JAVA: Simple pop does only return first item

Question

I got following code set up:

public class ListStack implements Stack {
    private class List {
        List next;
        Object object;

        public List(Object o, List n) {
            object = o;
            next = n;
        }
    }

    private List firstItem;
    private int size;

    public ListStack() {
        firstItem = new List(null, null);
        size = 0;
    }

    public List getEnd() {
        List endEl = firstItem;

        while (endEl.next != null) {
            endEl = endEl.next;
        }
        return endEl;
    }

    public boolean push(Object o) {
        List e1 = new List(o, null);
        this.getEnd().next = e1;
        size++;
        return true;
    }

    public Object pop() {
        if (this.firstItem.next == null) {
            return null;
        } else {
            List endEl;
            List tempEl;

            endEl = this.getEnd();
            tempEl = firstItem;
            while (tempEl.next != endEl) {
                tempEl = tempEl.next;
            }
            tempEl.next = null;
            size--;
            return tempEl.object;
        }
    }

    public int size() {
        return size;
    }

    public static void main(String[] args) {
        Stack s = new ListStack();

        Object test = new Object();
        Object test2 = new Object();

        System.out.println("pushing Object test to List: " + s.push(test));
        System.out.println("pushing Object test2 to List: " + s.push(test2));

        System.out.println("popping Object from List: " + s.pop());
        System.out.println("popping Object from List: " + s.pop());
        System.out.println("popping Object from List: " + s.pop());
    }
}

And this one:

 public interface Stack {  
     public int size();
     public boolean push(Object o);
     public Object pop();
 }

But its only giving me the first object and twice "null" but it should give me the two objects :( where is my mistake? It is asking for the last item and gives it back (.object) but only returns first object adress

Could someone please translate the comments from German to English? — wchargin
– wchargin, Commented May 6, 2013 at 21:19
@WChargin removed the comments in an edit, they were standard "what this method does" type comments. — kryger
– kryger, Commented May 6, 2013 at 21:23
@choice_sk: i think this is the solution , Try to write it an answer :) — Alya'a Gamal
– Alya'a Gamal, Commented May 6, 2013 at 21:30

ibecar · Accepted Answer · 2013-05-06 21:31:10Z

4

I think what your pop() function should return is endEl.object.

answered May 6, 2013 at 21:31

ibecar

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Comments

fjf2002 · Accepted Answer · 2013-05-06 21:29:21Z

1

Your code is way too long-winded. A stack is a data structure that can efficiently push and pop elements. But your code has to traverse the whole stack for both operations (i. e. runs in O(n) instead of O(1) time.).

Prepending to your list is much more efficient as appending.

Example for an efficient push:

public void push(Object o) {
    firstItem = new List(o, firstItem);
    size++;
}

answered May 6, 2013 at 21:29

fjf2002

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