How to handle null compare method arguments in Comparator?

Question

I have created an implementation of Comparator<Entity>, but when I use this comparator to sort an Array<Entity>. I will receive an java.lang.NullPointerException, because when I map the entity to a static collections which is already removed. Now my problem is I don't know what to return to skip the compare method.

public class CustomComparator implements Comparator<Entity> {

   public int compare(Entity e1, Entity e2) {
       if( e1== null || e2 == null) {
           return // don't know what to return to skip this method;
       }

       Vector2 e1Pos = Mapper.transform.get(e1).position;
       Vector2 e2Pos = Mapper.transform.get(e2).position;

   }

}

See http://stackoverflow.com/questions/2401606/comparator-with-null-values — c0der, Oct 01 '16 at 07:52
This can only happen if your `Array` contains null elements. Solution: don't. — user207421, Oct 01 '16 at 07:52

score 14 · Accepted Answer · answered Oct 01 '16 at 07:45

14

You can't "skip" the comparison. What would you expect the sorting code to do? You've got to provide it with a result.

Two options are common:

Throw a NullPointerException to indicate that you just don't support comparing null values. That's explicitly an option in the compare documentation
Decide that null comes before everything else, but is equal to itself

The latter implementation would be something like:

public int compare(Entity e1, Entity e2) {
    if (e1 == e2) {
        return 0;
    }
    if (e1 == null) {
        return -1;
    }
    if (e2 == null) {
        return 1;
    }
    Vector2 e1Pos = Mapper.transform.get(e1).position;
    Vector2 e2Pos = Mapper.transform.get(e2).position;
    return ...;
}

answered Oct 01 '16 at 07:45

Jon Skeet

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can you provide description what does 0, -1, 1 means? thank you it helps a lot! – Oct 01 '16 at 08:26
@RonJuneLopez: Did you read the documentation? If so, which part of that description of the return value didn't make sense? It's hard to give a second explanation without knowing what part of the first explanation doesn't make sense for you - and the documentation should **always** be the first place you look. – Jon Skeet Oct 01 '16 at 08:45

score 2 · Answer 2 · answered Oct 01 '16 at 08:38

To elaborate on Jon's answer, and answer Ron's question, one should always look at the spec before deciding what to do. In this case it says "Unlike Comparable, a comparator may optionally permit comparison of null arguments, while maintaining the requirements for an equivalence relation." See the comparator API. It elaborates on what is meant. I can't see any other reasonable solution.

How to handle null compare method arguments in Comparator?

2 Answers2

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