How to handle nulls when using Java collection sort

Question

When using Collection.sort in Java what should I return when one of the inner objects is null

Example:

Collections.sort(list, new Comparator<MyBean>() {
    public int compare(MyBean o1, MyBean o2) {
      return o2.getDate().compareTo(o1.getDate());
     } 

});

Lets say o2 is not null but o2.getDate() it is, so should I return 1 or -1 or 0 when adding a null validation?

Answer 1

Naturally, it's your choice. Whatever logic you write, it will define sorting rules. So 'should' isn't really the right word here.

If you want null to appear before any other element, something like this could do

public int compare(MyBean o1, MyBean o2) {
    if (o1.getDate() == null) {
        return (o2.getDate() == null) ? 0 : -1;
    }
    if (o2.getDate() == null) {
        return 1;
    }
    return o2.getDate().compareTo(o1.getDate());
} 

Answer 2

In Java 8 you can also use nullsFirst():

Comparator.nullsFirst(Date::compareTo).compare(dateOne, dateTwo);

Or nullsLast():

Comparator.nullsLast(Date::compareTo).compare(dateOne, dateTwo);

These methods will either sort null to the beginning or to the end. There is no wrong or right whether you consider null bigger or smaller than another objects. This is totally up to you, as others stated already.

Answer 3

Depending on whether the object is null, or the content of the object is null.

The object is null:

    import static java.util.Comparator.*;

    List<Data> listOfData = Arrays.asList(
           new Data("foo"),
           null,
           new Data("bar"),
           new Data("nyu"));

    listOfData.sort(nullsFirst(comparing(Data::getValue)));
    listOfData.forEach(System.out::println);
    //OUTPUT:
    // null
    // Data(bar)
    // Data(foo)
    // Data(nyu)

The content of the object is null:

    List<Data> listOfData = Arrays.asList(
           new Data("foo"),
           new Data(null),
           new Data("bar"),
           new Data("nyu"));


    listOfData.sort(nullsFirst(
         comparing(Data::getValue, nullsFirst(naturalOrder()))));

    listOfData.forEach(System.out::println);
    //OUTPUT:
    // Data(null)
    // Data(bar)
    // Data(foo)
    // Data(nyu)