How to handle null compare method arguments in Com

问题:

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;

   }

}

回答1:

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 ...;
}

回答2:

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.