CSCD 326 Data Structures I Stacks

Transcription

1 CSCD 326 Data Structures I Stacks 1

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5 Stack Interface public interface StackInterface { public boolean isempty(); // Determines whether the stack is empty. // Precondition: None. // Postcondition: Returns true if the stack is empty; otherwise returns false. public boolean isfull(); // Determines whether the stack is full. // Precondition: None. // Postcondition: Returns true if the stack is full; otherwise returns false. public void push(object newitem) throws StackException; // Adds an item to the top of a stack. // Precondition: newitem is the item to be added. // Postcondition: If insertion is successful, newitem is on the top of the stack. // Exception: Some implementations may throw // StackException when newitem cannot be placed on the stack. 5

6 Stack Interface (2) public Object pop() throws StackException; // Removes the top of a stack. // Precondition: None. // Postcondition: If the stack is not empty, the item // that was added most recently is removed from the stack and returned. // Exception: Throws StackException if the stack is empty. public void popall(); // Removes all the items from the stack. // Precondition: None. // PostCondition: Stack is empty. public Object peek() throws StackException; // Retrieves the top of a stack. // Precondition: None. // Postcondition: If the stack is not empty, the item that was added most //recently is returned. The stack is unchanged. // Exception: Throws StackException if the stack is empty. } // end StackInterface 6

7 List Stack Implementation // Assumes that the classes ListInterface and Node are available public class ListStack implements StackInterface { private Node top; // class Node is the same used in the LinkedList class except it stores // Objects rather than Comparables public ListStack() { top = null; } // end default constructor public boolean isempty() { return (top == null); } // end isempty public boolean isfull() { return false; } // end isfull 7

8 List Stack Implementation (2) public void push(object newitem) { if (!isfull()) { Node temp = new Node(newItem); temp.setnext(top); top = temp; } else { throw new StackException("StackException on " + "push: stack full"); } // end if } // end push public Object pop() throws StackException { if (!isempty()) { Object temp = top.getitem(); top = top.getnext(); return temp; } else { throw new StackException("StackException on " + "pop: stack empty"); } // end if } // end pop 8

9 List Stack Implementation (3) public void popall() { while(!isempty()) pop(); } // end popall public Object peek() throws StackException { if (!isempty()) { return top.getitem(); } else { throw new StackException("StackException on " + "peek: stack empty"); } // end else } // end peek } // end ListStack 9

10 Stack Exception public class StackException extends java.lang.runtimeexception { public StackException(String s) { super(s); } // end constructor } // end StackException 10

11 List Stack Test public class ListStackTest { public static void main(string[ ] args) { Integer items[] = new Integer[15]; System.out.println("Pushing:"); for (int i=0; i<15; i++) { items[i] = new Integer(i); if (!stack.isfull()) { System.out.print(" "+i); stack.push(items[i]); } // end if } // end for System.out.println("\nPopping:"); while (!stack.isempty()) { // cast result of pop to Integer System.out.print(" "+(Integer)(stack.pop())); } // end while System.out.println(); } // end main } 11

12 List Stack Test Output Pushing: Popping:

13 Array Stack Implementation public class ArrayStack implements StackInterface { final int MAX_STACK = 50; // maximum size of stack private Object items[ ]; private int top; public ArrayStack() { items = new Object[MAX_STACK]; top = -1; } // end default constructor public boolean isempty() { return top < 0; } // end isempty public boolean isfull() { return top == MAX_STACK-1; } // end isfull 13

14 Array Stack Implementation (2) public void push(object newitem) throws StackException { if (!isfull()) { items[++top] = newitem; } else { throw new StackException("StackException on " + "push: stack full"); } // end if } // end push public void popall() { items = new Object[MAX_STACK]; top = -1; } // end popall 14

15 Array Stack Implementation (3) public Object pop() throws StackException { if (!isempty()) { return items[top--]; } else { throw new StackException("StackException on " + "pop: stack empty"); } // end if } // end pop public Object peek() throws StackException { if (!isempty()) { return items[top]; } else { throw new StackException("Stack exception on " + "peek - stack empty"); } // end if } // end peek } // end ArrayStack 15

16 Array Stack Test public class ArrayStackTest { public static final int MAX_ITEMS = 15; public static void main(string[ ] args) { ArrayStack stack = new ArrayStack(); Integer items[ ] = new Integer[MAX_ITEMS]; System.out.println("Pushing:"); for (int i=0; i<max_items; i++) { items[i] = new Integer(i); if (!stack.isfull()) { System.out.print(" "+i); stack.push(items[i]); } // end if } // end for System.out.println("\nPopping:"); while (!stack.isempty()) { // cast result of pop to Integer System.out.print(" "+(Integer)(stack.pop())); } // end while System.out.println(); } // end main } // end ArrayStackTest 16

17 Array Stack Test Output Pushing: Popping: