A basic implementation of the Stack interface

  • The basic implementation of a Stack is using:

    1.  

    2.                                                                                                                                                                                     

    Schematically:

A basic implementation of the Stack interface

  • The basic implementation of a Stack is using:

    1. A fixed size array to store the data items

    2.                                                                                                                                                                                     

    Schematically:

A basic implementation of the Stack interface

  • The basic implementation of a Stack is using:

    1. A fixed size array to store the data items

    2. A stackTop index variable to record the first open position in the array

    Schematically:

A basic implementation of the Stack interface

  • The initial state of the stack when it is instantiated (= created):

  • Comment:

    • I choose to initialize stackTop = 0 to indicate and empty stack

    • An alternate choice can be to use stackTop = −1

A basic implementation of the Stack interface   variables and constructor

  • The instance variables and the constructor of an Integer stack: 

       public class IntegerStack implements Stack<Integer>
   {
       private Integer[] item;
       private int stackTop;

       public IntegerStack(int N) // Create a stack of size N
       {
           item = new Integer[N];
           stackTop = 0;
       }

       ....
   }

A basic implementation of the Stack interface   isEmpty()

  • The stack is empty when stackTop == 0: 

       public class IntegerStack implements Stack<Integer>
   {
       public boolean isEmpty( ) // Test if stack is empty
       {
           return stackTop == 0;
       }
       ....
   }

A basic implementation of the Stack interface   push(x)

  • Initial state of the stack:

  • State after push(3):

    Effect:   (1) item[stackTop] = 3;   (2) stackTop++;

A basic implementation of the Stack interface   push(x)

  • Initial state of the stack:

  • State after push(3):

    Effect:   (1) item[stackTop] = 0;   (2) stackTop++;

A basic implementation of the Stack interface   push(x)

  • Basic implementaion of the push( ) method: 

           public void push(Integer e) 
       {
          if ( isFull () )
          {
              System.out.println("Full");
              return ;
          }

          item[ stackTop ] = e;    // (1) store item
          stackTop++;              // (2) increment stackTop
       }

A basic implementation of the Stack interface   push(x)

  • Safer implementaion of the push( ) method: 

           public void push(Integer e) 
       {
          if ( isFull () )  // Implement this next
          {
              System.out.println("Full");
              return ;      // Or: throw an exception
          }

          item[ stackTop ] = e;    // (1) store item
          stackTop++;              // (2) increment stackTop
       }

A basic implementation of the Stack interface   isFull()

  • The stack is full when stackTop == item.lenght: 

       public class IntegerStack implements Stack<Integer>
   {
       public boolean isFull( ) // Test if stack is empty
       {
           return stackTop == item.length;
       }
       ....
   }

A basic implementation of the Stack interface   pop()

  • Initial state of the stack:

  • State after pop():

    Effect:   (1) stackTop--;   (2) return item[stackTop];

A basic implementation of the Stack interface   pop()

  • Basic implementaion of the pop( ) method: 

           public Integer pop( ) 
       {
          if ( isEmpty () )
          {
              System.out.println("Full");
              return ;
          }

          stackTop--;              // (1) decrement stackTop
          return item[ stackTop ]; // (2) return item
       }

A basic implementation of the Stack interface   pop()

  • Safer implementaion of the pop( ) method: 

           public Integer pop( ) 
       {
          if ( isEmpty () )
          {
              System.out.println("Empty");
              return null;         // Or: throw an exception
          }

          stackTop--;              // (1) decrement stackTop
          return item[ stackTop ]; // (2) return item
       }

Demo program

  • Demo program showing the functionality of the stack: 

    public class Demo
{
    public static void main(String[] args)
    {
        IntegerStack s = new IntegerStack(5);

        s.pop();    // Pops an empty stack !
        s.push(1);
        s.push(2);
        s.push(3);
        System.out.println( s );
        System.out.println( s.pop() );
        System.out.println( s );
        System.out.println( s.pop() );
        System.out.println( s );
        System.out.println( s.pop() );
        System.out.println( s );
    }

}

DEMO: demo/09-stack/01-array/Demo.java + IntegerStack.java + MyStackInterface.java