Chapter 2: Writing Java Programs Chapter 2 Writing

Chapter 2: Writing Java Programs Chapter 2 Writing

Chapter 2: Writing Java Programs Chapter 2 Writing Java Programs Java Programming FROM THE BEGINNING 1 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs 2.1 A Simple Java Program The following program displays the message Java rules! on the screen. JavaRules.java // Displays the message "Java rules!" public class JavaRules { public static void main(String[] args) { System.out.println("Java rules!"); }

} Java Programming FROM THE BEGINNING 2 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Java Programs in General Building blocks of a Java program: Classes. A class is a collection of related variables and/ or methods (usually both). A Java program consists of one or more classes. Methods. A method is a series of statements. Each class may contain any number of methods. Statements. A statement is a single command. Each method may contain any number of statements. Java Programming FROM THE BEGINNING 3

Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs 2.2 Executing a Java Program Steps involved in executing a Java program: Enter the program Compile the program Run the program With an integrated development environment, all three steps can be performed within the environment itself. Java Programming FROM THE BEGINNING 4 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs

Java Programming FROM THE BEGINNING 5 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Integrated Development Environments An integrated development environment (IDE) is an integrated collection of software tools for developing and testing programs. A typical IDE includes at least an editor, a compiler, and a debugger. A programmer can write a program, compile it, and execute it, all without leaving the IDE. Java Programming FROM THE BEGINNING 6

Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Entering a Java Program Any editor or word processor can be used to enter a program, including Notepad or WordPad. If you use a word processor, be careful to save the file as a Text Document or as Text Only. You can also enter the program from a DOS window using the DOS edit program. Java Programming FROM THE BEGINNING 7 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Compiling a Java Program Before compiling a program under Windows, open

a DOS window and use the cd (change directory) command to select the directory that contains the program: cd c:\myfiles\java\programs To compile the program, use the javac command: javac JavaRules.java The file name must match the program name, and the .java extension must be included. Java Programming FROM THE BEGINNING 8 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Running a Java Program If the compiler doesnt find any errors in the program, it creates a class file containing bytecode instructions. This file will have the same name as

the program but with .class as the extension. To run a program, use the java command, which executes the Java interpreter. java requires that the name of a class file be specified (without the .class extension): java JavaRules Java Programming FROM THE BEGINNING 9 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Java Programming FROM THE BEGINNING 10 Copyright 2000 W. W. Norton & Company.

Chapter 2: Writing Java Programs Using Multiple Windows If you use edit to create and modify programs, it's a good idea to have two DOS windows open simultaneously. You can run edit in one window and use the other window to compile and test the program. Its also a good idea to enable the doskey utility when you go into a DOS window. doskey remembers previous commands that have been entered in that window. Java Programming FROM THE BEGINNING 11 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs 2.3 Program Layout The JavaRules program raises a couple of issues:

Why do we put comments into programs, and what are the rules for doing so? How should we lay out a program? Does it matter where we put spaces and blank lines? Where should the curly braces go? Java Programming FROM THE BEGINNING 12 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Comments Comments are an important part of every program. They provide information thats useful for anyone who will need to read the program in the future. Typical uses of comments: To document who wrote the program, when it was written, what changes have been made to it, and so on. To describe the behavior or purpose of a particular part of the program, such as a variable or method.

To describe how a particular task was accomplished, which algorithms were used, or what tricks were employed to get the program to work. Java Programming FROM THE BEGINNING 13 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Types of Comments Single-line comments: // Comment style 1 Multiline comments: /* Comment style 2 */ Doc comments: /** Comment style 3 */ Doc comments are designed to be extracted by a special program, javadoc.

Java Programming FROM THE BEGINNING 14 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Problems with Multiline Comments Forgetting to terminate a multiline comment may cause the compiler to ignore part of a program: System.out.print("My "); /* forgot to close this comment... System.out.print("cat "); System.out.print("has "); /* so it ends here */ System.out.println("fleas"); Java Programming FROM THE BEGINNING 15

Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Single-line Comments Many programmers prefer // comments to /* */ comments, for several reasons: Ease of use Safety Program readability Ability to comment out portions of a program Java Programming FROM THE BEGINNING 16 Copyright 2000 W. W. Norton &

Company. Chapter 2: Writing Java Programs Tokens A Java compiler groups the characters in a program into tokens. The compiler then puts the tokens into larger groups (such as statements, methods, and classes). Tokens in the JavaRules program: public class JavaRules { public String [ ] args ) { System . "Java rules!" ) ; } } Java Programming FROM THE BEGINNING 17 static out . void main println (

Copyright 2000 W. W. Norton & Company. ( Chapter 2: Writing Java Programs Avoiding Problems with Tokens Always leave at least one space between tokens that would otherwise merge together: publicclassJavaRules { Dont put part of a token on one line and the other part on the next line: pub lic class JavaRules { Java Programming FROM THE BEGINNING 18 Copyright 2000 W. W. Norton & Company.

