stack

public interface Stack{
    public void pop();
    public void push(Object obj);
    public Object top();  
}

 

import java.util.NoSuchElementException;
public class ArrayStack implements Stack{
    private int capacity=101;
    private Object[] data;
    private int top=0;
   
    public ArrayStack(){
     this(101);
   
    }
 
 public ArrayStack(int capacity){
  this.capacity=capacity;
     data=new Object[capacity+1];
 }
  
    public void pop(){
     if(isEmpty())
        throw new NoSuchElementException("stack is empty");
      else{
        data[top]=null;
        top--;
      } 
     
    }

    public void push(Object item){
        top++;
        try{
            data[top]=item; 
         
        }catch(ArrayIndexOutOfBoundsException ex){
            top--;
            throw new ArrayIndexOutOfBoundsException("overflow"); 
        }
     
      }

      public boolean isEmpty(){
          if(top==0)
              return true;
          else
              return false;
              
      }

      public int size(){
         return top; 
      }
     
      public Object top(){
          if(isEmpty())
               throw new NoSuchElementException("stack is empty!");
          else
               return data[top] ; 
       
       
      }
     
     
     
      public static void main(String[] args){
          ArrayStack arrayStack = new ArrayStack(5);
          arrayStack.push(new Integer(1));
          arrayStack.push(new Integer(2));
          arrayStack.push(new Integer(2));
          System.out.println(arrayStack.size());
          arrayStack.pop();
          System.out.println(arrayStack.size());
      }
}