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Homework Help: Java NullPointerException

  1. Aug 7, 2011 #1
    For the following segment of code, I am getting a null pointer exception when trying to access any value in a list (doubly-linked with dummy nodes) AFTER a null value. For example, if I have the following list of integers:

    0, 1, 2, 3, null, 4, 5

    and I run the function list.contains(0) or list.contains(1)...list.contains(null), the program works as it should. When I try to call list.contains(4) or list.contains(5), I receive a null pointer exception. I know the order in which I use == and .equals() matters, but should I separate it into multiple if statements? I tried to do so, and I was not receiving any different results.

    for (Node cursor = _head._next; cursor != _tail; cursor = cursor._next){
    if ((o == null & cursor._data == null) || cursor._data.equals(o)) // o is the Object data
    return true;
     
  2. jcsd
  3. Aug 7, 2011 #2

    I like Serena

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    Homework Helper

    Hi magnifik! :smile:

    You cannot use the function cursor._data.equal() if cursor._data is a null pointer.
    Your current code does not prevent this (which manifests when you have o=4).

    I recommend turning it into an if-statement where you just check if cursor._data is a null pointer, and then take appropriate action.
     
  4. Aug 7, 2011 #3
    You can also consider using a try-catch statement similar to this:
    Code (Text):

    try {
        cursor._data.equals(o)
    }
    catch (NullPointerException e) {
        System.out.println("NullPointerException thrown");
    }
    I'm presuming that "I like Serena" is correct in that the exception is being caused by cursor._data.equal().
     
  5. Aug 8, 2011 #4
    thank you for the suggestion! my program functioned correctly after using a series of if statements rather than my single one :)
     
  6. Aug 8, 2011 #5

    I like Serena

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    Homework Helper

    Good! :smile:

    And here's one for the road:

    Code (Text):

    bool found = false;
    for (Node cursor = _head._next; (cursor != _tail) && !found; cursor = cursor._next)
    {
       // o is the Object data
       if (cursor._data == null)
       {
          found = (o == null);  
       }
       else
       {
          found = cursor._data.equals(o);
       }
    }
    return found;
     
     
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