Chapter 2: Writing Java Programs Indentation Programmers use indentation to indicate nesting. An increase in the amount of indentation indicates an additional level of nesting. The JavaRules program consists of a statement nested inside a method nested inside a class: Java Programming FROM THE BEGINNING 19 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs How Much Indentation? Common amounts of indentation:

2 spaces: the bare minimum 3 spaces: the optimal amount 4 spaces: what many programmers use 8 spaces: probably too much The JavaRules program with an indentation of four spaces: public class JavaRules { public static void main(String[] args) { System.out.println("Java rules!"); } } Java Programming FROM THE BEGINNING 20 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Brace Placement Brace placement is another important issue.

One technique is to put each left curly brace at the end of a line. The matching right curly brace is lined up with the first character on that line: public class JavaRules { public static void main(String[] args) { System.out.println("Java rules!"); } } Java Programming FROM THE BEGINNING 21 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Brace Placement Some programmers prefer to put left curly braces on separate lines: public class JavaRules { public static void main(String[] args)

{ System.out.println("Java rules!"); } } This makes it easier to verify that left and right braces match up properly. However, program files become longer because of the additional lines. Java Programming FROM THE BEGINNING 22 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Brace Placement To avoid extra lines, the line containing the left curly brace can be combined with the following line: public class JavaRules { public static void main(String[] args) { System.out.println("Java rules!"); }

} In a commercial environment, issues such as indentation and brace placement are often resolved by a coding standard, which provides a uniform set of style rules for all programmers to follow. Java Programming FROM THE BEGINNING 23 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs 2.4 Using Variables In Java, every variable must be declared before it can be used. Declaring a variable means informing the compiler of the variables name and its properties, including its type. int is one of Javas types. Variables of type int can store integers (whole numbers).

Java Programming FROM THE BEGINNING 24 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Declaring Variables Form of a variable declaration: The type of the variable The name of the variable A semicolon Example: int i; // Declares i to be an int variable Several variables can be declared at a time: int i, j, k; Its often better to declare variables individually.

Java Programming FROM THE BEGINNING 25 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Initializing Variables A variable is given a value by using =, the assignment operator: i = 0; Initializing a variable means to assign a value to the variable for the first time. Variables always need to be initialized before the first time their value is used. The Java compiler checks that variables declared in methods are initialized prior to their first use. Java Programming FROM THE BEGINNING 26

Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Initializers Variables can be initialized at the time theyre declared: int i = 0; 0 is said to be the initializer for i. If several variables are declared at the same time, each variable can have its own initializer: int i = 0, j, k = 1; Java Programming FROM THE BEGINNING 27 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs

Changing the Value of a Variable The assignment operator can be used both to initialize a variable and to change the value of the variable later in the program: i = 1; i = 2; // Value of i is now 1 // Value of i is now 2 Java Programming FROM THE BEGINNING 28 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Program: Printing a Lottery Number Lottery.java // Displays the winning lottery number

public class Lottery { public static void main(String[] args) { int winningNumber = 973; System.out.print("The winning number "); System.out.print("in today's lottery is "); System.out.println(winningNumber); } } Java Programming FROM THE BEGINNING 29 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs 2.5 Types A partial list of Java types: int An integer double A floating-point number boolean Either true or false char A character

Declarations of double, boolean, and char variables: double x, y; boolean b; char ch; Java Programming FROM THE BEGINNING 30 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Literals A literal is a token that represents a particular number or other value. Examples of int literals: 0 297 30303

Examples of double literals: 48.0 48. 4.8e1 4.8e+1 .48e2 480e-1 The only boolean literals are true and false. char literals are enclosed within single quotes: 'a' 'z' 'A' Java Programming FROM THE BEGINNING

