Added in API level 1

Comparator

@FunctionalInterface interface Comparator<T : Any!>
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 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>!, 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>!
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>!
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>!

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>!

Returns a comparator that imposes the reverse of the natural ordering.

open Comparator<T>!

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>!, 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>!
thenComparing(keyExtractor: Function<in T, out U>!)

Returns a lexicographic-order comparator with a function that extracts a Comparable sort key.

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.

comparing

Added in API level 24
open static fun <T : Any!, U : Any!> comparing(
    keyExtractor: Function<in T, out U>!,
    keyComparator: Comparator<in U>!
): Comparator<T>!

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.

The returned comparator is serializable if the specified function and comparator are both serializable.

Parameters
<T> the type of element to be compared
<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
Return
Comparator<T>! a comparator that compares by an extracted key using the specified Comparator
Exceptions
java.lang.NullPointerException if either argument is null

comparing

Added in API level 24
open static fun <T : Any!, U : Comparable<U>!> comparing(keyExtractor: Function<in T, out U>!): Comparator<T>!

Accepts a function that extracts a Comparable sort key from a type T, and returns a Comparator<T> that compares by that sort key.

The returned comparator is serializable if the specified function is also serializable.

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
Return
Comparator<T>! a comparator that compares by an extracted key
Exceptions
java.lang.NullPointerException if the argument is null

comparingDouble

Added in API level 24
open static fun <T : Any!> comparingDouble(keyExtractor: ToDoubleFunction<in T>!): Comparator<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.

The returned comparator is serializable if the specified function is also serializable.

Parameters
<T> the type of element to be compared
keyExtractor ToDoubleFunction<in T>!: the function used to extract the double sort key
Return
Comparator<T>! a comparator that compares by an extracted key
Exceptions
java.lang.NullPointerException if the argument is null

comparingInt

Added in API level 24
open static fun <T : Any!> comparingInt(keyExtractor: ToIntFunction<in T>!): Comparator<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.

The returned comparator is serializable if the specified function is also serializable.

Parameters
<T> the type of element to be compared
keyExtractor ToIntFunction<in T>!: the function used to extract the integer sort key
Return
Comparator<T>! a comparator that compares by an extracted key
Exceptions
java.lang.NullPointerException if the argument is null

comparingLong

Added in API level 24
open static fun <T : Any!> comparingLong(keyExtractor: ToLongFunction<in T>!): Comparator<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.

The returned comparator is serializable if the specified function is also serializable.

Parameters
<T> the type of element to be compared
keyExtractor ToLongFunction<in T>!: the function used to extract the long sort key
Return
Comparator<T>! a comparator that compares by an extracted key
Exceptions
java.lang.NullPointerException if the argument is null

naturalOrder

Added in API level 24
open static fun <T : Comparable<T>!> naturalOrder(): Comparator<T>!

Returns a comparator that compares Comparable objects in natural order.

The returned comparator is serializable and throws NullPointerException when comparing null.

Parameters
<T> the Comparable type of element to be compared
Return
Comparator<T>! a comparator that imposes the natural ordering on Comparable objects.

nullsFirst

Added in API level 24
open static fun <T : Any!> nullsFirst(comparator: Comparator<in T>!): Comparator<T>!

Returns a null-friendly comparator that considers null to be less than non-null. When both are null, they are considered equal. If both are non-null, the specified Comparator is used to determine the order. If the specified comparator is null, then the returned comparator considers all non-null values to be equal.

The returned comparator is serializable if the specified comparator is serializable.

Parameters
<T> the type of the elements to be compared
comparator Comparator<in T>!: a Comparator for comparing non-null values
Return
Comparator<T>! a comparator that considers null to be less than non-null, and compares non-null objects with the supplied Comparator.

nullsLast

Added in API level 24
open static fun <T : Any!> nullsLast(comparator: Comparator<in T>!): Comparator<T>!

