Added in API level 1

Comparator

@FunctionalInterface interface Comparator<T : Any!>

java.util.Comparator

Known Direct Subclasses

ApplicationInfo.DisplayNameComparator, Collator, Collator, PackageItemInfo.DisplayNameComparator, ResolveInfo.DisplayNameComparator

ApplicationInfo.DisplayNameComparator
Collator	[icu enhancement] ICU's replacement for `java.text.Collator`.
Collator	The `Collator` class performs locale-sensitive `String` comparison.
PackageItemInfo.DisplayNameComparator
ResolveInfo.DisplayNameComparator

Known Indirect Subclasses

RuleBasedCollator, RuleBasedCollator

RuleBasedCollator	RuleBasedCollator is a concrete subclass of Collator.
RuleBasedCollator	The class is a concrete subclass of `Collator` that provides a simple, data-driven, table collator.

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)}.

In other words, when the imposed ordering is consistent with equals, the equivalence classes defined by the equivalence relation of the equals method and the equivalence classes defined by the quotient of the compare method are the same.

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.

Summary

Public methods
abstract Int	`compare(o1: T, o2: T)` Compares its two arguments for order.
open static Comparator<T>!	`comparing(keyExtractor: Function<in T, out U>!)` Accepts a function that extracts a `Comparable` sort key from a type `T`, and returns a `Comparator<T>` that compares by that sort key.
open static Comparator<T>!	`comparing(keyExtractor: Function<in T, out U>!, keyComparator: Comparator<in U>!)` 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`.
open static Comparator<T>!	`comparingDouble(keyExtractor: ToDoubleFunction<in T>!)` Accepts a function that extracts a `double` sort key from a type `T`, and returns a `Comparator<T>` that compares by that sort key.
open static Comparator<T>!	`comparingInt(keyExtractor: ToIntFunction<in T>!)` Accepts a function that extracts an `int` sort key from a type `T`, and returns a `Comparator<T>` that compares by that sort key.
open static Comparator<T>!	`comparingLong(keyExtractor: ToLongFunction<in T>!)` Accepts a function that extracts a `long` sort key from a type `T`, and returns a `Comparator<T>` that compares by that sort key.
open static Comparator<T>!	`naturalOrder()` Returns a comparator that compares `Comparable` objects in natural order.
open static Comparator<T>!	`nullsFirst(comparator: Comparator<in T>!)` Returns a null-friendly comparator that considers `null` to be less than non-null.
open static Comparator<T>!	`nullsLast(comparator: Comparator<in T>!)` Returns a null-friendly comparator that considers `null` to be greater than non-null.
open static Comparator<T>!	`reverseOrder()` Returns a comparator that imposes the reverse of the natural ordering.
open Comparator<T>!	`reversed()` Returns a comparator that imposes the reverse ordering of this comparator.
open Comparator<T>!	`thenComparing(other: Comparator<in T>!)` Returns a lexicographic-order comparator with another comparator.
open Comparator<T>!	`thenComparing(keyExtractor: Function<in T, out U>!)` Returns a lexicographic-order comparator with a function that extracts a `Comparable` sort key.
open Comparator<T>!	`thenComparing(keyExtractor: Function<in T, out U>!, keyComparator: Comparator<in U>!)` Returns a lexicographic-order comparator with a function that extracts a key to be compared with the given `Comparator`.
open Comparator<T>!	`thenComparingDouble(keyExtractor: ToDoubleFunction<in T>!)` Returns a lexicographic-order comparator with a function that extracts a `double` sort key.
open Comparator<T>!	`thenComparingInt(keyExtractor: ToIntFunction<in T>!)` Returns a lexicographic-order comparator with a function that extracts an `int` sort key.
open Comparator<T>!	`thenComparingLong(keyExtractor: ToLongFunction<in T>!)` Returns a lexicographic-order comparator with a function that extracts a `long` sort key.

Public methods

compare

Added in API level 1

abstract fun compare(
    o1: T, 
    o2: T
): Int

Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.

The implementor must ensure that signum(compare(x, y)) == -signum(compare(y, x)) for all x and y. (This implies that compare(x, y) must throw an exception if and only if compare(y, x) throws an exception.)

The implementor must also ensure that the relation is transitive: ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0.

Finally, the implementor must ensure that compare(x, y)==0 implies that signum(compare(x, z))==signum(compare(y, z)) for all z.

Parameters
`o1`	T: the first object to be compared.
`o2`	T: the second object to be compared.

Return
`Int`	a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.

Exceptions
`java.lang.NullPointerException`	if an argument is null and this comparator does not permit null arguments
`java.lang.ClassCastException`	if the arguments' types prevent them from being compared by this comparator.

Parameters
`<T>`	the type of element to be compared
`<U>`	the type of the `Comparable` sort key
`keyExtractor`	Function<in T, out U>!: the function used to extract the `Comparable` sort key

Parameters
`<T>`	the type of the elements to be compared
`comparator`	Comparator<in T>!: a `Comparator` for comparing non-null values

Parameters
`<U>`	the type of the sort key
`keyExtractor`	Function<in T, out U>!: the function used to extract the sort key
`keyComparator`	Comparator<in U>!: the `Comparator` used to compare the sort key

Comparator

Summary

Public methods

compare

comparing

comparing

comparingDouble

comparingInt

comparingLong

naturalOrder

nullsFirst

nullsLast

reverseOrder

reversed

thenComparing

thenComparing

thenComparing

thenComparingDouble

thenComparingInt

thenComparingLong