Java Glossary : Comparator

Last updated 2004-06-28 by Roedy Green ©1996-2004 Canadian Mind Products

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Comparator

java.util.Comparator is used to define a special sort order for a class. In contrast java.lang.Comparable is used to define the natural sort order of a class. To implement java.util.Comparator you must write two methods. It typically describes a small delegate object passed to a sort to describe some special sort order. The compare method compares two objects of the class you want to sort. The equal method compares this java.util.Comparator to another to see if they represent the same order. It does not compare two objects of the class you want to sort.

public final int compare (Object o1, Object o2);
public boolean equals (Object obj);

Here is a typical java.util.Comparator.compare routine:

view

If you are using a java.util.Comparator only once, you might implement it as an anonymous inner class like this:

...

Collections.sort( myArrayList, new Comparator()
                     {
                     public int compare( Object a, Object b )
                        {
                        return( (String)a ).compareToIgnoreCase( (String) b );
                        }
                     } );

Here is a very general purpose Comparator for sorting rows of a Table (array of arrays or Vector of arrays) where each element is some sort of Object that implements Comparable.

view

String.CASE_INSENSITIVE_ORDER is a Comparator you can use without writing any code. Unforntunately there is no String.CASE_SENSITIVE_ORDER. The reason is String's Comparable implementation provides it naturally without a Comparator. You can use the following code to sort by natural Comparable order when a Comparator is required.

public class Natural implements Comparator
   {

   /**
   * Compare two objects by their natural orders
   *
   * @param a first object to be compared
   *
   * @param b second object to be compared
   *
   * @return +1 if a>b, 0 if a=b, -1 if a<b
   */
   public final int compare ( Object a, Object b )
      {
      return( (Comparable )a ).compareTo ( b );
      }
   }

Collator ¤ Comparable ¤ compare ¤ signum ¤ sort

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