Module java.base

Package java.util

Interface Comparator<T>

Type Parameters:

T - the type of objects that may be compared by this comparator

All Known Implementing Classes:

Collator, RuleBasedCollator

Functional Interface:

This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.

@FunctionalInterface
public interface Comparator<T>

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 or Arrays.sort) to allow precise control over the sort order. Comparators can also be used to control the order of certain data structures (such as sorted sets or sorted maps), or to provide an ordering for collections of objects that don't have a natural ordering.

The ordering imposed by a comparator c on a set of elements S is said to be consistent with equals if and only if c.compare(e1, e2)==0 has the same boolean value as e1.equals(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, suppose one adds two elements a and b such that (a.equals(b) && c.compare(a, b) != 0) to an empty TreeSet with comparator c. The second add operation will return true (and the size of the tree set will increase) because a and b are not equivalent from the tree set's perspective, even though this is contrary to the specification of the Set.add method.

Note: It is generally a good idea for comparators to also 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 imposed ordering that a given comparator c imposes on a given set of objects S is:

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

The quotient for this total order is:

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

It follows immediately from the contract for compare that the quotient is an equivalence relation on S, and that the imposed 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 ordering is the equivalence relation defined by the objects' equals(Object) method(s):

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

Unlike Comparable, a comparator may optionally permit comparison of null arguments, while maintaining the requirements for an equivalence relation.

This interface is a member of the Java Collections Framework.

Since:

1.2

See Also:

Comparable, Serializable

Method Summary

Modifier and Type	Method	Description
int	compare(T o1, T o2)	Compares its two arguments for order.
static <T,U extends Comparable<? super U>> Comparator<T>	comparing(Function<? super T,? extends U> keyExtractor)	Accepts a function that extracts a Comparable sort key from a type T, and returns a Comparator<T> that compares by that sort key.
static <T,U> Comparator<T>	comparing(Function<? super T,? extends U> keyExtractor, Comparator<? super U> keyComparator)	Accepts a function that extracts a sort key from a type T, and returns a Comparator<T> that compares by that sort key using the specified Comparator.
static <T> Comparator<T>	comparingDouble(ToDoubleFunction<? super T> keyExtractor)	Accepts a function that extracts a double sort key from a type T, and returns a Comparator<T> that compares by that sort key.
static <T> Comparator<T>	comparingInt(ToIntFunction<? super T> keyExtractor)	Accepts a function that extracts an int sort key from a type T, and returns a Comparator<T> that compares by that sort key.
static <T> Comparator<T>	comparingLong(ToLongFunction<? super T> keyExtractor)	Accepts a function that extracts a long sort key from a type T, and returns a Comparator<T> that compares by that sort key.
boolean	equals(Object obj)	Indicates whether some other object is "equal to" this comparator.
static <T extends Comparable<? super T>> Comparator<T>	naturalOrder()	Returns a comparator that compares Comparable objects in natural order.
static <T> Comparator<T>	nullsFirst(Comparator<? super T> comparator)	Returns a null-friendly comparator that considers null to be less than non-null.
static <T> Comparator<T>	nullsLast(Comparator<? super T> comparator)	Returns a null-friendly comparator that considers null to be greater than non-null.
default Comparator<T>	reversed()	Returns a comparator that imposes the reverse ordering of this comparator.
static <T extends Comparable<? super T>> Comparator<T>	reverseOrder()	Returns a comparator that imposes the reverse of the natural ordering.
default Comparator<T>	thenComparing(Comparator<? super T> other)	Returns a lexicographic-order comparator with another comparator.
default <U extends Comparable<? super U>> Comparator<T>	thenComparing(Function<? super T,? extends U> keyExtractor)	Returns a lexicographic-order comparator with a function that extracts a Comparable sort key.
default <U> Comparator<T>	thenComparing(Function<? super T,? extends U> keyExtractor, Comparator<? super U> keyComparator)	Returns a lexicographic-order comparator with a function that extracts a key to be compared with the given Comparator.
default Comparator<T>	thenComparingDouble(ToDoubleFunction<? super T> keyExtractor)	Returns a lexicographic-order comparator with a function that extracts a double sort key.
default Comparator<T>	thenComparingInt(ToIntFunction<? super T> keyExtractor)	Returns a lexicographic-order comparator with a function that extracts an int sort key.
default Comparator<T>