Comparator : Java Glossary

by Roedy Green ©1996-2008 Canadian Mind Products
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Comparator

java.util.Comparator is used to define a special sort order for a class.

How do you remember that java.util. Comparable. compareTo defines a natural order where java.util. Comparator. compare defines an auxiliary order? I think about it this way:

“Does it makes sense to compare two Oranges? If so they are Comparable. I will need a method that compares this Orange Object to (compareTo) another. If I want do define an alternate order, I need to invent some sort of independent machine, a Comparator to do it. The comparison is not inherent to Oranges, so there is no this. Instead, I compare two Orange Objects.”

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 equals 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. Normally you also redefine equals when you implement Comparator even though it is not part of the Comparator interface. There are no generics involved in overriding equals.

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

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

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

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.

String.CASE_INSENSITIVE_ORDER is a Comparator you can use without writing any code. Unfortunately 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.

You can use Collections. reverseOrder() to generate a Comparator that sorts in revese order to the usual Comparable. You can use Collections. reverseOrder( Comparable t ) to generate a Comparator that sorts in reverse order to a given Comparator. If you have a Comparable, you could roll your own Comparator that sorts in descending order like this:

Beware of using subtraction in writing Comparators rather than < > if there is any possibility you will be using numbers large enough to overflow and give the wrong sign comparing them with subtraction.

Comparator Using Generics

You don’t need any casts since the compiler checks that no non-Pair classes ever get inside the ArrayList<Pair>.

case sensitive
Collator
Comparable
compare
generics
interned Strings
signum
sort

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