Java Foundations

Java Foundations

Chapter 4 Linked Structures - Stacks Chapter Scope Object references used as links Linked vs. array-based structures Managing linked lists Linked implementation of a stack Java Software Structures, 4th Edition, Lewis/Chase 4-2 Linked Structures An alternative to array-based implementations are linked structures A linked structure uses object references to create links between objects Recall that an object reference variable holds the address of an object Java Software Structures, 4th Edition, Lewis/Chase 4-3

Linked Structures A Person object, for instance, could contain a reference to another Person object A series of Person objects would make up a linked list: Attributes | next Java Software Structures, 4th Edition, Lewis/Chase Attributes | next 4-4 Linked Structures Links could also be used to form more complicated, nonlinear structures Java Software Structures, 4th Edition, Lewis/Chase 4-5

Linked Lists There are no index values built into linked lists To access each node in the list you must follow the references (represented by the reference variable called next) from one node to the next Person current = first; while (current != null) { System.out.println(current); current = current.next; } Java Software Structures, 4th Edition, Lewis/Chase 4-6 Linked Lists Care must be taken to maintain the integrity of the links To insert a node at the front of the list, first point the new node to the front node, then reassign the front reference Java Software Structures, 4th Edition, Lewis/Chase

4-7 Linked Lists To delete the first node, reassign the front reference accordingly If the deleted node is needed elsewhere, a reference to it must be established before reassigning the front pointer Java Software Structures, 4th Edition, Lewis/Chase 4-8 Linked Lists So far we've assumed that the list contains nodes that are self-referential (Person points to a Person) But often we'll want to make lists of objects (String objects for examples) that don't contain such references Solution: have a separate Node class that forms the list and holds a reference to the objects (elements) being stored

Java Software Structures, 4th Edition, Lewis/Chase 4-9 Linked Lists There are many variations on the basic linked list concept For example, we could create a doubly-linked list with next and previous references in each node and a separate pointer to the rear of the list Java Software Structures, 4th Edition, Lewis/Chase 4 - 10 Stacks Revisited In the previous chapter we developed our own array-based version of a stack, and we also used the java.util.Stack class from the Java API The API's stack class is derived from Vector, which has many non-stack abilities It is, therefore, not the best example of inheritance, because a stack is not a vector

It's up to the user to use a Stack object only as intended Java Software Structures, 4th Edition, Lewis/Chase 4 - 11 Stacks Revisited Stack characteristics can also be found by using the Deque interface from the API Deque interface supports the insertion and removal of elements at both end points The LinkedList class implements the Deque interface Deque stands for double-ended queue, and will be explored further later For now, we will use the stack characteristics of a Deque to solve the problem of traversing a maze Java Software Structures, 4th Edition, Lewis/Chase 4 - 12 Using Stacks: Traversing a Maze (skip to slide 24)

Suppose a two-dimensional maze is represented as a grid of 1 (path) and 0 (wall) Goal: traverse from the upper left corner to the bottom right (no diagonal moves) 9 1 1 1 0 1 1 1 1 1 13 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 0

1 1 Java Software Structures, 4th Edition, Lewis/Chase 0 1 1 0 0 0 1 0 1 1 1 1 1 1 0 1 0 1

1 0 0 1 1 0 1 0 1 0 1 1 1 1 0 1 0 1 0 1 0 0

0 1 1 0 1 0 1 1 1 1 1 0 0 1 1 1 0 0 0 1 1 0

1 1 0 1 1 1 0 1 0 1 1 0 0 1 1 0 1 0 1 1 1

0 1 1 1 1 0 1 4 - 13 Traversing a Maze Using a stack, we can perform a backtracking algorithm to find a solution to the maze An object representing a position in the maze is pushed onto the stack when trying a path If a dead end is encountered, the position is popped and another path is tried We'll change the integers in the maze grid to represent tried paths (2) and the successful path (3) Java Software Structures, 4th Edition, Lewis/Chase 4 - 14 import java.util.*;

import java.io.*; /** * Maze represents a maze of characters. The goal is to get from the * top left corner to the bottom right, following a path of 1's. Arbitrary * constants are used to represent locations in the maze that have been TRIED * and that are part of the solution PATH. * * @author Lewis and Chase * @version 4.0 */ public class Maze { private static final int TRIED = 2; private static final int PATH = 3; private int numberRows, numberColumns; private int[][] grid; Java Software Structures, 4th Edition, Lewis/Chase 4 - 15 /** * Constructor for the Maze class. Loads a maze from the given file. * Throws a FileNotFoundException if the given file is not found.

