[rhm54]

Greetings,

I am extremely new to Java and programming in General but I am trying to implement a stack using two queues for a homework assignment. I think I have the right idea but for some reason when the program runs I get:

Exception in thread "main" java.lang.NullPointerException
at com.shadowconcept.mcdougal.StackImplementationUsin gTwoQueues.push(StackImplementationUsingTwoQueues. java:27)
at com.shadowconcept.mcdougal.StackImplementationUsin gTwoQueues.main(StackImplementationUsingTwoQueues. java:57)


Here is the code:

Code:

package com.shadowconcept.mcdougal;

import java.util.*;

public class StackImplementationUsingTwoQueues {

	Queue<Integer> q1;
	Queue<Integer> q2;
	
	public int pop() {
		if (q1 == null) {
			System.out.println("The stack is empty, nothing to return");
			int i = 0;
			return i;
		}
		else {
			int temp = q1.remove();
			if (q2 != null){
				q1.add(q2.remove());
			}
			return temp;
		}
	}
	
	public void push(int data){
		if (q1 == null){
			q1.add(data);
		}
		else {
			q2.add(q1.remove());
			q1.add(data);
		}
	}
	
	public int top(){
		if (q1 == null){
			int temp = 0;
			System.out.println("The Stack is empty returning value 0");
			return temp;
		}
		return q1.peek();
	}
	
	public int size(){
		if (q1 == null && q2 == null){
			int size = 0;
			return size;
		}
		else{
			int size = q1.size() + q2.size();
			return size;
		}
	}
 	
	public static void main(String[] args) {
		StackImplementationUsingTwoQueues s1 = new StackImplementationUsingTwoQueues();
		s1.push(8);
		s1.push(10);
		s1.push(3);
		s1.pop();
		s1.pop();

	}

}