
 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 asCollections.sort
orArrays.sort
) to allow precise control over the sort order. Comparators can also be used to control the order of certain data structures (such assorted sets
orsorted maps
), or to provide an ordering for collections of objects that don't have anatural ordering
.The ordering imposed by a comparator
c
on a set of elementsS
is said to be consistent with equals if and only ifc.compare(e1, e2)==0
has the same boolean value ase1.equals(e2)
for everye1
ande2
inS
.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 setS
. If the ordering imposed byc
onS
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 ofequals
.For example, suppose one adds two elements
a
andb
such that(a.equals(b) && c.compare(a, b) != 0)
to an emptyTreeSet
with comparatorc
. The secondadd
operation will return true (and the size of the tree set will increase) becausea
andb
are not equivalent from the tree set's perspective, even though this is contrary to the specification of theSet.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 (likeTreeSet
,TreeMap
). In order for the data structure to serialize successfully, the comparator (if provided) must implementSerializable
.For the mathematically inclined, the relation that defines the imposed ordering that a given comparator
c
imposes on a given set of objectsS
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 forcompare
that the quotient is an equivalence relation onS
, and that the imposed ordering is a total order onS
. When we say that the ordering imposed byc
onS
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
All Methods Static Methods Instance Methods Abstract Methods Default Methods 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 aComparable
sort key from a typeT
, and returns aComparator<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 typeT
, and returns aComparator<T>
that compares by that sort key using the specifiedComparator
.static <T> Comparator<T>
comparingDouble(ToDoubleFunction<? super T> keyExtractor)
Accepts a function that extracts adouble
sort key from a typeT
, and returns aComparator<T>
that compares by that sort key.static <T> Comparator<T>
comparingInt(ToIntFunction<? super T> keyExtractor)
Accepts a function that extracts anint
sort key from a typeT
, and returns aComparator<T>
that compares by that sort key.static <T> Comparator<T>
comparingLong(ToLongFunction<? super T> keyExtractor)
Accepts a function that extracts along
sort key from a typeT
, and returns aComparator<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 comparesComparable
objects in natural order.static <T> Comparator<T>
nullsFirst(Comparator<? super T> comparator)
Returns a nullfriendly comparator that considersnull
to be less than nonnull.static <T> Comparator<T>
nullsLast(Comparator<? super T> comparator)
Returns a nullfriendly comparator that considersnull
to be greater than nonnull.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 lexicographicorder comparator with another comparator.default <U extends Comparable<? super U>>
Comparator<T>thenComparing(Function<? super T,? extends U> keyExtractor)
Returns a lexicographicorder comparator with a function that extracts aComparable
sort key.default <U> Comparator<T>
thenComparing(Function<? super T,? extends U> keyExtractor, Comparator<? super U> keyComparator)
Returns a lexicographicorder comparator with a function that extracts a key to be compared with the givenComparator
.default Comparator<T>
thenComparingDouble(ToDoubleFunction<? super T> keyExtractor)
Returns a lexicographicorder comparator with a function that extracts adouble
sort key.default Comparator<T>
thenComparingInt(ToIntFunction<? super T> keyExtractor)
Returns a lexicographicorder comparator with a function that extracts anint
sort key.default Comparator<T>
thenComparingLong(ToLongFunction<? super T> keyExtractor)
Returns a lexicographicorder comparator with a function that extracts along
sort key.



Method Detail

compare
int compare(T o1, T o2)
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
sgn(compare(x, y)) == sgn(compare(y, x))
for allx
andy
. (This implies thatcompare(x, y)
must throw an exception if and only ifcompare(y, x)
throws an exception.)The implementor must also ensure that the relation is transitive:
((compare(x, y)>0) && (compare(y, z)>0))
impliescompare(x, z)>0
.Finally, the implementor must ensure that
compare(x, y)==0
implies thatsgn(compare(x, z))==sgn(compare(y, z))
for allz
.It is generally the case, but not strictly required that
(compare(x, y)==0) == (x.equals(y))
. Generally speaking, any comparator that violates this condition should clearly indicate this fact. The recommended language is "Note: this comparator imposes orderings that are inconsistent with equals."In the foregoing description, the notation
sgn(
expression)
designates the mathematical signum function, which is defined to return one of1
,0
, or1
according to whether the value of expression is negative, zero, or positive, respectively. Parameters:
o1
 the first object to be compared.o2
 the second object to be compared. Returns:
 a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.
 Throws:
NullPointerException
 if an argument is null and this comparator does not permit null argumentsClassCastException
 if the arguments' types prevent them from being compared by this comparator.

