Java in a Nutshell, 5th Edition

Name

Comparator<T>

Synopsis

This interface defines a compare( ) method that specifies a total ordering for a set of objects, allowing those objects to be sorted. The Comparator is used when the objects to be ordered do not have a natural ordering defined by the Comparable interface, or when you want to order them using something other than their natural ordering. Comparator has been made generic in Java 5.0 and the type variable T represents the type of objects being compared.

The compare( ) method is passed two objects. If the first argument is less than the second argument or should be placed before the second argument in a sorted list, compare( ) should return a negative integer. If the first argument is greater than the second argument or should be placed after the second argument in a sorted list, compare( ) should return a positive integer. If the two objects are equivalent or if their relative position in a sorted list does not matter, compare( ) should return 0. Comparator implementations may assume that both Object arguments are of appropriate types and cast them as desired. If either argument is not of the expected type, the compare( ) method throws a ClassCastException.

Note that the magnitude of the numbers returned by compare( ) does not matter, only whether they are less than, equal to, or greater than zero. In most cases, you should implement a Comparator so that compare(o1,o2) returns 0 if and only if o1.equals(o2) returns true. This is particularly important when ...