator SortedSet The Collection inteface stores groups of Objects, with duplicates allowed The Set interface extends Collection but forbids duplicates The List interface extends Collection, allows duplicates, and introduces positional indexing. Map is a separate hierarchy Collection implementations

Note that Java does not provide any direct implementations of Collection. Rather, concrete implementations are based on other interfaces which extend Collection, such as Set, List, etc. Still, the most general code will be written using Collection to type variables. Collection Interface boolean add(Object o); boolean addAll(Collection c);

void clear(); boolean contains(Object o); boolean containsAll(Collection c); boolean equals(Object o); int hashCode(); boolean isEmpty(); Iterator iterator(); boolean remove(Object o); boolean removeAll(Collection c); boolean retainAll(Collection c); int size(); Object[] toArray();

Object[] toArray(Object[] a); Optional operation, throw UnsupportedOperationException Comments on Collection methods Note the iterator() method, which returns an Object which implements the Iterator interface. Iterator objects are used to traverse

elements of the collection in their natural order. Iterator has the following methods: boolean hasNext(); // are there any more elements? Object next(); // return the next element void remove(); // remove the next element AbstractCollection Class java.util.AbstractCollection Abstract class which is partial implementation of

of Collection interface Implements all methods except iterator() and size() Makes it much less work to implement Collections Interface List interface An interface that extends the Collections interface. An ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list.

Unlike Set, allows duplicate elements. Provides a special Iterator called ListIterator for looping through elements of the List. Additional methods in List Interface List extends Collection with additional methods for performing index-based operations:

void add(int index, Object element) boolean addAll(int index, Collection collection) Object get(int index) int indexOf(Object element)

int lastIndexOf(Object element) Object remove(int index) Object set(int index, Object element) List/ListIterator Interface The List interface also provides for working with a subset of the collection, as well as iterating through the entire list in a position friendly manner: ListIterator listIterator() ListIterator listIterator(int startIndex)

List subList(int fromIndex, int toIndex) ListIterator extends Iterator and adds methods for bi-directional traversal as well as adding/removing elements from the underlying collection. Concrete List Implementations There are two concrete implementations of the List interface

LinkedList ArrayList Which is best to use depends on specific needs. Recall that linked lists are optimal for inserting/removing elements. ArrayLists are good for traversing. Note that LinkedList and ArrayList both extend abstract partial implementations of the List interface.

LinkedList Class The LinkedList class offeres a few additional methods for directly manipulating the ends of the list:

void addFirst(Object) void addLast(Object); Object getFirst(); Object getLast(); Object removeFirst(); Object removeLast(); These methods make it natural to implement other simpler data structures, like Stacks and Queues. LinkedList examples

See heavily commented LinkedList Example in course notes A few things to be aware of: it is really bad to use the positional indexing features copiously of LinkedList if you care at all about performance. This is because the LinkedList has no memory and must always traverse the chain from the beginning. Elements can be changed both with the List and ListIterator objects. That latter is often more convenient. You can create havoc by creating several iterators that you use to mutate the List. There is some protection built-in, but best is to

have only one iterator that will actually mutate the list structure. ArrayList Class Also supports the List interface, so top-level code can pretty much invisibly use this class or LinkedList (minus a few additional operations in LinkedList). However, ArrayList is much better for using positional index access methods. At the same time, ArrayList is much worse at inserting elements.

This behavior follows from how ArrayLists are structured: they are just like Vectors. More on ArrayList Additional methods for managing size of underlying array

size, isEmpty, get, set, iterator, and listIterator methods all run in constant time. Adding n elements take O[n] time. Can explicitly grow capacity in anticipation of adding many elements. Note: legacy Vector class almost identical. Main differences are naming and synchronization. See short ArrayList example. Set Interface Set also extends Collection, but it prohibits duplicate

items (this is what defines a Set). No new methods are introduced; specifically, none for index-based operations (elements of Sets are not ordered). Concrete Set implementations contain methods that forbid adding two equal Objects.

More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element Java has two implementations: HashSet, TreeSet HashSets and hash tables Lists allow for ordered elements, but searching

them is very slow. Can speed up search tremendously if you dont care about ordering. Hash tables let you do this. Drawback is that you have no control over how elements are ordered. hashCode() computes integer (quickly) which corresponds to position in hash table. Independent of other objects in table. HashSet Class Hash tables can be used to implement several

important data structures. Simplest of these is HashSet add elements with add(Object) method contains(Object) is redefined to first look for duplicates. if duplicate exists, Object is not added What determines a duplicate? careful here, must redefine both hashCode() and equals(Object)! HashSet

Look HashSetExample.java Play around with some additional methods. Try creating your own classes and override hashCode method. Do Some timings. Be certain you understand this for final! Tree Sets Another concrete set implementation in Java is TreeSet.

Similar to HashSet, but one advantage: While elements are added with no regard for order, they are returned (via iterator) in sorted order. What is sorted order? this is defined either by having class implement Comparable interface, or passing a Comparator object to the TreeSet Constructor. Latter is more flexible: doesnt lock in specific sorting rule, for example. Collection could be sorted in one place by name, another by age, etc.

