Java Instance Variable does not retain state

Question

I have a straightforward implementation of stack, using Linked Lists, however during testing the code I noticed that the instance variable first does not retain state and gets reverted to NULL during subsequent push operations. While N retains its values, the variable first does not. Can someone please help?

import java.util.NoSuchElementException;

public class Stack <T> {

    private Node first;
    private Node current;
    private int N;

    private class Node {
        T item;
        Node next;
    }

    public void push(T item) {
        Node oldFirst = first;

        Node first = new Node();
        first.item = item;
        first.next = oldFirst;

        N++;
    }

    public T pop()  throws NoSuchElementException{
        try {
            T item = first.item;
            first = first.next;
            N--;
            return item;
        } catch (java.lang.NullPointerException error) {
            throw new NoSuchElementException("Stack is empty.");
        }
    }

}

Test Client:

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class TestStack {

   /**
     * Test the LIFO property of the stack i.e. the item pushed last into the
     * stack is the one to be popped first from the stack.
     */
    @Test
    void testLIFOPropertyWithIntegerStack() {
        int[] testClient = new int[] {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
        Stack<Integer> stack = new Stack<> ();

        for (int item : testClient) {
            stack.push(item);
        }

        int index = testClient.length - 1;
        while (stack.size() != 0) {
            int item = stack.pop();
            assertEquals(item, testClient[index--]);
        }
    }

}

Answer 1

There is a problem in your push method. Your Node first = new Node(); is hiding the field first in your stack.

So, change:

Node first = new Node();

to

// This is your object field
first = new Node();

And the test will pass. Remember that you need to implement size() since your test method uses it.

public int size(){
    return N;
}