* * @param filename the name of the file to load * @throws FileNotFoundException if the given file is not found */ public Maze(String filename) throws FileNotFoundException { Scanner scan = new Scanner(new File(filename)); numberRows = scan.nextInt(); numberColumns = scan.nextInt(); grid = new int[numberRows][numberColumns]; // create the array object for (int i = 0; i < numberRows; i++) for (int j = 0; j < numberColumns; j++) grid[i][j] = scan.nextInt(); // get the maze data } Java Software Structures, 4th Edition, Lewis/Chase 4 - 16 /** * Marks the specified position in the maze as TRIED * * @param row the index of the row to try * @param col the index of the column to try

*/ public void tryPosition(int row, int col) { grid[row][col] = TRIED; } /** * Return the number of rows in this maze * * @return the number of rows in this maze */ public int getRows() { return grid.length; } /** * Return the number of columns in this maze * * @return the number of columns in this maze */ public int getColumns() { return grid[0].length; }

Java Software Structures, 4th Edition, Lewis/Chase 4 - 17 /** * Marks a given position in the maze as part of the PATH * * @param row the index of the row to mark as part of the PATH * @param col the index of the column to mark as part of the PATH */ public void markPath(int row, int col) { grid[row][col] = PATH; } /** * Determines if a specific location is valid. A valid location * is one that is on the grid, is not blocked, and has not been TRIED. * * @param row the row to be checked * @param column the column to be checked * @return true if the location is valid */ public boolean validPosition(int row, int column) {

boolean result = false; // check if cell is in the bounds of the matrix if (row >= 0 && row < grid.length && column >= 0 && column < grid[row].length) // check if cell is not blocked and not previously tried if (grid[row][column] == 1) result = true; return result; } Java Software Structures, 4th Edition, Lewis/Chase 4 - 18 /** * Returns the maze as a string. * * @return a string representation of the maze */ public String toString() { String result = "\n"; for (int row=0; row < grid.length; row++) { for (int column=0; column < grid[row].length; column++) result += grid[row][column] + "";

result += "\n"; } return result; } } Java Software Structures, 4th Edition, Lewis/Chase 4 - 19 import java.util.*; /** * MazeSolver attempts to traverse a Maze using a stack. The goal is to get from the * given starting position to the bottom right, following a path of 1's. Arbitrary * constants are used to represent locations in the maze that have been TRIED * and that are part of the solution PATH. * * @author Lewis and Chase * @version 4.0 */ public class MazeSolver { private Maze maze; /**

* Constructor for the MazeSolver class. */ public MazeSolver(Maze maze) { this.maze = maze; } Java Software Structures, 4th Edition, Lewis/Chase 4 - 20 /** * Attempts to traverse the maze using a stack. Inserts special * characters indicating locations that have been TRIED and that * eventually become part of the solution PATH. * * @param row row index of current location * @param column column index of current location * @return true if the maze has been solved */ public boolean traverse() { boolean done = false; int row, column;

Position pos = new Position(); Deque stack = new LinkedList(); stack.push(pos); while (!(done) && !stack.isEmpty()) { pos = stack.pop(); maze.tryPosition(pos.getx(),pos.gety()); // this cell has been tried if (pos.getx() == maze.getRows()-1 && pos.gety() == maze.getColumns()-1) done = true; // the maze is solved else { push_new_pos(pos.getx() - 1,pos.gety(), stack); push_new_pos(pos.getx() + 1,pos.gety(), stack); push_new_pos(pos.getx(),pos.gety() - 1, stack); push_new_pos(pos.getx(),pos.gety() + 1, stack); } } return done; } Java Software Structures, 4th Edition, Lewis/Chase 4 - 21

/** * Push a new attempted move onto the stack * @param x represents x coordinate * @param y represents y coordinate * @param stack the working stack of moves within the grid * @return stack of moves within the grid */ private void push_new_pos(int x, int y, Deque stack) { Position npos = new Position(); npos.setx(x); npos.sety(y); if (maze.validPosition(x,y)) stack.push(npos); } } Java Software Structures, 4th Edition, Lewis/Chase 4 - 22 import java.util.*; import java.io.*;