Comparable interface Many java classes already implement this. Try String, Character, Integer, etc. Your own classes will have to do this explicitly: Comparable defines the method public int compareTo(Object other); Comparator defines the method public int compare(Object a, Object b); As we discussed before, be aware of the general

contracts of these interfaces. See TreeSetExample.java Maps Maps are similar to collections but are actually represented by an entirely different class hierarchy. Maps store objects by key/value pairs: map.add(1234, Andrew); ie Object Andrew is stored by Object key 1234

Keys may not be duplicated Each key may map to only one value Java Map interface Methods can be broken down into three groups: querying altering obtaining different views

Fairly similar to Collection methods, but Java designers still thought best to make separate hierarchy no simple answers here. Map methods Here is a list of the Map methods:

void clear() boolean containsKey(Object) boolean containsValue(Object)

Set entrySet() boolean get(Object) boolean isEmpty() Set keySet() Object put(Object, Object) void putall(Map) Object remove(Object) int size() Collection values() Map Implementations We wont go into too much detail on Maps.

Java provides several common class implementations: HashMap a hashtable implementation of a map good for quick searching where order doesnt matter must override hashCode and equals TreeMap A tree implementation of a map Good when natural ordering is required

Must be able to define ordering for added elements. JNI Linking Java and C code JNI Stands for Java Native Interface Set of tools/code that allows user to call native

methods from Java. Includes bindings for C/C++. Can be used to call C/C++ from Java (typical), or Java from C (invocation API) Differs from spawning executable data is passed to/from C/C++ method Question: why is this difficult? Reasons for using JNI Feature not available in java language

(rare). Code already written in another language, dont want to rewrite (typical). Java is slow (how slow?) Other language has no additional features per se, but has much better syntax for handling certain operations (Fortran for math). Problems with JNI Only provides C/C++ bindings. Going to

Fortran, COBOL, Ada, etc. requires extra step. Not portable Mapping is not trivial Can be unsafe Cannot run from applet (security issues)

Machinery for using JNI Steps to follow Basic steps to calling native code 1. Write java class with at least one method declared with native keyword. Provide no implementation public native void sayHello();

Example above is most simple, but method may pass any parameters or have any return type. 2. Add a call to System.loadLibrary(libname) in the class that declares the native method: static{ System.loadLibrary(hello);}//static means called only once. Steps, cont.

3. Compile the class javac Hello.java 4. Produce the C/C++ header files using the javah utility: Javah Hello This produces the header file Hello.h

5. Write your implementation file by first copying the function signature produced in the include file. Also, #include the header file. #include Hello.h Steps, cont. 6. Write the implementation in C/C++. This will require using JNI methods to access the data or possibly casts to convert to basic C/C++ types 7. Best technique: Break into two steps. Think of

your C/C++ function as a wrapper which accesses the Java data and maps it to C data using JNI methods, then shoves the converted data into a prewritten standalone C program. Steps, cont. 8. Compile your native method(s) as a shared object (or DLL on Windows). WARNING: Be sure to point your linker to the include files in /jdk1.3/include and jdk1.3/include/linux (for example). WARNING: Mixing languages is much easier using a

straight C wrapper rather than C++. 9. Set the environment variable LD_LIBRARY_PATH to the shared object directory Run main Java class. C language bindings What does Java pass to my method? What does Java pass to my C function?

JNIEnv* : A pointer to the the JNI environment. This pointer is a handle to the current thread in the JVM, and contains mapping functions and other housekeeping information. jobject : A reference to the object that called the native code. (like this pointer). Any arguments specified by the method. Pause for some nice pictures

Legacy C calls Java Java calls legacy JNI is inter-language glue Simple examples on union HelloWorld Example: No data passed Hello.java Hello.cc Max example : Only native dataypes Utils.java

utils.cc Advanced Max example: Arrays Utils.java utils.cc Max Java-C-Fortran: max.f General Strategy Keep interface as simple as Native datatype mappings Java Type boolean

byte char short int long float double void Native Type jboolean jbyte

jchar jshort jint jlong jfloat jdouble void Size in bits 8, unsigned 8 16, unsigned

16 32 64 32 64 N/a Java object Mappings Object passed by reference All objects have type jobject as: Object mappings, cont.

For example, if a method getLine exists in a class call Prompt, then: private native String getLine(String Prompt); is mapped into JNIExport jstring JNICALL Java_Prompt_getLine(JNIEnv *, jobject, jstring); But how to access data/methods from object that is passed in?

JNI Advice Can seem like a bewildering number of functions. Do not try to learn it all. Keep interfaces very simple. Preferably, only native datatypes, Strings, and arrays. Be careful about Copies vs. rerences Freeing memory Best not to allocate memory from with native

code. Accessing java strings Do NOT do the following: JNIEXPORT jstring JNICALL Java_Prompt_getLine(JNIEnv *env, jobject obj, jstring prompt){ printf(%s, prompt); } Why is this bad?

Right way to access Strings Must use special methods in env structure char *str = (*env)->GetStringUTFChars(env,prompt,0); /* this maps into regular C char* */ printf(%s, str); /* now it is ok to print */ (*env)->ReleaseStringUTFChars(env, prompt, str); /* must release String to avoid memory leaks */ Returning Strings Previous technique allows us to use a String passed in from Java.

