How to fix the Findbugs issue "Null value is guaranteed to be dereferenced" NP_GUARANTEED_DEREF

Question

Hi I have got some code that is reported as having the NP_GUARANTEED_DEREF issue by Findbugs. Now looking at my code I don't quite understand what is wrong with it, can anyone suggest what the problem is.

public void test() {
  String var = "";
  int index = 2;
  if (index == -1) {
    var = String.class.getName();
    if (var.length() == 0) {
      var = null;
    }
  } else {
    var = Integer.class.getName();
    if (var.length() == 0) {
      var = null;
    }
  }
  if (var == null) {// FINBUGS reports on this line NP_GUARANTEED_DEREF
    /*
     * There is a statement or branch that if executed guarantees that a value
     * is null at this point, and that value that is guaranteed to be
     * dereferenced (except on forward paths involving runtime exceptions).
     */
    throw new NullPointerException("NULL");
  }
}

Now drilling into the Error in Findbugs it highlights the two assignments to var = null; as cause for the bug but I don't quite understand why. It is not like I am actually doing anything with the var object I am just doing a Null check. The example is taken from real production code but stripped of anything that wasn't needed to reproduce the error. What I am wondering if this is a false positive or not. And if not what would be an appropriate fix.

Here is the link to the Findbugs Bug Detail: http://findbugs.sourceforge.net/bugDescriptions.html#NP_GUARANTEED_DEREF

[UPDATE] After recieving some feedback on this issue I have now logged this as a False Positive in the Findbugs Bugtracker on Sourceforge the link is https://sourceforge.net/tracker/?func=detail&aid=3277814&group_id=96405&atid=614693

Conversation about the problem will continue there.

Answer 1

I see. I can confirm the same FB behavior on my computer. Looks strange indeed. What's funny, that if you replaced throw new NullPointerException with throw new RuntimeException the bug marker would disappear.

Now I think I understand what they've meant. The wording of the message is not exact, but they are warning you against a NPE. I guess they consider explicitly throwing NPE a bad practice.