What to do with null fields in compare()?

Question

In Java, I use a class in which some fields can be null. For example:

class Foo {     String bar;     //.... } 

I want to write a BarComparator for this class,

    private static class BarComparator             implements Comparator<Foo> {         public int compare( final Foo o1, final Foo o2 )         {             // Implementation goes here         }     } 

Is there a standard way to deal with the fact that any of o1, o2, o1.bar, o2.bar can be null, without writing lots of nested if...else?

Cheers!

Answer 1

I guess you could wrap the call to the field compareTo method with a small static method to sort nulls high or low:

static <T extends Comparable<T>> int cp(T a, T b) {      return          a==null ?          (b==null ? 0 : Integer.MIN_VALUE) :          (b==null ? Integer.MAX_VALUE : a.compareTo(b)); } 

Simple usage (multiple fields is as you would normally):

public int compare( final Foo o1, final Foo o2 ) {     return cp(o1.field, o2.field); } 

Answer 2

Thanks for the replies! The generic method and the Google Comparators look interesting.

And I found that there's a NullComparator in the Apache Commons Collections (which we're currently using):

private static class BarComparator         implements Comparator<Foo> {     public int compare( final Foo o1, final Foo o2 )     {         // o1.bar & o2.bar nulleness is taken care of by the NullComparator.         // Easy to extend to more fields.         return NULL_COMPARATOR.compare(o1.bar, o2.bar);     }      private final static NullComparator NULL_COMPARATOR =                                             new NullComparator(false); } 

Note: I focused on the bar field here to keep it to the point.