What if we want to return a String? Can use NewStringUTF as: char buf[128]; /* allocate memory for local char* in C */ scanf(%s, buf); /* read into char* from stdio */ return( (*env)->NewStringUTF(env, buf)); /* construct and return the Java String */ Other JNI String methods GetStringChars Takes the Java String and returns a pointer to an array of Unicode characters that comprise it. ReleaseStringChars

Releases the pointer to the array of Unicode characters NewString Constructs a new String object from an array of Unicode Characters GetStringLength Returns the length of a string of Unicode characters Java arrays Note that you can NOT do the following: JNIExport jint JNICALL Java_IntArray_sumArray(JNIEnv *env, jobject obj, jintArray arr){ int i, sum = 0;

for (i = 0; i<10; i++){ sum += arr[i]; /* NO! why not? } ... Must use java methods to access array data in C Array methods The previous example should be written as: jsize len = (*env)->GetArrayLength(env,arr); jint *body = (*env)->GetIntArrayElements(env,arr,0); for (i=0;i

} (*env)->ReleastIntArrayElements(env,arr,body,0); /* very important copies back to java array if copy had to be made */ Array methods, cont. Note that there are analagous functions for float, byte, double, etc: GetArrayElements ReleaseArrayElements

Important: These Get functions may copy the entire array. If this is undesirable, use Get/SetArrayRegion functions Function for accessing arrays Function GetBooleanArrayElements GetByteArrayElements GetShortArrayElements GetIntArrayElements GetLongArrayElements

GetFloatArrayElements GetDoubleArrayElements GetObjectArrayElements Array Type boolean byte short int long float double

object Functions for releasing arrays Function Array Type ReleaseBooleanArrayElements ReleaseByteArrayElements ReleaseShortArrayElements RelaseIntArrayElements ReleaseLongArrayElements

ReleaseFloatArrayElements ReleaseDoubleArrayElements ReleaseObjectArrayElements boolean byte short int long float double object

Calling java methods What if you pass a java object to a C routine and wish to call back a method on the Java object. Good to avoid this when you can but sometimes it is very important. Need to use the jobject reference that is passed in by java. Steps to follow

Native method calls JNI function GetObjectClass returns the jclass object that is type of that obj Native method calls JNI function GetMethodID returns jmethodID of method in class (0 for no such method) Finally, native method calls JNI function CallVoidMethod. invokes an instance of method with void return type. You pass object, methodID, and actual arguments.

A simple alternative spawning a system executable Advantages Infinitely simpler Portable Can use any native language Disadvantages Can only pass data to and from vi stdout Must reload executable for each invocation Legacy Collections java.util.Vector

Still useable, but typically ArrayList is preffered. Only major difference is if you are using muliple threads java.util.HashTable Still useable, but typically HashMap is preferred. Again, different if using multiple threads. Multithreaded programming with Java

What are threads? Like several concurrent subprograms running within the same address space. Within a program, individual threads are explicitly spawned and give the appearance of simultaneously running sequences of commands. On a single proc machine, the simultaneous running is an illusion cpu is time splicing. Differ from separate processes in that each process runs in its own address space shared memory model

Why use threads? Single-user programs/clients continuously responsive user interfaces accept input when event handler is bus Can actually speed up certain tasks Servers Allows multiple client connections simultaneously

General examples User clicks GUI button to download web page (occurs in separate thread so GUI isnt frozen) Massive numerical problems split among processors assumes each thread runs on separate processor; not necessarily the case Server spawns thread for each client connection and main thread goes back to accept()

User clicks button which begins time-consuming database lookup. Client can accept more input while lookup is taking place. Concrete example Consider two versions of a program which animates a ball bouncing around in a window. In one version, the animation takes place in the event handler thread. Thus, the gui is frozen for the whole animation In a second version, the animation takes place in a

new thread spawned in the event handler. This gives the event thread a chance to operate. Run Bounce.java and BounceThread.java Second concrete example Imagine a GUI program that performs a time-consuming task in the event handler (such as the AuctionSimulator). How can the GUI remain responsive? If we do task in a separate thread and sleep it periodically, user interface thread will

appear live. See FrameThread.java and FrameNoThread.java Machinery How to setup threads in Java How to create separate threads in Java -- technique I Extend Thread. Specifically ... Create a class that extends Thread and place the work that the Thread will carry out in the run()

method (ie override the run method). Create an object from your class that extends the Thread class. Call the start() method on the Thread object. The new Thread then unters the runnable state (it may or may not run depending on resources/ priority). How to create threads Technique 2 Implement Runnable. Specifically ... Create a class that implements the Runnable

interface. Place all of the work that the Thread will perform in the run() method. Create an object from the Runnable class. Create a Thread object and pass the Runnable object to the constructor. Call the start() method on Thread object. See simple examples under basic directory. Simple Example Thread Class ThreadExample{ public static void main(String[] args){ System.out.println(Main thread started);

MyFirstThread t = new MyFirstThread() t.start(); } } Class MyFirstThread extends Thread{ void run(){ System.out.println(in new thread ); } } Example using second technique class ThreadTest{

public static void main(String[] args){ System.out.println(main thread started ); MyRunnableObj r = new MyRunnableObj(); Thread t = new Thread(r); t.start(); } } Class MyRunnableObj implements Runnable{ public void run(){ System.out.println(new thread started ); } }

What happens to new threads? Main thread continues New threads execute the run method and die when they are finished If any thread calls System.exit(0), it will kill all threads. Think of each run() as its own main Program does not exit until all non-daemon threads die.

