T
 the type of objects that may be compared by this comparatorCollator
, RuleBasedCollator
@FunctionalInterface public interface Comparator<T>
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.
Comparable
,
Serializable
Modifier and Type  Method  Description 

int 
compare(T o1,
T o2) 
Compares its two arguments for order.

static <T,U extends Comparable<? super U>> 
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>> 
naturalOrder() 
Returns a comparator that compares
Comparable objects in natural
order. 
static <T> Comparator<T> 
nullsFirst(Comparator<? super T> comparator) 
Returns a nullfriendly comparator that considers
null to be
less than nonnull. 
static <T> Comparator<T> 
nullsLast(Comparator<? super T> comparator) 
Returns a nullfriendly comparator that considers
null 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>> 
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>> 
thenComparing(Function<? super T,? extends U> keyExtractor) 
Returns a lexicographicorder 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 lexicographicorder 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 lexicographicorder comparator with a function that
extracts a
double sort key. 
default Comparator<T> 
thenComparingInt(ToIntFunction<? super T> keyExtractor) 
Returns a lexicographicorder comparator with a function that
extracts an
int sort key. 
default Comparator<T> 
thenComparingLong(ToLongFunction<? super T> keyExtractor) 
Returns a lexicographicorder comparator with a function that
extracts a
long sort key. 
int compare(T o1, T o2)
In the foregoing description, the notation
sgn(
expression)
designates the mathematical
signum function, which is defined to return one of 1
,
0
, or 1
according to whether the value of
expression is negative, zero or positive.
The implementor must ensure that sgn(compare(x, y)) ==
sgn(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 sgn(compare(x, z))==sgn(compare(y, z))
for all
z
.
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."
o1
 the first object to be compared.o2
 the second object to be compared.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.boolean equals(Object obj)
Object.equals(Object)
. Additionally, this method can return
true
only if the specified object is also a comparator
and it imposes the same ordering as this comparator. Thus,
comp1.equals(comp2)
implies that sgn(comp1.compare(o1,
o2))==sgn(comp2.compare(o1, o2))
for every object reference
o1
and o2
.
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.
equals
in class Object
obj
 the reference object with which to compare.true
only if the specified object is also
a comparator and it imposes the same ordering as this
comparator.Object.equals(Object)
,
Object.hashCode()
default Comparator<T> reversed()
default Comparator<T> thenComparing(Comparator<? super T> other)
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.
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);
other
 the other comparator to be used when this comparator
compares two objects that are equal.NullPointerException
 if the argument is null.default <U> Comparator<T> thenComparing(Function<? super T,? extends U> keyExtractor, Comparator<? super U> keyComparator)
Comparator
.
thenComparing(comparing(keyExtractor, cmp))
.U
 the type of the sort keykeyExtractor
 the function used to extract the sort keykeyComparator
 the Comparator
used to compare the sort keyNullPointerException
 if either argument is null.comparing(Function, Comparator)
,
thenComparing(Comparator)
default <U extends Comparable<? super U>> Comparator<T> thenComparing(Function<? super T,? extends U> keyExtractor)
Comparable
sort key.
thenComparing(comparing(keyExtractor))
.U
 the type of the Comparable
sort keykeyExtractor
 the function used to extract the Comparable
sort keyComparable
sort key.NullPointerException
 if the argument is null.comparing(Function)
,
thenComparing(Comparator)
default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor)
int
sort key.
thenComparing(comparingInt(keyExtractor))
.keyExtractor
 the function used to extract the integer sort keyint
sort keyNullPointerException
 if the argument is null.comparingInt(ToIntFunction)
,
thenComparing(Comparator)
default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor)
long
sort key.
thenComparing(comparingLong(keyExtractor))
.keyExtractor
 the function used to extract the long sort keylong
sort keyNullPointerException
 if the argument is null.comparingLong(ToLongFunction)
,
thenComparing(Comparator)
default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor)
double
sort key.
thenComparing(comparingDouble(keyExtractor))
.keyExtractor
 the function used to extract the double sort keydouble
sort keyNullPointerException
 if the argument is null.comparingDouble(ToDoubleFunction)
,
thenComparing(Comparator)
static <T extends Comparable<? super T>> Comparator<T> reverseOrder()
The returned comparator is serializable and throws NullPointerException
when comparing null
.
T
 the Comparable
type of element to be comparedComparable
objects.Comparable
static <T extends Comparable<? super T>> Comparator<T> naturalOrder()
Comparable
objects in natural
order.
The returned comparator is serializable and throws NullPointerException
when comparing null
.
T
 the Comparable
type of element to be compared
Comparable
objects.Comparable
static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator)
null
to be
less than nonnull. When both are null
, they are considered
equal. If both are nonnull, the specified Comparator
is used
to determine the order. If the specified comparator is null
,
then the returned comparator considers all nonnull values to be equal.
The returned comparator is serializable if the specified comparator is serializable.
T
 the type of the elements to be comparedcomparator
 a Comparator
for comparing nonnull valuesnull
to be less than
nonnull, and compares nonnull objects with the supplied
Comparator
.static <T> Comparator<T> nullsLast(Comparator<? super T> comparator)
null
to be
greater than nonnull. When both are null
, they are considered
equal. If both are nonnull, the specified Comparator
is used
to determine the order. If the specified comparator is null
,
then the returned comparator considers all nonnull values to be equal.
The returned comparator is serializable if the specified comparator is serializable.
T
 the type of the elements to be comparedcomparator
 a Comparator
for comparing nonnull valuesnull
to be greater than
nonnull, and compares nonnull objects with the supplied
Comparator
.static <T,U> Comparator<T> comparing(Function<? super T,? extends U> keyExtractor, Comparator<? super U> keyComparator)
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.
Comparator
that compares
Person
objects by their last name ignoring case differences,
Comparator<Person> cmp = Comparator.comparing(
Person::getLastName,
String.CASE_INSENSITIVE_ORDER);
T
 the type of element to be comparedU
 the type of the sort keykeyExtractor
 the function used to extract the sort keykeyComparator
 the Comparator
used to compare the sort keyComparator
NullPointerException
 if either argument is nullstatic <T,U extends Comparable<? super U>> Comparator<T> comparing(Function<? super T,? extends U> keyExtractor)
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.
Comparator
that compares
Person
objects by their last name,
Comparator<Person> byLastName = Comparator.comparing(Person::getLastName);
T
 the type of element to be comparedU
 the type of the Comparable
sort keykeyExtractor
 the function used to extract the Comparable
sort keyNullPointerException
 if the argument is nullstatic <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor)
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.
T
 the type of element to be comparedkeyExtractor
 the function used to extract the integer sort keyNullPointerException
 if the argument is nullcomparing(Function)
static <T> Comparator<T> comparingLong(ToLongFunction<? super T> keyExtractor)
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.
T
 the type of element to be comparedkeyExtractor
 the function used to extract the long sort keyNullPointerException
 if the argument is nullcomparing(Function)
static <T> Comparator<T> comparingDouble(ToDoubleFunction<? super T> keyExtractor)
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.
T
 the type of element to be comparedkeyExtractor
 the function used to extract the double sort keyNullPointerException
 if the argument is nullcomparing(Function)
Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developertargeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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