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Thread: Help with using iterator remove on double linked list

  1. #1
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    Default Help with using iterator remove on double linked list

    I am trying to remove data from a double linked list using the remove method using a iterator and I keep getting a null pointer exception.

    This is what I have

    System.out.print("Enter student ID: ");
    String studentID2 = in.next();
    Iterator<Student> iter = students.iterator();
    while (iter.hasNext()) {
    Student stu = (Student) iter.next();
    if(stu.getStudentID().equals(studentID2)){
    iter.remove();
    }
    }

    Says the error is in my remove method

    public void remove() {
    lastItemRet = lastItemRet.prev;
    lastItemRet.next = nextItem; The error is on this line.
    nextItem.prev = lastItemRet;
    }


  2. #2
    Administrator copeg's Avatar
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    Default Re: Help with using iterator remove on double linked list

    Post an SSCCE and the full error code, or else you get guesses. My guess, nextItem is not defined anywhere or is out of scope of the location you are trying to access it.

  3. #3
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    Default Re: Help with using iterator remove on double linked list

    The full error I get isThe full error I get is


    Exception in thread "main" java.lang.NullPointerException
    at DLList$Node.access$3(DLList.java:105)
    at DLList$ListIter.remove(DLList.java:182)
    at Menu.display(Menu.java:46)
    at Prog3.main(Prog3.java:17)

  4. #4
    Administrator copeg's Avatar
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    Default Re: Help with using iterator remove on double linked list

    What could be null on that line? Could lastItemRet be null? Add some println's in there to try and debug, or use a debugger to step through the code and see what is null. Without an SSCCE it is hard to comment on a fix.

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