Thread States Four states a Thread can be in: New When you create with new operator but havent run yet. Runnable When you invoke start() method. Note that Thread is not necessarily running, could be waiting. Blocked

When sleep() is called Blocking operation such as input/output wait() is called by the Thread object Thread tries to obtain a lock on a locked object Thread states, cont. Dead Dies a natural death because the run method exits normally

Dies abruptly after an uncaught exception terminates the run method Use isAlive() method to determine if Thread is currently alive (either runnable or blocked). Thread Priority The execution of multiple threads on a single CPU is called scheduling. The Java runtime supports a very simple, deterministic scheduling algorithm known

as fixed priority scheduling. Thread Priority Each Java thread is given a numeric priority between MIN_PRIORITY and MAX_PRIORITY. When multiple threads are ready to be executed, the thread with the highest priority is chosen for execution. Only when that thread stops, or is suspended for some reason, will a lower priority thread start executing. Scheduling of the CPU is fully preemptive. If a thread with a higher priority than the currently executing

thread needs to execute, the higher priority thread is immediately scheduled. Thread Priority The Java runtime will not preempt the currently running thread for another thread of the same priority. In other words, the Java runtime does not time-slice. However, the system implementation of threads underlying the Java Thread class may support timeslicing. Do not write code that relies on time-slicing. In addition, a given thread may, at any time, give up its right to execute by calling the yield method.

Threads can only yield the CPU to other threads of the same priority--attempts to yield to a lower priority thread are ignored. Thread Priority When all the runnable threads in the system have the same priority, the scheduler chooses the next thread to run in a simple, non-preemptive, round-robin scheduling order.

A common scenario: polling vs callbacks How do we implement the following? Thread1 spawns Thread2 Thread1 does work Thread2 does work that results in new value of variable (or new data in file, etc). Thread1 finishes work and needs update value (or file) from Thread2s work. How can we synchronize activities? Whats so difficult?

Atomic processes, sharing resources, synchronization, deadlock Bottom Line Any time a writeable variable is visible to more than one thread, potential problems exist. Simple example: two clients try to purchase item at same time. Order or execution unpredictable If (itemsLeft > itemsRequested) not reliable!

Must create thread-safe programs More on this later Managing Threads Everything in either Object or Thread class Two classes of methods: Those defined in Object wait(), notify(), notifyAll() Those defined in Thread class join(), sleep(), interrupt(), isAlive(), yield(), etc.

All involve situations where threads communicate with each other in some way. Will discuss later Producer/Consumer example One thread is called the Producer. Producer shoves consecutive integers into a Cubbyhole object as fast as it can. Other thread is called Consumer. Consumer

grabs from the Cubbyhole object as fast as it can. Consumer would like to grab once for each put by the Producer, but what if one goes faster than the other? Producer Class public class Producer extends Thread { private CubbyHole cubbyhole; private int number; public Producer(CubbyHole c, int number) { cubbyhole = c;

this.number = number; } public void run() { for (int i = 0; i < 10; i++) { cubbyhole.put(i); System.out.println("Producer #" + this.number + " put: " + i); try { sleep((int)(Math.random() * 100)); } catch (InterruptedException e) { } }}}

Consumer class public class Consumer extends Thread { private CubbyHole cubbyhole; private int number; public Consumer(CubbyHole c, int number) { cubbyhole = c; this.number = number; } public void run() { int value = 0; for (int i = 0; i < 10; i++) { value = cubbyhole.get();

System.out.println("Consumer #" + this.number + " got: " + value); }}} Comments Note that these classes of themselves do not forbid a race condition. This is done by shychronizing access to the Cubbyhole object. We want to guarantee that the Consumer thread cant get until the Producer has

produced. Need to study wait() and notify() methods. How to synchronize code This topic confuses many beginning Java programmers Two forms: synchronized(objReference){ }//synchronize a block synchronized void methodName(){}//synch a method Former is more general, but causes confusion. Best

to use simple form whenever possible. Second form is equivalent to: synchronized(this) for entire method body Synchronizing a method Fairly straightforward rules: When a given thread is executing a synchronized method in an object, no other thread can execute an other synchronized method on the SAME OBJECT! We say that the first thread obtains a lock on the object and doesnt release it until it finishes the synched

method Beware that only code which tries to obtain a lock on the object will have to wait. For example, if a thread wishes to call a non-synched method in the object, it will not be blocked. Synchronizing a block Remember the following rules: When a thread encounters a synchronized block of code, it attempts to obtain a lock on the object that is being synchronized upon. Consider the first thread in a program that

encounters the lock. Since it is the first thread, it will successfully obtain the lock. Now consider a second thread that encounters any synchronized block of code that synchronzies on the same object. Synchronizing a block, cont. This second thread will attempt to obtain the lock on objReference. It will fail to obtain the lock unless the first thread has released it. This means that the first thread must have finished its synchronized

block of code. If the second thread cannot obtain the lock, it must wait until the first thread releases it. wait() and notify()/notifyAll() A common scenario is as follows: A thread enters a synchronized block of code (and thus obtains the object lock). The code cannot continue the sychronized block until some other thread has done some work on the same object and left it in a new state Thus, the first thread wants to temporarily

relinquish the lock to another thread. wait() and notify()/notifyAll() Called when a thread needs to wait for some other thread(s) to complete a task before continuing. The current thread simply calls wait(), and the thread freezes until another object calls notify() upon the waiting object (or notifyAll()). notifyAll() is much safer since notify()

chooses randomly! wait() and notify()/notifyAll() Beware, wait() can only be called from a synchronized method or block of code. When wait is called on an object, its lock is released until it is notified. Moving out of blocked state Must use the opposite route that put the

