I keep getting wrong output or null pointer exception in my towers program

Question

I'm working on solving towers game by using stacks and linked lists and moving blocks from tower to tower by using recursion.

I encountered a problem in my problem which causes java.lang.NullPointerException. My guess why this was happening was that I try to pop value from a stack even when there are no entries. After I put bound control I still receive that error.

Th error points to the line with deleteFirst() method but I don't see why this would happens even after I check if lists are empty.

My task here was just to pass towers or LinkedStack objects then move the content of theirs in tower game fashion.

Errors:

Exception in thread "main" java.lang.NullPointerException
    at LinkList.deleteFirst(towers.java:47) // code with: **first = first.next;**
    at LinkedStack.pop(towers.java:82) // code with: return theList.deleteFirst();
    at LinkListApp.doTowers(towers.java:146) // code with: A.pop();
    at LinkListApp.doTowers(towers.java:140) // doTowers(a-1, A, C, B);
    at LinkListApp.main(towers.java:121) // doTowers(nDisks, linkA, linkB, linkC);

What am I doing wrong here? I can't make this work. as it should.

FullStackTrace would be more helpful than the explanation of error. Please post it. — Rohit Jain, Oct 30 '12 at 18:43
I see no comment in your code stating // *** this is where the NPE occurs***. Again, please correct this deficiency. Let's make the problem as unambiguous as possible. — Hovercraft Full Of Eels, Oct 30 '12 at 18:44
Indicating the whole method as the site of the NPE is insufficient. — Marko Topolnik, Oct 30 '12 at 18:46
@digitaljoel Yes I copied pasted the wrong part of the code. — assylias, Oct 30 '12 at 18:51

digitaljoel · Accepted Answer · 2012-10-30T19:13:56.623

2

Your doTowers call makes calls to A.pop() and C.pop(), neither of which is protected. Your LinkedStack directly calls theList.deleteFirst() without an empty check, and your deleteFirst method calls first = first.next without checking if first is null or not. It would be good to make your LinkedList smart enough that deleteFirst doesn't throw a NPE in this case, then you won't need special checks all over the place in the upper layers. To do that, change deleteFirst to something like

public long deleteFirst()
{
    if ( first != null ) {
        Link temp = first;
        first = first.next;
        return temp.dData;
    }
    else {
        return whateverIndicatesTheListIsAlreadyEmptyWhichMayBeHardWithReturnTypelong;
    }
}

edited Oct 30 '12 at 19:13

answered Oct 30 '12 at 18:46

digitaljoel

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Yes, I tried that by wrapping pop method in if statement like this: `if (!A.isEmpty()) A.pop();`. But I still get null pointer exception. When I change to `if (A.isEmpty()) A.pop();` it never deletes fields, but with no errors. – HelpNeeder Oct 30 '12 at 18:55
This fixed the null pointer exception. I tried to check for this in doTowers method which didn't work. Thanks. Now, I just need to work on the output. – HelpNeeder Oct 30 '12 at 19:26

I keep getting wrong output or null pointer exception in my towers program

1 Answers1