'Z' '0' 31 '9' '%' '.' Copyright 2000 W. W. Norton & Company. ' ' Chapter 2: Writing Java Programs Using Literals as Initializers Literals are often used as initializers: double x = 0.0, y = 1.0; boolean b = true; char ch = 'f';

Java Programming FROM THE BEGINNING 32 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs 2.6 Identifiers Identifiers (names chosen by the programmer) are subject to the following rules: Identifiers may contain letters (both uppercase and lowercase), digits, and underscores (_). Identifiers begin with a letter or underscore. Theres no limit on the length of an identifier. Lowercase letters are not equivalent to uppercase letters. (A language in which the case of letters matters is said to be case-sensitive.) Java Programming FROM THE BEGINNING 33

Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Multiword Identifiers When an identifier consists of multiple words, its important to mark the boundaries between words. One way to break up long identifiers is to use underscores between words: last_index_of Another technique is to capitalize the first letter of each word after the first: lastIndexOf This technique is the one commonly used in Java. Java Programming FROM THE BEGINNING 34 Copyright 2000 W. W. Norton & Company.

Chapter 2: Writing Java Programs Conventions A rule that we agree to follow, even though its not required by the language, is said to be a convention. A common Java convention is beginning a class name with an uppercase letter: Color FontMetrics String Names of variables and methods, by convention, never start with an uppercase letter. Java Programming FROM THE BEGINNING 35 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs

Keywords The following keywords cant be used as identifiers because Java has already given them a meaning: abstract double int super boolean else interface switch break extends long synchronized byte final native this case finally new throw catch float

package throws char for private transient class goto protected try const if public void continue implements return volatile default import short while do instanceof static

null, true, and false are also reserved. Java Programming FROM THE BEGINNING 36 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs 2.7 Performing Calculations In general, the right side of an assignment can be an expression. A literal is an expression, and so is a variable. More complicated expressions are built out of operators and operands. In the expression 5 / 9, the operands are 5 and 9, and the operator is /. The operands in an expression can be variables, literals, or other expressions. Java Programming FROM THE BEGINNING

37 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Operators Javas arithmetic operators: + * / % Addition Subtraction Multiplication Division Remainder Examples: 6+2 6-2 6*2 6/2

8 4 12 3 Java Programming FROM THE BEGINNING 38 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Integer Division If the result of dividing two integers has a fractional part, Java throws it away (we say that it truncates the result).

Examples: 1/2 0 5/3 1 Java Programming FROM THE BEGINNING 39 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs double Operands +, -, *, and / accept double operands: 6.1 6.1 6.1 6.1 + * /

2.5 2.5 2.5 2.5 8.6 3.6 15.25 2.44 int and double operands can be mixed: 6.1 6.1 6.1 6.1 + * /

2 2 2 2 8.1 4.1 12.2 3.05 Java Programming FROM THE BEGINNING 40 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs

Binary Operators The +, -, *, and / operators are said to be binary operators, because they require two operands. Theres one other binary arithmetic operator: % (remainder). The % operator produces the remainder when the left operand is divided by the right operand: 13 % 3 1 % is normally used with integer operands. Its a good idea to put a space before and after each binary operator. Java Programming FROM THE BEGINNING 41 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Unary Operators Java also has two unary arithmetic operators: + Plus

- Minus Unary operators require just one operand. The unary + and - operators are often used in conjunction with literals (-3, for example). Java Programming FROM THE BEGINNING 42 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Round-Off Errors Calculations involving floating-point numbers can sometimes produce surprising results. If d is declared as follows, its value will be 0.09999999999999987 rather than 0.1: double d = 1.2 - 1.1; Round-off errors such as this occur because some numbers (1.2 and 1.1, for example) cant be stored

in double form with complete accuracy. Java Programming FROM THE BEGINNING 43 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Operator Precedence Whats the value of 6 + 2 * 3? (6 + 2) * 3, which yields 24? 6 + (2 * 3), which yields 12? Operator precedence resolves issues such as this. *, /, and % take precedence over + and -. Examples: 5+2/2 8*3-5 6-1*7 9/4+6 6+2%3

5 + (2 / 2) (8 * 3) - 5 6 - (1 * 7) (9 / 4) + 6 6 + (2 % 3) Java Programming FROM THE BEGINNING 44 6

