Jakarta Commons Cookbook

4.5. Comparing Nulls

Problem

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.

Solution

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.

Tip

If a Comparator does not handle null values gracefully, wrap it in a NullComparator and avoid a NullPointerException.

Discussion

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 );

See Also

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).