You need to sort an object array that contains null elements, and you want to have control over whether the null values are at the beginning or end of a sorted array.

Wrap your Comparator in a NullComparator from Commons Collections. The NullComparator can sort null values higher or lower than non-null values, depending on options passed into the constructor. The following example shows a custom BookComparator wrapped with a NullComparator:

import org.apache.commons.collections.comparators.NullComparator;
import java.util.*;
Comparator bookComparator = new BookComparator( );
               Comparator nullComparator = new NullComparator( BookComparator );
Book[] bookArray = new Book[] { new Book( ), null, new Book( ) };
Arrays.sort( bookArray, nullComparator );

This example sorts an array of Book objects, placing null at the end of the sorted array; after the sort, bookArray contains two Book objects at index zero and index one and a null reference at index two.

To configure NullComparator to sort null values as less than non-null values, pass false to the constructor of NullComparator; to sort null values as greater than non-null values, pass true. By default, null values are sorted higher:

// null is less than non-null
Comparator nullComparator = new NullComparator( bookComparator, false );
// null is greater than non-null (default)
Comparator nullComparator = new NullComparator( bookComparator, true );

While the NullComparator usually decorates another instance of Comparator, the NullComparator can also be used by itself to compare null and non-null objects, as in the following example:

Comparator nullHighComparator = new NullComparator( );
Comparator nullLowComparator = new NullComparator(false);
// Returns 1
nullHighComparator.compare( null, new Double(3.0) );
// Returns -1
nullLowComparator.compare( null, new Double(3.0) );
// Returns 0
nullLowComparator.compare( null, null );

Both ReverseComparator and NullComparator are objects that decorate an existing Comparator. Take note of the decorator pattern as it is a common pattern used throughout Commons Collections. For more information about the decorator design pattern, read Design Patterns: Elements of Reusable Object-Oriented Software (Addison Wesley).