19 1 8 8 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Associativity Precedence rules are of no help when it comes to determining the value of 1 - 2 - 3. Associativity rules come into play when precedence rules alone arent enough. The binary +, -, *, /, and % operators are all left associative: 2 + 3 - 4 (2 + 3) - 4 1 2 * 3 / 4 (2 * 3) / 4 1 Java Programming FROM THE BEGINNING 45 Copyright 2000 W. W. Norton &

Company. Chapter 2: Writing Java Programs Parentheses in Expressions Parentheses can be used to override normal precedence and associativity rules. Parentheses in the expression (6 + 2) * 3 force the addition to occur before the multiplication. Its often a good idea to use parentheses even when theyre not strictly necessary: (x * x) + (2 * x) - 1 However, dont use too many parentheses: ((x) * (x)) + ((2) * (x)) - (1) Java Programming FROM THE BEGINNING 46 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs

Assignment Operators The assignment operator (=) is used to save the result of a calculation in a variable: area = height * width; The type of the expression on the right side of an assignment must be appropriate for the type of the variable on the left side of the assignment. Assigning a double value to an int variable is not legal. Assigning an int value to a double variable is OK, however. Java Programming FROM THE BEGINNING 47 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Using Assignment to Modify a Variable Assignments often use the old value of a variable as part of the expression that computes the new value.

The following statement adds 1 to the variable i: i = i + 1; Java Programming FROM THE BEGINNING 48 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Compound Assignment Operators The compound assignment operators make it easier to modify the value of a variable. A partial list of compound assignment operators: += -= *= /= %= Combines addition and assignment Combines subtraction and assignment

Combines multiplication and assignment Combines division and assignment Combines remainder and assignment Java Programming FROM THE BEGINNING 49 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Compound Assignment Operators Examples: i i i i i += -= *=

/= %= 2; 2; 2; 2; 2; // // // // // Same Same Same Same Same Java Programming FROM THE BEGINNING as

as as as as 50 i i i i i = = = = = i i i i i

+ * / % 2; 2; 2; 2; 2; Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Program: Converting from Fahrenheit to Celsius FtoC.java // Converts a Fahrenheit temperature to Celsius public class FtoC { public static void main(String[] args) { double fahrenheit = 98.6; double celsius = (fahrenheit - 32.0) * (5.0 / 9.0); System.out.print("Celsius equivalent: ");

System.out.println(celsius); } } Java Programming FROM THE BEGINNING 51 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs 2.8 Constants A constant is a value that doesnt change during the execution of a program. Constants can be named by assigning them to variables: double freezingPoint = 32.0; double degreeRatio = 5.0 / 9.0; To prevent a constant from being changed, the word final can be added to its declaration: final double freezingPoint = 32.0; final double degreeRatio = 5.0 / 9.0;

Java Programming FROM THE BEGINNING 52 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Naming Constants The names of constants are often written entirely in uppercase letters, with underscores used to indicate boundaries between words: final double FREEZING_POINT = 32.0; final double DEGREE_RATIO = 5.0 / 9.0; Java Programming FROM THE BEGINNING 53 Copyright 2000 W. W. Norton & Company.

Chapter 2: Writing Java Programs Adding Constants to the FtoC Program FtoC2.java // Converts a Fahrenheit temperature to Celsius public class FtoC2 { public static void main(String[] args) { final double FREEZING_POINT = 32.0; final double DEGREE_RATIO = 5.0 / 9.0; double fahrenheit = 98.6; double celsius = (fahrenheit - FREEZING_POINT) * DEGREE_RATIO; System.out.print("Celsius equivalent: "); System.out.println(celsius); } } Java Programming FROM THE BEGINNING 54 Copyright 2000 W. W. Norton & Company.

Chapter 2: Writing Java Programs Advantages of Naming Constants Advantages of naming constants: Programs are easier to read. The alternative is a program full of magic numbers. Programs are easier to modify. Inconsistencies and typographical errors are less likely. Always create meaningful names for constants. Theres no point in defining a constant whose name signifies its value: final int TWELVE = 12; Java Programming FROM THE BEGINNING 55 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs 2.9 Methods

A method is a series of statements that can be executed as a unit. A method does nothing until it is activated, or called. To call a method, we write the name of the method, followed by a pair of parentheses. The methods arguments (if any) go inside the parentheses. A call of the println method: System.out.println("Java rules!"); Java Programming FROM THE BEGINNING 56 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Methods in the Math Class The Math class contains a number of methods for performing mathematical calculations. These methods are called by writing Math.name, where name is the name of the method.