Returns a null-friendly comparator that considers null to be greater than non-null. When both are null, they are considered equal. If both are non-null, the specified Comparator is used to determine the order. If the specified comparator is null, then the returned comparator considers all non-null values to be equal.

The returned comparator is serializable if the specified comparator is serializable.

Parameters
<T> the type of the elements to be compared
comparator Comparator<in T>!: a Comparator for comparing non-null values
Return
Comparator<T>! a comparator that considers null to be greater than non-null, and compares non-null objects with the supplied Comparator.

reverseOrder

Added in API level 24
open static fun <T : Comparable<T>!> reverseOrder(): Comparator<T>!

Returns a comparator that imposes the reverse of the natural ordering.

The returned comparator is serializable and throws NullPointerException when comparing null.

Parameters
<T> the Comparable type of element to be compared
Return
Comparator<T>! a comparator that imposes the reverse of the natural ordering on Comparable objects.

reversed

Added in API level 24
open fun reversed(): Comparator<T>!

Returns a comparator that imposes the reverse ordering of this comparator.

Return
Comparator<T>! a comparator that imposes the reverse ordering of this comparator.

thenComparing

Added in API level 24
open fun thenComparing(other: Comparator<in T>!): Comparator<T>!

Returns a lexicographic-order comparator with another comparator. If this Comparator considers two elements equal, i.e. compare(a, b) == 0, other is used to determine the order.

The returned comparator is serializable if the specified comparator is also serializable.

Parameters
other Comparator<in T>!: the other comparator to be used when this comparator compares two objects that are equal.
Return
Comparator<T>! a lexicographic-order comparator composed of this and then the other comparator
Exceptions
java.lang.NullPointerException if the argument is null.

thenComparing

Added in API level 24
open fun <U : Any!> thenComparing(
    keyExtractor: Function<in T, out U>!,
    keyComparator: Comparator<in U>!
): Comparator<T>!

Returns a lexicographic-order comparator with a function that extracts a key to be compared with the given Comparator.

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
Return
Comparator<T>! a lexicographic-order comparator composed of this comparator and then comparing on the key extracted by the keyExtractor function
Exceptions
java.lang.NullPointerException if either argument is null.

thenComparing

Added in API level 24
open fun <U : Comparable<U>!> thenComparing(keyExtractor: Function<in T, out U>!): Comparator<T>!

Returns a lexicographic-order comparator with a function that extracts a Comparable sort key.

Parameters
<U> the type of the Comparable sort key
keyExtractor Function<in T, out U>!: the function used to extract the Comparable sort key
Return
Comparator<T>! a lexicographic-order comparator composed of this and then the Comparable sort key.
Exceptions
java.lang.NullPointerException if the argument is null.

thenComparingDouble

Added in API level 24
open fun thenComparingDouble(keyExtractor: ToDoubleFunction<in T>!): Comparator<T>!

Returns a lexicographic-order comparator with a function that extracts a double sort key.

Parameters
keyExtractor ToDoubleFunction<in T>!: the function used to extract the double sort key
Return
Comparator<T>! a lexicographic-order comparator composed of this and then the double sort key
Exceptions
java.lang.NullPointerException if the argument is null.

thenComparingInt

Added in API level 24
open fun thenComparingInt(keyExtractor: ToIntFunction<in T>!): Comparator<T>!

Returns a lexicographic-order comparator with a function that extracts an int sort key.

Parameters
keyExtractor ToIntFunction<in T>!: the function used to extract the integer sort key
Return
Comparator<T>! a lexicographic-order comparator composed of this and then the int sort key
Exceptions
java.lang.NullPointerException if the argument is null.

thenComparingLong

Added in API level 24
open fun thenComparingLong(keyExtractor: ToLongFunction<in T>!): Comparator<T>!

Returns a lexicographic-order comparator with a function that extracts a long sort key.

Parameters
keyExtractor ToLongFunction<in T>!: the function used to extract the long sort key
Return
Comparator<T>! a lexicographic-order comparator composed of this and then the long sort key
Exceptions
java.lang.NullPointerException if the argument is null.