equals
boolean equals(Object obj)
Indicates whether some other object is "equal to" this comparator. This method must obey the general contract ofObject.equals(Object)
. Additionally, this method can returntrue
only if the specified object is also a comparator and it imposes the same ordering as this comparator. Thus,comp1.equals(comp2)
implies thatsgn(comp1.compare(o1, o2))==sgn(comp2.compare(o1, o2))
for every object referenceo1
ando2
.Note that it is always safe not to override
Object.equals(Object)
. However, overriding this method may, in some cases, improve performance by allowing programs to determine that two distinct comparators impose the same order. Overrides:
equals
in classObject
 Parameters:
obj
 the reference object with which to compare. Returns:
true
only if the specified object is also a comparator and it imposes the same ordering as this comparator. See Also:
Object.equals(Object)
,Object.hashCode()

reversed
default Comparator<T> reversed()
Returns a comparator that imposes the reverse ordering of this comparator. Returns:
 a comparator that imposes the reverse ordering of this comparator.
 Since:
 1.8

thenComparing
default Comparator<T> thenComparing(Comparator<? super T> other)
Returns a lexicographicorder comparator with another comparator. If thisComparator
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.
 API Note:
 For example, to sort a collection of
String
based on the length and then caseinsensitive natural ordering, the comparator can be composed using following code,Comparator<String> cmp = Comparator.comparingInt(String::length) .thenComparing(String.CASE_INSENSITIVE_ORDER);
 Parameters:
other
 the other comparator to be used when this comparator compares two objects that are equal. Returns:
 a lexicographicorder comparator composed of this and then the other comparator
 Throws:
NullPointerException
 if the argument is null. Since:
 1.8

thenComparing
default <U> Comparator<T> thenComparing(Function<? super T,? extends U> keyExtractor, Comparator<? super U> keyComparator)
Returns a lexicographicorder comparator with a function that extracts a key to be compared with the givenComparator
. Implementation Requirements:
 This default implementation behaves as if
thenComparing(comparing(keyExtractor, cmp))
.  Type Parameters:
U
 the type of the sort key Parameters:
keyExtractor
 the function used to extract the sort keykeyComparator
 theComparator
used to compare the sort key Returns:
 a lexicographicorder comparator composed of this comparator and then comparing on the key extracted by the keyExtractor function
 Throws:
NullPointerException
 if either argument is null. Since:
 1.8
 See Also:
comparing(Function, Comparator)
,thenComparing(Comparator)

thenComparing
default <U extends Comparable<? super U>> Comparator<T> thenComparing(Function<? super T,? extends U> keyExtractor)
Returns a lexicographicorder comparator with a function that extracts aComparable
sort key. Implementation Requirements:
 This default implementation behaves as if
thenComparing(comparing(keyExtractor))
.  Type Parameters:
U
 the type of theComparable
sort key Parameters:
keyExtractor
 the function used to extract theComparable
sort key Returns:
 a lexicographicorder comparator composed of this and then the
Comparable
sort key.  Throws:
NullPointerException
 if the argument is null. Since:
 1.8
 See Also:
comparing(Function)
,thenComparing(Comparator)

thenComparingInt
default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor)
Returns a lexicographicorder comparator with a function that extracts anint
sort key. Implementation Requirements:
 This default implementation behaves as if
thenComparing(comparingInt(keyExtractor))
.  Parameters:
keyExtractor
 the function used to extract the integer sort key Returns:
 a lexicographicorder comparator composed of this and then the
int
sort key  Throws:
NullPointerException
 if the argument is null. Since:
 1.8
 See Also:
comparingInt(ToIntFunction)
,thenComparing(Comparator)

thenComparingLong
default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor)
Returns a lexicographicorder comparator with a function that extracts along
sort key. Implementation Requirements:
 This default implementation behaves as if
thenComparing(comparingLong(keyExtractor))
.  Parameters:
keyExtractor
 the function used to extract the long sort key Returns:
 a lexicographicorder comparator composed of this and then the
long
sort key  Throws:
NullPointerException
 if the argument is null. Since:
 1.8
 See Also:
comparingLong(ToLongFunction)
,thenComparing(Comparator)

thenComparingDouble
default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor)
Returns a lexicographicorder comparator with a function that extracts adouble
sort key. Implementation Requirements:
 This default implementation behaves as if
thenComparing(comparingDouble(keyExtractor))
.  Parameters:
keyExtractor
 the function used to extract the double sort key Returns:
 a lexicographicorder comparator composed of this and then the
double
sort key  Throws:
NullPointerException
 if the argument is null. Since:
 1.8
 See Also:
comparingDouble(ToDoubleFunction)
,thenComparing(Comparator)

reverseOrder
static <T extends Comparable<? super T>> Comparator<T> reverseOrder()
Returns a comparator that imposes the reverse of the natural ordering.The returned comparator is serializable and throws
NullPointerException
when comparingnull
. Type Parameters:
T
 theComparable
type of element to be compared Returns:
 a comparator that imposes the reverse of the natural
ordering on
Comparable
objects.  Since:
 1.8
 See Also:
Comparable