The methods in the Math class return a value when they have completed execution. Java Programming FROM THE BEGINNING 57 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs The pow and sqrt Methods The pow method raises a number to a power: Math.pow(2.0, 3.0) 8.0 Math.pow(-2.0, 3.0) 8.0 Math.pow(2.0, -1.0) 0.5 The sqrt method computes the square root of a number: Math.sqrt(2.0) 1.4142135623730951 Math.sqrt(4.0) 2.0 Both pow and sqrt return values of type

double. Java Programming FROM THE BEGINNING 58 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs The abs and max Methods The abs method computes the absolute value of a number: Math.abs(2.0) 2.0 Math.abs(-2.0) 2.0 Math.abs(2) 2 Math.abs(-2) 2 The max method finds the larger of two numbers: Math.max(3.0, 5.5) 5.5 Math.max(10.0, -2.0) 10.0 Math.max(12, -23) 12 Math.max(-5, -2) 2 Java Programming

FROM THE BEGINNING 59 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs The min Method The min method finds the smaller of two numbers: Math.min(3.0, 5.5) 3.0 Math.min(10.0, -2.0) 2.0 Math.min(12, -23) 23 Math.min(-5, -2) 5 The value returned by abs, max, and min depends on the type of the argument: If the argument is an int, the methods return an int. If the argument is a double, the methods return a double. Java Programming FROM THE BEGINNING

60 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs The round Method The round method rounds a double value to the nearest integer: Math.round(4.1) 4 Math.round(4.5) 5 Math.round(4.9) 5 Math.round(5.5) 6 Math.round(-4.1) 4 Math.round(-4.5) 4 Math.round(-4.9) 5 Math.round(-5.5) 5 round returns a long value rather than an int value. Java Programming FROM THE BEGINNING 61

Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Using the Result of a Method Call The value returned by a method can be saved in a variable for later use: double y = Math.abs(x); Another option is to use the result returned by a method directly, without first saving it in a variable. For example, the statements double y = Math.abs(x); double z = Math.sqrt(y); can be combined into a single statement: double z = Math.sqrt(Math.abs(x)); Java Programming FROM THE BEGINNING 62 Copyright 2000 W. W. Norton &

Company. Chapter 2: Writing Java Programs Using the Result of a Method Call Values returned by methods can also be used as operands in expressions. Example (finding the roots of a quadratic equation): double (-b double (-b root1 = + Math.sqrt(b * b - 4 * a * c)) / (2 * a); root2 = - Math.sqrt(b * b - 4 * a * c)) / (2 * a); Because the square root of b2 4ac is used twice, it would be more efficient to save it in a variable: double discriminant = Math.sqrt(b * b - 4 * a * c); double root1 = (-b + discriminant) / (2 * a); double root2 = (-b - discriminant) / (2 * a);

Java Programming FROM THE BEGINNING 63 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Using the Result of a Method Call The value returned by a method can be printed without first being saved in a variable: System.out.println(Math.sqrt(2.0)); Java Programming FROM THE BEGINNING 64 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs

2.10 Input and Output Most programs require both input and output. Input is any information fed into the program from an outside source. Output is any data produced by the program and made available outside the program. Java Programming FROM THE BEGINNING 65 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Displaying Output on the Screen Properties of System.out.print and System.out.println: Can display any single value, regardless of type. The argument can be any expression, including a variable, literal, or value returned by a method. println always advances to the next line after displaying its argument; print does not.

Java Programming FROM THE BEGINNING 66 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Displaying a Blank Line One way to display a blank line is to leave the parentheses empty when calling println: System.out.println("Hey Joe"); System.out.println(); // Write a blank line The other is to insert \n into a string thats being displayed by print or println: System.out.println("A hop,\na skip,\n\nand a jump"); Each occurrence of \n causes the output to begin on a new line. Java Programming

FROM THE BEGINNING 67 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Escape Sequences The backslash character combines with the character after it to form an escape sequence: a combination of characters that represents a single character. The backslash character followed by n forms \n, the new-line character. Java Programming FROM THE BEGINNING 68 Copyright 2000 W. W. Norton & Company.

