What should int compareTo() return when the parameter string is null?

Question

It is said that when input parameter is null, compareTo() should throw a NullPointerException. However, I am implementing a class which needs to compare fields with the type of String. These fields need not to be mandatory. I wonder in this case,

1) What should I return when the input is null? Should any not-null strings lexicographically bigger or smaller than null?

and

2) If this is considered bad practice, is there any supporting arguments? Should I force the user to use empty strings instead? If using empty string, won't that confuse the case in which the field is not applicable and the case in which the field is empty? And if exception must be thrown, then except from warning the user in the manual, what else could/shall I do?

EDIT: I might not express myself clearly here, but in the program I am implementing, the strings that could be null are all fields or a class, which should not be null. In other words, the objects comparedTo() uses could not be null, just their private fields could be. So in this case, I believe if I implement compareTo() properly, it would not violate the transitive requirement since classes with null fields would be considered the same always. Am I right or am I interpreting this wrong?

Thank you all for the answers!

Answer 1

From javadoc for Comparable

Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false.

Answer 2

Yes, there is no problem allowing null for instance fields - just make sure its sorting order is defined. Most natural would be putting it either before or after all real strings, but you could do anything here, just do it consistently. (For example, you could sort null like "null".)

Here is an example implementation for a single member:

class Example implements Comparable<Example> {

   @Nullable
   private String member;

   // TODO: getter, setter, constructor, ...

   public int compareTo(Example that) {
      if(this.member == null)
         if(that.member == null)
            return 0; //equal
         else
            return -1; // null is before other strings
       else // this.member != null
         if(that.member == null)
            return 1;  // all other strings are after null
         else
            return this.member.compareTo(that.member);
   }
}

Please note that the specification of Comparable.compareTo() only has a constraint for o.compareTo(null) (which should behave just like - null.compareTo(o), i.e. throw a NullPointerException), but not about how null fields are handled (it doesn't mention fields at all, so a class could return whatever it wants, as long as the antisymmetry, reflexivity and transitivity is ensured).