naturalOrder
static <T extends Comparable<? super T>> Comparator<T> naturalOrder()
Returns a comparator that comparesComparable
objects in natural order.The returned comparator is serializable and throws
NullPointerException
when comparingnull
. Type Parameters:
T
 theComparable
type of element to be compared Returns:
 a comparator that imposes the natural ordering on
Comparable
objects.  Since:
 1.8
 See Also:
Comparable

nullsFirst
static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator)
Returns a nullfriendly comparator that considersnull
to be less than nonnull. When both arenull
, they are considered equal. If both are nonnull, the specifiedComparator
is used to determine the order. If the specified comparator isnull
, then the returned comparator considers all nonnull values to be equal.The returned comparator is serializable if the specified comparator is serializable.
 Type Parameters:
T
 the type of the elements to be compared Parameters:
comparator
 aComparator
for comparing nonnull values Returns:
 a comparator that considers
null
to be less than nonnull, and compares nonnull objects with the suppliedComparator
.  Since:
 1.8

nullsLast
static <T> Comparator<T> nullsLast(Comparator<? super T> comparator)
Returns a nullfriendly comparator that considersnull
to be greater than nonnull. When both arenull
, they are considered equal. If both are nonnull, the specifiedComparator
is used to determine the order. If the specified comparator isnull
, then the returned comparator considers all nonnull values to be equal.The returned comparator is serializable if the specified comparator is serializable.
 Type Parameters:
T
 the type of the elements to be compared Parameters:
comparator
 aComparator
for comparing nonnull values Returns:
 a comparator that considers
null
to be greater than nonnull, and compares nonnull objects with the suppliedComparator
.  Since:
 1.8

comparing
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 typeT
, and returns aComparator<T>
that compares by that sort key using the specifiedComparator
.The returned comparator is serializable if the specified function and comparator are both serializable.
 API Note:
 For example, to obtain a
Comparator
that comparesPerson
objects by their last name ignoring case differences,Comparator<Person> cmp = Comparator.comparing( Person::getLastName, String.CASE_INSENSITIVE_ORDER);
 Type Parameters:
T
 the type of element to be comparedU
 the type of the sort key Parameters:
keyExtractor
 the function used to extract the sort keykeyComparator
 theComparator
used to compare the sort key Returns:
 a comparator that compares by an extracted key using the
specified
Comparator
 Throws:
NullPointerException
 if either argument is null Since:
 1.8

comparing
static <T,U extends Comparable<? super U>> Comparator<T> comparing(Function<? super T,? extends U> keyExtractor)
Accepts a function that extracts aComparable
sort key from a typeT
, and returns aComparator<T>
that compares by that sort key.The returned comparator is serializable if the specified function is also serializable.
 API Note:
 For example, to obtain a
Comparator
that comparesPerson
objects by their last name,Comparator<Person> byLastName = Comparator.comparing(Person::getLastName);
 Type Parameters:
T
 the type of element to be comparedU
 the type of theComparable
sort key Parameters:
keyExtractor
 the function used to extract theComparable
sort key Returns:
 a comparator that compares by an extracted key
 Throws:
NullPointerException
 if the argument is null Since:
 1.8

comparingInt
static <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor)
Accepts a function that extracts anint
sort key from a typeT
, and returns aComparator<T>
that compares by that sort key.The returned comparator is serializable if the specified function is also serializable.
 Type Parameters:
T
 the type of element to be compared Parameters:
keyExtractor
 the function used to extract the integer sort key Returns:
 a comparator that compares by an extracted key
 Throws:
NullPointerException
 if the argument is null Since:
 1.8
 See Also:
comparing(Function)

comparingLong
static <T> Comparator<T> comparingLong(ToLongFunction<? super T> keyExtractor)
Accepts a function that extracts along
sort key from a typeT
, and returns aComparator<T>
that compares by that sort key.The returned comparator is serializable if the specified function is also serializable.
 Type Parameters:
T
 the type of element to be compared Parameters:
keyExtractor
 the function used to extract the long sort key Returns:
 a comparator that compares by an extracted key
 Throws:
NullPointerException
 if the argument is null Since:
 1.8
 See Also:
comparing(Function)

comparingDouble
static <T> Comparator<T> comparingDouble(ToDoubleFunction<? super T> keyExtractor)
Accepts a function that extracts adouble
sort key from a typeT
, and returns aComparator<T>
that compares by that sort key.The returned comparator is serializable if the specified function is also serializable.
 Type Parameters:
T
 the type of element to be compared Parameters:
keyExtractor
 the function used to extract the double sort key Returns:
 a comparator that compares by an extracted key
 Throws:
NullPointerException
 if the argument is null Since:
 1.8
 See Also:
comparing(Function)