Chapter 2: Writing Java Programs Escape Sequences Another common escape sequence is \", which represents " (double quote): System.out.println("He yelled \"Stop!\" and we stopped."); In order to print a backslash character as part of a string, the string will need to contain two backslash characters: System.out.println("APL\\360"); Java Programming FROM THE BEGINNING 69 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Printing Multiple Items The + operator can be used to combine multiple items into a single string for printing purposes: System.out.println("Celsius equivalent: " + celsius);

At least one of the two operands for the + operator must be a string. Java Programming FROM THE BEGINNING 70 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Obtaining Input from the User The SimpleIO class (not part of standard Java) simplifies the task of obtaining input. The SimpleIO.prompt method is first used to prompt the user: SimpleIO.prompt("Enter Fahrenheit temperature: "); Next, the SimpleIO.readLine method is used to read the users input: String userInput = SimpleIO.readLine();

Java Programming FROM THE BEGINNING 71 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Obtaining Input from the User If the users input represents a number, it will need to be converted to numeric form. The Convert class (not part of standard Java) provides a toDouble method that converts a string to a double value: double fahrenheit = Convert.toDouble(userInput); To convert a string to an integer, the Integer.parseInt method is used instead: int n = Integer.parseInt(userInput); Java Programming FROM THE BEGINNING

72 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Packages Java allows classes to be grouped into larger units known as packages. The package that contains SimpleIO and Convert is named jpb (Java Programming: From the Beginning). Java comes with a large number of standard packages. Java Programming FROM THE BEGINNING 73 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs

Import Declarations Accessing the classes that belong to a package is done by using an import declaration: import package-name . * ; Import declarations go at the beginning of a program. A program that needs SimpleIO or Convert would begin with the line import jpb.*; Java Programming FROM THE BEGINNING 74 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Application Programming Interfaces The packages that come with Java belong to the Java Application Programming Interface (API). In general, an API consists of code that someone

else has written but that we can use in our programs. Typically, an API allows an application programmer to access a lower level of software. In particular, an API often provides access to the capabilities of a particular operating system or windowing system. Java Programming FROM THE BEGINNING 75 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Program: Converting from Fahrenheit to Celsius (Revisited) FtoC3.java // Converts a Fahrenheit temperature entered by the user to // Celsius import jpb.*; public class FtoC3 { public static void main(String[] args) {

final double FREEZING_POINT = 32.0; final double DEGREE_RATIO = 5.0 / 9.0; } } SimpleIO.prompt("Enter Fahrenheit temperature: "); String userInput = SimpleIO.readLine(); double fahrenheit = Convert.toDouble(userInput); double celsius = (fahrenheit - FREEZING_POINT) * DEGREE_RATIO; System.out.println("Celsius equivalent: " + celsius); Java Programming FROM THE BEGINNING 76 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs 2.11 Case Study:

Computing a Course Average The CourseAverage program will calculate a class average, using the following percentages: Programs 30% Quizzes10% Test 1 15% Test 2 15% Final exam 30% The user will enter the grade for each program (0 20), the score on each quiz (010), and the grades on the two tests and the final (0100). There will be eight programs and five quizzes. Java Programming FROM THE BEGINNING 77 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Output of the CourseAverage Program Welcome to the CSc 2310 average calculation program.

Enter Enter Enter Enter Enter Enter Enter Enter Program Program Program Program Program Program Program Program Enter Enter Enter Enter Enter Quiz

Quiz Quiz Quiz Quiz 1 2 3 4 5 1 2 3 4 5 6 7 8 score: score: score: score: score:

score: score: score: score: score: score: score: score: 20 19 15 18.5 20 20 18 20 9 10 5.5 8 9.5

Enter Test 1 score: 78 Enter Test 2 score: 92 Enter Final Exam score: 85 Course average: 88 Java Programming FROM THE BEGINNING 78 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Design of the CourseAverage Program 1. Print the introductory message ("Welcome to the CSc 2310 average calculation program"). 2. Prompt the user to enter eight program scores. 3. Compute the program average from the eight scores. 4. Prompt the user to enter five quiz scores. 5. Compute the quiz average from the five scores. 6. Prompt the user to enter scores on the tests and final exam. 7. Compute the course average from the program average, quiz average, test scores, and final exam score.

