Implementing a Stack Using two Queues

[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();

	}

}

[jelly]

I suspect that the reason your code is crashing is due to the code

Code:

		if (q1 == null){
			q1.add(data);
		}

you test the q1 reference to be null, and having found it is, you then try to use it. I suspect there are two things you need to change, firstly you need to instantiate the Queues,currently you create reference variables ( q1 and q2 ) that are capable of referring to a Queue but you never actually create the Queues. Maybe something like

Code:

	Queue<Integer> q1 = new LinkedList<Integer>();
	Queue<Integer> q2= new LinkedList<Integer>();

also you are getting confused between

Code:

q1 == null

which says that the variable is not currently referencing a Queue and

Code:

q1.peek() == null

which implies the Queue is empty - though there are edge cases where this may not be true, just don't go there yet

[rhm54]

Thanks for your help! The variables not being intialized and the null comparison were at fault. Now I seem to have a logic problem that I cannot wrap my head around. The problem appears to be in the push method. I push on 5 numbers 1, 2, 3, 4, 5 in that order so when I pop() them I should get 5, 4, 3, 2, 1 however I get this instead:

1
5
3
2
The stack is empty, nothing to return
0

Here is the code:

Code:

package com.shadowconcept.mcdougal;

import java.util.*;

public class StackImplementationUsingTwoQueues {

	Queue<Integer> q1 = new LinkedList<Integer>();
	Queue<Integer> q2 = new LinkedList<Integer>();
	
	public int pop() {
		if (q1.peek() == null) {
			System.out.println("The stack is empty, nothing to return");
			int i = 0;
			return i;
		}
		else {
			int pop = q1.remove();
			return pop;
		}
	}
	
	public void push(int data){
		if (q1.peek() == null){
			q1.add(data);
		}
		else {
			for (int i = 0; i < q1.size(); i++){
				q2.add(q1.remove());
			}
			q1.add(data);
			for (int j = 0; j < q2.size(); j++){
				q1.add(q2.remove());
			}
		}	
	}
	
	public int top(){
		if (q1.peek() == null){
			int temp = 0;
			System.out.println("The Stack is empty returning value 0");
			return temp;
		}
		return q1.peek();
	}
	
	public int size(){
		return q1.size();
		}
 	
	public static void main(String[] args) {
		StackImplementationUsingTwoQueues s1 = new StackImplementationUsingTwoQueues();
		s1.push(1);
		s1.push(2);
		s1.push(3);
		s1.push(4);
		s1.push(5);
		System.out.println(s1.pop());
		System.out.println(s1.pop());
		System.out.println(s1.pop());
		System.out.println(s1.pop());
		System.out.println(s1.pop());
		}

}