Thread into the blocked state If put to sleep, specified time interval must elapse. If waiting for i/o operation, operation must have finished. If the thread called wait(), then another thread must call notify/notifyAll. If waiting for a lock, then owning thread must have relinquished the lock. Synchronized Cubbyhole

public class CubbyHole { private int contents; private boolean available = false; public synchronized int get() { while (available == false) { try { wait(); } catch (InterruptedException e) { } } available = false; notifyAll(); return contents;

} Cubbyhole, cont. public synchronized void put(int value) { while (available == true) { try { wait(); } catch (InterruptedException e) { } } contents = value; available = true; notifyAll();

} } Interrupting Threads Thread terminates when run method ends. So, run method should check once in a while whether there is more work to do in regular way. However, what if thread is sleeping or otherwise blocked? This is where interrupt() method comes

into play. Interrupting, cont. When interrupt method is called on thread that is blocking, the blocking call is terminated by an InterruptException. public void run(){ try{ while (more work to do){ do work } catch(InterrupetedException e){ //thread was interrupted during sleep or wait

//do whatever you wish here } Interrupting, cont. Problem with previous code is that interrupts only succeed if thread is blocked in call to sleep() or wait(). Can get around this by calling interrupted() periodically to see if thread has recently been interrupted: while (!interrupted() && more work to do){

// do work } Deadlock Dining Philosophers example http://java.sun.com/docs/books/tutorial/ essential/threads/deadlock.html Atomic operations -- Bank Account example See BankTest.java and SynchBankTest.java

Tips on writing thread-safe code Obvious way is to use synchronization (see next slide) Performance problems Can lead to deadlock General rule: dont overuse if possible Local variables Each thread gets its own copy Immutable sequences

Objects which cant be changed (String, Integer, etc.) Re-entrant threads Javas model supports a concept called re- entrant threads. If a thread t obtains a lock on object o by calling a synchronized method in o (say m1), and then from within m1 calls a second sychronized method in o m2, deadlock is guaranteed not to occur.

Other useful methods join() tells the calling thread to halt execution until the second thread (whose yield method is called), has completed. Thread t = new Thread(runnableObj); t.join(); Additional Topics Priorities Thread Groups Creating Thread pools

Threads and Swing Higher-level methods Timer class Distributed Objects Higher level alternative to sockets RMI or CORBA Nested Classes Nested Classes An nested class is a class that is defined

inside another class. To this point we have only studied top-level classes. at most one public per file arbitrarily man package-scope per file either package or public ONLY Nested classes introduced in jdk1.1 Why use nested classes? Simplifies many coding tasks can define small classes on the fly near the

objects created from them + concise syntax can access outer classes ivs automatically no need to pass a this pointer to the constructor of separate outer class can be hidden from other classes in the same package However, price to pay in terms of complexity, number of gotchas, etc. Pre jdk1.1 In jdk1.0, the clean and simple class rules were

ballyhooed as a major improvement over C++ Addition of inner classes complicates things significantly However, they do make certain code much less awkard to write, particularly when writing GUIs Still, you do not have to use them, but they can be quite cool and I do recommend it in moderation! Types of nested classes Inner classes local

anonymous or named non-local named only Static nested classes non-local named only Non-local inner classes Simply a nested class that does not have the static attribute and is not defined within a class method.

Can be private, public, package, protected, abstract, etc. just like any class member. Think of outer class as owning inner class inner class can only be instantiated via outer class reference (including this) Inner class has access to all outer class ivs, private or otherwise! Simple non-local inner class example class Outer{ private int x1;

Outer(int x1){ this.x1 = x1; } public void foo(){ System.out.println(fooing);} public class Inner{ private int x1 = 0; void foo(){ System.out.println(Outer value of x1: + Outer.this.x1); System.out.println(Inner value of x1: + this.x1); } }

Simple example, cont -- driver Rules for instantiation a little funny public class TestDrive{ public static void main(String[] args){ Outer outer = new Outer(); // can create in regular way Inner inner = outer.new Inner(); //must call new through //outer object handle inner.foo(); // note that this can only be done if inner is visible // according to the regular scoping rules } }

When to use non-local inner classes Most typically used when inner class is instatiated from outer class. If classes naturally belong together, it is cumbersome to pass a this pointer to a separate outer class just so second class can access first classs properties/methods. Note that inner class can access outer classs private data, making them even more

powerful than mechanism implied above! Local inner classes Inner classes may also be defined within class methods. These are called local inner classes.

Principle advantage is scoping: such classes are completely inaccessible anywhere but the method itself where they are defined. Thus, they have no visibility attribute (public, etc.) Also, can NOT access local variables other than those declared with final attribute. Local anonymous inner classes Local inner classes can be taken a step further it is not required to give them an explicit name.

This is very convenient when you want to use a class only once and the code that it contains is succinct. Great example is defining Swing callback functions. Anonymous class example but.addActionListener( new ActionListener(){ public void actionPerformed(actionEvent ae){ //do work here }

} ); Final Exam Review Questions Question0 class A { static void display ( ){ System.out.println ( "Class A" ) ; } } class B extends A { static void display ( ) { System.out.println ( "Class B" ); }

} When we create an object and call the display ( ) method as:. A aa = new B() ; aa.display ( ) What is displayed ?? What if display is non-static?? Question1 1. 2. 3.

4. 5. Given a class A with a protected iv ivp, which of the following are possible ways of accessing ivp? using the this pointer within a subclass of A within the same package using the this pointer within a subclass of B outside of the package using an instance of A from within any class in the same package using an instance of A from within any class

using an instance of A from within any class in the CLASSPATH Question2 1. 2. 3. 4. 5. Given a class A with a private iv ivp, which of the

following are possible ways of accessing ivp? using the this pointer within a subclass of A within the same package using the this pointer within a subclass of B outside of the package using an instance of A from within any class in the same package using an instance of A from within any class using an instance of A from within any class in the CLASSPATH Question3

1. 2. 3. 4. List the visibility keywords in order of most to least restrictive. private default protected public public protected default private private protected default public public default private protected

Question4 If an iv has private scope in some superclass, then no subclass can access the iv using the this pointer. Question5 Immutable classes may contain accessor methods

Question6 Immutable classes are Threadsafe. Question7 class X{ public boolean equals(Object o){ X anX = (X) o; if (anX.iv1 == this.iv1) return true; return false; } ? int iv1; What can be inferred about iv1?

private, protected, default, public, all, some?? Question8-10 Assuming the class X is exactly as it appears on the previous slide (with public in place of ?), is the following valid? X x2 = (X) anX.clone(); If so, is this clone and adequate clone? If not, how would you modify the class X to make it cloneable? Is X serializable as-is?

Question11 Is it appropriate to store anX in HashMap, again assuming that X is defined verbatim as you see? Is it legal to store anX in a HashMap? Question12 Is it possible for a subclass to override a private superclass method?

Question13 Can a static variable be used in a non-static context? Question14 Can a static method access a non-static variable? Question15

Can a class with all static methods be instantiated? Questions16-18 public class ClassA { public void methodOne(int i) { } public void methodTwo(int i) { } public static void methodThree(int i) { } public static void methodFour(int i) { } } public class ClassB extends ClassA {

public static void methodOne(int i) { } public void methodTwo(int i) { } public void methodThree(int i) { } public static void methodFour(int i) { } } a. Which method overrides a method in the superclass? b. Which method hides a method in the superclass? c. What do the other methods do? Question19-24 Exception handling (T or F) All java Exceptions must either be caught or

thrown. Classes can throw Exceptions. Static methods can throw Exceptions. main can be declared to throw Exception rethrowing Exceptions is bad programming style

Every method that throws an Exception should be placed in its own try-catch block It is bad programming style to have empty catch blocks, at least in production code. Question25 1. 2. 3. 4.

Which best describes and Applet? A thin client A fat client A lightweight web server A full-blown application server Question 26-31 All applets extend either Applet or JApplet. Applets need a main method to run All applets must override at least the start, stop, init, and destroy methods.

start, stop, init, and destroy methods are declared abstract in the Applet class Unsigned applets cannot make socket connections to any server. Unsigned applets cannot access files on the local directory Somewhat irritating certification type questions Question 1) Which of the following lines will compile without

warning or error? 1) float f=1.3; 2) char c="a"; 3) byte b=257; 4) boolean b=null; 5) int i=10; What will happen if you try to compile and run the following code public class MyClass { public static void main(String arguments[]) { amethod(arguments);

} public void amethod(String[] arguments) { System.out.println(arguments); System.out.println(arguments[1]); }} 1) error Can't make static reference to void amethod. 2) error method main not correct 3) error array must include parameter 4) amethod must be declared with String Which of the following will compile without error 1)

import java.awt.*; package Mypackage; class Myclass {} 2) package MyPackage; import java.awt.*; class MyClass{} 3) /*This is a comment */ package MyPackage; import java.awt.*; class MyClass{}

What will happen when you compile and run the following code public class MyClass{ static int i; public static void main(String argv[]){ System.out.println(i); }} 1) Error Variable i may not have been initialized 2) null 3) 1 4) 0

What will be the result of attempting to compile and run the following code? abstract class MineBase { abstract void amethod(); static int i; } public class Mine extends MineBase { public static void main(String argv[]){ int[] ar=new int[5]; for(i=0;i < ar.length;i++) System.out.println(ar[i]); }}

1) a sequence of 5 0's will be printed 2) Error: ar is used before it is initialized 3) Error Mine must be declared abstract 4) IndexOutOfBoundes Error What will be printed out if you attempt to compile and run the following cod int i=1; switch (i) { case 0: System.out.println("zero"); break; case 1:

System.out.println("one"); case 2: System.out.println("two"); default: System.out.println("default"); } 1) one 2) one, default 3) one, two, default Which of the following statements are true? 1) Methods cannot be overriden to be more private

2) Static methods cannot be overloaded 3) Private methods cannot be overloaded 4) An overloaded method cannot throw exceptions not checked in the base class What will happen if you attempt to compile and run the following code? class Base {} class Sub extends Base {} class Sub2 extends Base {} public class CEx{ public static void main(String argv[]){

Base b=new Base(); Sub s=(Sub) b; }} 1) Compile and run without error 2) Compile time Exception 3) Runtime Exception What will happen when you attempt to compile and run the following code?. class Background implements Runnable{ int i=0; public int run(){ while(true){

i++; System.out.println("i="+i); } //End while return 1; }//End run }//End class 1) It will compile and the run method will print out the increasing value of i. 2) It will compile and calling start will print out the increasing value of i. 3) The code will cause an error at compile time. 4) Compilation will cause an error because while cannot take What will be the result when you try to compile and run the following code?

private class Base{ Base(){ int i = 100; System.out.println(i);}} public class Pri extends Base{ static int i = 200; public static void main(String argv[]){ Pri p = new Pri(); System.out.println(i);}} 1) Error at compile time 2) 200 3) 100 followed by 200

4) 100 JNI JNI (Java Native Interface) JNI Stands for Java Native Interface Set of tools/code that allows user to call native

methods from Java. Includes bindings for C/C++. Can be used to call C/C++ from Java (typical), or Java from C (invocation API) Differs from spawning executable data is passed to/from C/C++ method Question: why is this difficult? Reasons for using JNI Feature not available in java language

(rare). Code already written in another language, dont want to rewrite (typical). Java is slow (how slow?) Other language has no additional features per se, but has much better syntax for handling certain operations (Fortran for math). Problems with JNI Only provides C/C++ bindings. Going to

Fortran, COBOL, Ada, etc. requires extra step. Not portable Mapping is not trivial Can be unsafe Cannot run from applet (security issues)

Machinery for using JNI Steps to follow Basic steps to calling native code 1. Write java class with at least one method declared with native keyword. Provide no implementation public native void sayHello();

Example above is most simple, but method may pass any parameters or have any return type. 2. Add a call to System.loadLibrary(libname) in the class that declares the native method: static{ System.loadLibrary(hello);}//static means called only once. Steps, cont.

3. Compile the class javac Hello.java 4. Produce the C/C++ header files using the javah utility: Javah Hello This produces the header file Hello.h

5. Write your implementation file by first copying the function signature produced in the include file. Also, #include the header file. #include Hello.h Steps, cont. 6. Write the implementation in C/C++. This will require using JNI methods to access the data or possibly casts to convert to basic C/C++ types 7. Best technique: Break into two steps. Think of

your C/C++ function as a wrapper which accesses the Java data and maps it to C data using JNI methods, then shoves the converted data into a prewritten standalone C program. Steps, cont. 8. Compile your native method(s) as a shared object (or DLL on Windows). WARNING: Be sure to point your linker to the include files in /jdk1.3/include and jdk1.3/include/linux (for example). WARNING: Mixing languages is much easier using a

straight C wrapper rather than C++. 9. Set the environment variable LD_LIBRARY_PATH to the shared object directory Run main Java class. C language bindings What does Java pass to my method? What does Java pass to my C function?

JNIEnv* : A pointer to the the JNI environment. This pointer is a handle to the current thread in the JVM, and contains mapping functions and other housekeeping information. jobject : A reference to the object that called the native code. (like this pointer). Any arguments specified by the method. Pause for some nice pictures

Legacy C calls Java Java calls legacy JNI is inter-language glue Simple examples on union HelloWorld Example: No data passed Hello.java Hello.cc Max example : Only native dataypes Utils.java

utils.cc Advanced Max example: Arrays Utils.java utils.cc Max Java-C-Fortran: max.f General Strategy Keep interface as simple as Native datatype mappings Java Type boolean

byte char short int long float double void Native Type jboolean jbyte

jchar jshort jint jlong jfloat jdouble void Size in bits 8, unsigned 8 16, unsigned

16 32 64 32 64 N/a Java object Mappings Object passed by reference All objects have type jobject as: Object mappings, cont.

For example, if a method getLine exists in a class call Prompt, then: private native String getLine(String Prompt); is mapped into JNIExport jstring JNICALL Java_Prompt_getLine(JNIEnv *, jobject, jstring); But how to access data/methods from object that is passed in?

JNI Advice Can seem like a bewildering number of functions. Do not try to learn it all. Keep interfaces very simple. Preferably, only native datatypes, Strings, and arrays. Be careful about Copies vs. rerences Freeing memory Best not to allocate memory from with native

code. Accessing java strings Do NOT do the following: JNIEXPORT jstring JNICALL Java_Prompt_getLine(JNIEnv *env, jobject obj, jstring prompt){ printf(%s, prompt); } Why is this bad?

Right way to access Strings Must use special methods in env structure char *str = (*env)->GetStringUTFChars(env,prompt,0); /* this maps into regular C char* */ printf(%s, str); /* now it is ok to print */ (*env)->ReleaseStringUTFChars(env, prompt, str); /* must release String to avoid memory leaks */ Returning Strings Previous technique allows us to use a String passed in from Java.

What if we want to return a String? Can use NewStringUTF as: char buf[128]; /* allocate memory for local char* in C */ scanf(%s, buf); /* read into char* from stdio */ return( (*env)->NewStringUTF(env, buf)); /* construct and return the Java String */ Other JNI String methods GetStringChars Takes the Java String and returns a pointer to an array of Unicode characters that comprise it. ReleaseStringChars

Releases the pointer to the array of Unicode characters NewString Constructs a new String object from an array of Unicode Characters GetStringLength Returns the length of a string of Unicode characters Java arrays Note that you can NOT do the following: JNIExport jint JNICALL Java_IntArray_sumArray(JNIEnv *env, jobject obj, jintArray arr){ int i, sum = 0;

for (i = 0; i<10; i++){ sum += arr[i]; /* NO! why not? } ... Must use java methods to access array data in C Array methods The previous example should be written as: jsize len = (*env)->GetArrayLength(env,arr); jint *body = (*env)->GetIntArrayElements(env,arr,0); for (i=0;i

} (*env)->ReleastIntArrayElements(env,arr,body,0); /* very important copies back to java array if copy had to be made */ Array methods, cont. Note that there are analagous functions for float, byte, double, etc: GetArrayElements ReleaseArrayElements

Important: These Get functions may copy the entire array. If this is undesirable, use Get/SetArrayRegion functions Function for accessing arrays Function GetBooleanArrayElements GetByteArrayElements GetShortArrayElements GetIntArrayElements GetLongArrayElements

GetFloatArrayElements GetDoubleArrayElements GetObjectArrayElements Array Type boolean byte short int long float double

object Functions for releasing arrays Function ReleaseBooleanArrayElements ReleaseByteArrayElements ReleaseShortArrayElements RelaseIntArrayElements ReleaseLongArrayElements ReleaseFloatArrayElements ReleaseDoubleArrayElements ReleaseObjectArrayElements

Array Type boolean byte short int long float double object Calling java methods

What if you pass a java object to a C routine and wish to call back a method on the Java object. Good to avoid this when you can but sometimes it is very important. Need to use the jobject reference that is passed in by java. Steps to follow Native method calls JNI function GetObjectClass returns the jclass object that is type of that obj

Native method calls JNI function GetMethodID returns jmethodID of method in class (0 for no such method) Finally, native method calls JNI function CallVoidMethod. invokes an instance of method with void return type. You pass object, methodID, and actual arguments. A simple alternative spawning a system executable

Advantages Infinitely simpler Portable Can use any native language Disadvantages Can only pass data to and from vi stdout Must reload executable for each invocation Spawning Executable -technique Process p = Runtime.exec(some_exec); Use p to manage process:

p.getInputStream(); p.getOutputStream(); p.kill(); p.halt(); Part 2 Extending Web Server Functionality Servlets and JSP

Web Servers A server program that listens on a standard port and handles http protocol. Http protocol consists mainly of requests for documents Documents are typically html files Two most important http protocol elements: GET (request document, may upload data) POST (request document, upload data).

This protocol is hidden from you by browser. Early Web Servers Earliest web sites were static: Browser requests page Server hands over page CGI (Common Gateway Interface) scripts defined a standard for extending functionality http GET/POST data could be passed to and processed in separate function (C, Perl, Python, etc.) This often included call to back-end database and

response to user via modified html document Shortcomings of CGI E-Commerce became more popular and web sites became more heavily used. This brought to the fore some shortcomings of CGI: New process spawned for every hit not scalable No concept of sesssion or state Pretty low level

Security risks (C in particular) Servlets Javas answer to CGI, very simple, high level Requirements: a servlet-enabled web server When specified by your web page, web

page passes http requests to java method (assuming everything is setup properly) Servlet method then has access to all of Java capabilities jdbc very important here. Servlet then writes html back to user. Servlets, cont. Important: a web server takes care of all interactions with the servlet servlet extends functionality. On the client side, servlet pages are

typically requested in one of two ways: As a regular URL address As a link in a regular html document Details are server-dependent Writing servlets All servlets extend the Servlet class. All http servlets (by far most typical) should extend the HttpServlet class. In extending HttpServlet, you typically

override the following methods: init, doGet,doPost, destroy (very common) doPut, doDelete, doOptions, doTrace (rare) Main HttpServlet Methods init() called once when servlet is loaded by server. Contains any initializations that are common to all requests. doGet(HttpServletRequest, HttpServletResponse)

Called each time the servlet receives an http GET request posted by a client. Passes two objects, one representing the information of the request, the other used to configure a response. Main HttpServlet Methods, cont. doPost(HttpServletRequest, HttpServletResponse) Same as doGet but for an http POST request.

destroy() Called before servlet is unloaded from memory. Performs any final cleanup, freeing memory, closing connections, etc. Service Method Important: The method service(HttpServletRequest, HttpServletResponse) is also called for each servlet invocation. Service() in turn calls doGet and doPost,

etc. It is best not to override service even if you want to handle doGet and doPost identically. Simply have one call the other. HttpServletRequest Object Passed when browser calls doGet and doPost. Most import methods for beginning servlet programming getParameter(String paramName) getParameterNames()

getParameterValues() Makes getting data from web pages very simple. Many other methods for images, cookies, etc. HttpServletResponse Object Passed when browser calls doGet or doPost Most import methods for beginning servlet programming: getWriter(); Get Writer for communicating back to client

setContentType(String); Typically use text/html, indicating that html will be sent back to the browser Examples Use html form to connect to servlet, accept form data, and then echo it back with the output stream. Postdata.java : Java servlet

Form.html : HTML front-end Basic steps very simple just need to know a little HTML and a few Java methods General Comments Recall that each request for a servlet gets its own thread but access the same methods. Thus, synchronization issues arise.

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