8. Round the course average to the nearest integer and display it. Java Programming FROM THE BEGINNING 79 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Design of the CourseAverage Program double variables can be used to store scores and averages. Computing the course average involves scaling the program average and quiz average so that they lie between 0 and 100: courseAverage = .30 programAverage 5 + .10 quizAverage 10 + .15 test1 + .15 test2 + .30 finalExam Math.round can be used to round the course average to the nearest integer. Copyright 2000 W. W. Norton &

Java Programming FROM THE BEGINNING 80 Company. Chapter 2: Writing Java Programs CourseAverage.java // // // // // // // // // // // // // // //

// // // Program name: CourseAverage Author: K. N. King Written: 1998-04-05 Modified: 1999-01-27 Prompts the user to enter eight program scores (0-20), five quiz scores (0-10), two test scores (0-100), and a final exam score (0-100). Scores may contain digits after the decimal point. Input is not checked for validity. Displays the course average, computed using the following formula: Programs Quizzes Test 1 Test 2 Final exam 30% 10% 15% 15% 30%

The course average is rounded to the nearest integer. Java Programming FROM THE BEGINNING 81 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs import jpb.*; public class CourseAverage { public static void main(String[] args) { // Print the introductory message System.out.println("Welcome to the CSc 2310 average " + "calculation program.\n"); // Prompt the user to enter eight program scores SimpleIO.prompt("Enter Program 1 score: "); String userInput = SimpleIO.readLine(); double program1 = Convert.toDouble(userInput); SimpleIO.prompt("Enter Program 2 score: "); userInput = SimpleIO.readLine(); double program2 = Convert.toDouble(userInput); SimpleIO.prompt("Enter Program 3 score: ");

userInput = SimpleIO.readLine(); double program3 = Convert.toDouble(userInput); Java Programming FROM THE BEGINNING 82 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs SimpleIO.prompt("Enter Program 4 score: "); userInput = SimpleIO.readLine(); double program4 = Convert.toDouble(userInput); SimpleIO.prompt("Enter Program 5 score: "); userInput = SimpleIO.readLine(); double program5 = Convert.toDouble(userInput); SimpleIO.prompt("Enter Program 6 score: "); userInput = SimpleIO.readLine(); double program6 = Convert.toDouble(userInput); SimpleIO.prompt("Enter Program 7 score: "); userInput = SimpleIO.readLine(); double program7 = Convert.toDouble(userInput); SimpleIO.prompt("Enter Program 8 score: ");

userInput = SimpleIO.readLine(); double program8 = Convert.toDouble(userInput); Java Programming FROM THE BEGINNING 83 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs // Compute the program average from the eight scores double programAverage = (program1 + program2 + program3 + program4 + program5 + program6 + program7 + program8) / 8; // Prompt the user to enter five quiz scores SimpleIO.prompt("\nEnter Quiz 1 score: "); userInput = SimpleIO.readLine(); double quiz1 = Convert.toDouble(userInput); SimpleIO.prompt("Enter Quiz 2 score: "); userInput = SimpleIO.readLine(); double quiz2 = Convert.toDouble(userInput); SimpleIO.prompt("Enter Quiz 3 score: "); userInput = SimpleIO.readLine();

double quiz3 = Convert.toDouble(userInput); Java Programming FROM THE BEGINNING 84 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs SimpleIO.prompt("Enter Quiz 4 score: "); userInput = SimpleIO.readLine(); double quiz4 = Convert.toDouble(userInput); SimpleIO.prompt("Enter Quiz 5 score: "); userInput = SimpleIO.readLine(); double quiz5 = Convert.toDouble(userInput); // Compute the quiz average from the five scores double quizAverage = (quiz1 + quiz2 + quiz3 + quiz4 + quiz5) / 5; // Prompt the user to enter scores on the tests and final // exam SimpleIO.prompt("\nEnter Test 1 score: "); userInput = SimpleIO.readLine(); double test1 = Convert.toDouble(userInput);

Java Programming FROM THE BEGINNING 85 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs SimpleIO.prompt("Enter Test 2 score: "); userInput = SimpleIO.readLine(); double test2 = Convert.toDouble(userInput); SimpleIO.prompt("Enter Final Exam score: "); userInput = SimpleIO.readLine(); double finalExam = Convert.toDouble(userInput); // Compute the course average from the program average, // quiz average, test scores, and final exam score. // The program average (0-20) is multiplied by 5 to put // it on a scale of 0 to 100. The quiz average (0-10) is // multiplied by 10 for the same reason. double courseAverage = .30 * programAverage * 5 + .10 * quizAverage * 10 + .15 * test1 + .15 * test2 +

.30 * finalExam; Java Programming FROM THE BEGINNING 86 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs // Round the course average to the nearest integer and // display it System.out.println("\nCourse average: " + Math.round(courseAverage)); } } Java Programming FROM THE BEGINNING 87 Copyright 2000 W. W. Norton & Company.

