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}.
{(x, y) such that c.compare(x, y) == 0}.
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)}.
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 |
open static Comparator<T>! |
Accepts a function that extracts a |
open static Comparator<T>! |
comparingDouble(keyExtractor: ToDoubleFunction<in T>!) Accepts a function that extracts a |
open static Comparator<T>! |
comparingInt(keyExtractor: ToIntFunction<in T>!) Accepts a function that extracts an |
open static Comparator<T>! |
comparingLong(keyExtractor: ToLongFunction<in T>!) Accepts a function that extracts a |
open static Comparator<T>! |
Returns a comparator that compares |
open static Comparator<T>! |
nullsFirst(comparator: Comparator<in T>!) Returns a null-friendly comparator that considers |
open static Comparator<T>! |
nullsLast(comparator: Comparator<in T>!) Returns a null-friendly comparator that considers |
open static Comparator<T>! |
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>!, keyComparator: Comparator<in U>!) Returns a lexicographic-order comparator with a function that extracts a key to be compared with the given |
open Comparator<T>! |
thenComparing(keyExtractor: Function<in T, out U>!) Returns a lexicographic-order comparator with a function that extracts a |
open Comparator<T>! |
thenComparingDouble(keyExtractor: ToDoubleFunction<in T>!) Returns a lexicographic-order comparator with a function that extracts a |
open Comparator<T>! |
thenComparingInt(keyExtractor: ToIntFunction<in T>!) Returns a lexicographic-order comparator with a function that extracts an |
open Comparator<T>! |
thenComparingLong(keyExtractor: ToLongFunction<in T>!) Returns a lexicographic-order comparator with a function that extracts a |
Public methods
compare
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
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
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
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 |
See Also
comparingInt
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 |
See Also
comparingLong
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 |
See Also
naturalOrder
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. |
See Also
nullsFirst
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
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
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. |
See Also
reversed
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
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
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
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
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
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
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. |