java.util.Comparator

A comparison function, which imposes a total ordering on some collection of objects. Comparators can be passed to a sort method (such as Collections.sort) to allow precise control over the sort order. Comparators can also be used to control the order of certain data structures (such as TreeSet or TreeMap).

The ordering imposed by a Comparator c on a set of elements S is said to be consistent with equals if and only if (compare((Object)e1, (Object)e2)==0) has the same boolean value as e1.equals((Object)e2) for every e1 and e2 in S.

Caution should be exercised when using a comparator capable of imposing an ordering inconsistent with equals to order a sorted set (or sorted map). Suppose a sorted set (or sorted map) with an explicit Comparator c is used with elements (or keys) drawn from a set S. If the ordering imposed by c on S is inconsistent with equals, the sorted set (or sorted map) will behave "strangely." In particular the sorted set (or sorted map) will violate the general contract for set (or map), which is defined in terms of equals.

For example, if one adds two keys a and b such that (a.equals((Object)b) && c.compare((Object)a, (Object)b) != 0) to a sorted set with comparator c, the second add operation will return false (and the size of the sorted set will not increase) because a and b are equivalent from the sorted set's perspective.

Note: It is generally a good idea for comparators to implement java.io.Serializable, as they may be used as ordering methods in serializable data structures (like TreeSet, TreeMap). In order for the data structure to serialize successfully, the comparator (if provided) must implement Serializable.

For the mathematically inclined, the relation that defines the total order that a given comparator c imposes on a given set of objects S is:

       {(x, y) such that c.compare((Object)x, (Object)y) <= 0}.
 

The quotient for this total order is:

       {(x, y) such that c.compare((Object)x, (Object)y) == 0}.
 

It follows immediately from the contract for compare that the quotient is an equivalence relation on S, and that the natural ordering is a total order on S. When we say that the ordering imposed by c on S is consistent with equals, we mean that the quotient for the natural ordering is the equivalence relation defined by the objects' equals(Object) method(s):

       {(x, y) such that x.equals((Object)y)}.
 

This interface is a member of the Java Collections Framework.