Chapter 2: Writing Java Programs Style Issues Style issues raised by the CourseAverage program: Comment blocks Blank lines Short comments Long lines Java Programming FROM THE BEGINNING 88 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs

Improving the Program The values of most variables in the CourseAverage program are used only once. When a variables value is used only once, the variable can often be eliminated by substituting the value thats assigned to it. The lines String userInput = SimpleIO.readLine(); double program1 = Convert.toDouble(userInput); could be replaced by double program1 = Convert.toDouble(SimpleIO.readLine()); Java Programming FROM THE BEGINNING 89 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Improving the Program

The number of variables in CourseAverage can be reduced by keeping a running total of all scores entered so far, rather than storing each score in a separate variable: // Prompt the user to enter eight program scores. SimpleIO.prompt("Enter Program 1 score: "); String userInput = SimpleIO.readLine(); double programTotal = Convert.toDouble(userInput); SimpleIO.prompt("Enter Program 2 score: "); userInput = SimpleIO.readLine(); programTotal += Convert.toDouble(userInput); ... // Compute the program average from the eight scores. double programAverage = programTotal / 8; Java Programming FROM THE BEGINNING 90 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs

2.12 Debugging Debugging is the process of finding bugs in a program and fixing them. Types of errors: Compile-time errors Run-time errors (called exceptions in Java) Incorrect behavior Java Programming FROM THE BEGINNING 91 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Fixing Compile-Time Errors Strategies for fixing compile-time errors: Read error messages carefully. Example: Buggy.java:8: Undefined variable: i System.out.println(i); ^

Buggy.java:10: Variable j may not have been initialized System.out.println(j); ^ Pay attention to line numbers. Fix the first error. Java Programming FROM THE BEGINNING 92 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Fixing Compile-Time Errors Dont trust the compiler (completely). The error isnt always on the line reported by the compiler. Also, the error reported by the compiler may not accurately indicate the nature of the error. Example: System.out.print("Value of i: ") System.out.println(i);

A semicolon is missing at the end of the first statement, but the compiler reports a different error: Buggy.java:8: Invalid type expression. System.out.print("Value of i: ") ^ Buggy.java:9: Invalid declaration. System.out.println(i); ^ Java Programming FROM THE BEGINNING 93 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Fixing Run-Time Errors When a run-time error occurs, a message will be displayed on the screen. Example: Exception in thread "main" java.lang.NumberFormatException: foo at java.lang.Integer.parseInt(Compiled Code)

at java.lang.Integer.parseInt(Integer.java:458) at Buggy.main(Buggy.java:11) Once we know what the nature of the error is and where the error occurred, we can work backwards to determine what caused the error. Java Programming FROM THE BEGINNING 94 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Fixing Behavioral Errors Errors of behavior are the hardest problems to fix, because the problem probably lies either in the original algorithm or in the translation of the algorithm into a Java program. Other than simply checking and rechecking the algorithm and the program, there are two approaches to locating the source of a behavioral

problem, depending on whether a debugger is available. Java Programming FROM THE BEGINNING 95 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Using a Debugger A debugger doesnt actually locate and fix bugs. Instead, it allows the programmer to see inside a program as it executes. Things to look for while debugging: Order of statement execution Values of variables Key features of a debugger: Step Breakpoint Watch

Java Programming FROM THE BEGINNING 96 Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Debugging Without a Debugger The JDK includes a debugger, named jdb. A debugger isnt always necessary, however. If a run-time error occurs in a Java program, the message displayed by the Java interpreter may be enough to identify the bug. Also, System.out.println can be used to print the values of variables for the purpose of debugging: System.out.println("Value of a: " + a + " Value of b: " + b); Java Programming FROM THE BEGINNING 97

Copyright 2000 W. W. Norton & Company. Chapter 2: Writing Java Programs Choosing Test Data Testing a program usually requires running it more than once, using different input each time. One strategy, known as boundary-value testing, involves entering input at the extremes of what the program considers to be legal. Boundary-value testing is both easy to do and surprisingly good at revealing bugs. Java Programming FROM THE BEGINNING 98 Copyright 2000 W. W. Norton & Company.

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