/** * MazeTester determines if a maze can be traversed. * * @author Lewis and Chase * @version 4.0 */ public class MazeTester { /** * Creates a new maze, prints its original form, attempts to * solve it, and prints out its final form. */ public static void main(String[] args) throws FileNotFoundException { Scanner scan = new Scanner(System.in); System.out.print("Enter the name of the file containing the maze: "); String filename = scan.nextLine(); Maze labyrinth = new Maze(filename); System.out.println(labyrinth); MazeSolver solver = new MazeSolver(labyrinth); if (solver.traverse()) System.out.println("The maze was successfully traversed!"); else System.out.println("There is no possible path.");

System.out.println(labyrinth); } } Java Software Structures, 4th Edition, Lewis/Chase 4 - 23 Implementing a Stack using Links Let's now implement our own version of a stack that uses a linked list to hold the elements Our LinkedStack class stores a generic type T and implements the same StackADT interface used previously A separate LinearNode class forms the list and hold a reference to the element stored An integer count will store how many elements are currently in the stack Java Software Structures, 4th Edition, Lewis/Chase 4 - 24 package jsjf; public interface StackADT

{ public void push(T element); public T pop(); public T peek(); public boolean isEmpty(); public int size(); public String toString(); } } Java Software Structures, 4th Edition, Lewis/Chase 4 - 25 Implementing a Stack using Links Since all activity on a stack happens on one end, a single reference to the front of the list will represent the top of the stack A LinearNode object A LinkedStack object

Java Software Structures, 4th Edition, Lewis/Chase 4 - 26 Implementing a Stack using Links The stack after A, B, C, and D are pushed, in that order: Java Software Structures, 4th Edition, Lewis/Chase 4 - 27 Implementing a Stack using Links After E is pushed onto the stack: push E onto the stack Java Software Structures, 4th Edition, Lewis/Chase 4 - 28 package jsjf; /**

* Represents a node in a linked list. * * @author Lewis and Chase * @version 4.0 */ public class LinearNode element next { private LinearNode next; private T element; /** * Creates an empty node. */ public LinearNode() { next = null; element = null; }

a LinearNode // Constructor 1 /** * Creates a node storing the specified element. * @param elem element to be stored */ public LinearNode(T elem) // Constructor 2 { next = null; element = elem; } Java Software Structures, 4th Edition, Lewis/Chase element next elem 4 - 29 /** * Returns the node that follows this one.

* @return reference to next node */ public LinearNode getNext() { return next; node } /** * Sets the node that follows this one. * @param node node to follow this one */ element next public void setNext(LinearNode node) { next = node; } /** * Returns the element stored in this node. * @return element stored at the node

*/ public T getElement() { element next return element; } /** * Sets the element stored in this node. * @param elem element to be stored at this node */ public void setElement(T elem) { element = elem; } elem } Java Software Structures, 4th Edition, Lewis/Chase

4 - 30 package jsjf; import jsjf.exceptions.*; import java.util.Iterator; /** * Represents a linked implementation of a stack. * * @author Lewis and Chase * @version 4.0 */ public class LinkedStack implements StackADT { private int count; private LinearNode top; /** * Creates an empty stack. */ public LinkedStack() { count = 0; top = null; }

Java Software Structures, 4th Edition, Lewis/Chase 4 - 31 /** * Adds the specified element to the top of this stack. * @param element element to be pushed on stack */ public void push(T element) { LinearNode temp = new LinearNode(element); // 1. Create a new node temp.setNext(top); // 2. Set the next field of the new node to the top element top = temp; // 3. Change top to point to the new node count++; // See next slide for this process } /** * Removes the element at the top of this stack and returns a * reference to it. * @return element from top of stack * @throws EmptyCollectionException if the stack is empty

*/ public T pop() throws EmptyCollectionException { if (isEmpty()) throw new EmptyCollectionException("stack"); T result = top.getElement(); top = top.getNext(); // Note: A LinkedStack object does not have direct access count--; // to the next field of a LinearNode return result; } Java Software Structures, 4th Edition, Lewis/Chase 4 - 32 Implementing a Stack using Links t temp E new node

before Java Software Structures, 4th Edition, Lewis/Chase after 4 - 33

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