Interface Comparator<T>
 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.
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.
 Since:
 1.2
 See Also:

Method Summary
Modifier and TypeMethodDescriptionint
Compares its two arguments for order.static <T,
U extends Comparable<? super U>>
Comparator<T> 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
Indicates whether some other object is "equal to" this comparator.static <T extends Comparable<? super T>>
Comparator<T> 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> 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 Details

compare
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 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 thatsignum(compare(x, z))==signum(compare(y, z))
for allz
. API Note:
 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."  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
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 thatsignum
(comp1.compare(o1, o2))==signum(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. 
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
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:

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:

thenComparingInt
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:

thenComparingLong
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:

thenComparingDouble
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:

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:

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:

nullsFirst
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
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
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:

comparingLong
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:

comparingDouble
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:
