T
- the type of objects that may be compared by this comparator@FunctionalInterface public static interface Remote.Comparator<T> extends Comparator<T>, Serializable
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.
Modifier and Type | Method and Description |
---|---|
static <T,U extends Comparable<? super U>> |
comparing(Remote.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> Remote.Comparator<T> |
comparing(Remote.Function<? super T,? extends U> keyExtractor,
Remote.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 Remote.Comparator . |
static <T> Remote.Comparator<T> |
comparingDouble(Remote.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> Remote.Comparator<T> |
comparingInt(Remote.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> Remote.Comparator<T> |
comparingLong(Remote.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. |
static <T extends Comparable<? super T>> |
naturalOrder()
Returns a comparator that compares
Comparable objects in
natural order. |
static <T> Remote.Comparator<T> |
nullsFirst(Remote.Comparator<T> comparator)
Returns a null-friendly comparator that considers
null to be
less than non-null. |
static <T> Remote.Comparator<T> |
nullsLast(Remote.Comparator<T> comparator)
Returns a null-friendly comparator that considers
null to be
greater than non-null. |
default Remote.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 Remote.Comparator<T> |
thenComparing(Remote.Comparator<? super T> other)
Returns a lexicographic-order comparator with another comparator.
|
default <U extends Comparable<? super U>> |
thenComparing(Remote.Function<? super T,? extends U> keyExtractor)
Returns a lexicographic-order comparator with a function that
extracts a
Comparable sort key. |
default <U> Remote.Comparator<T> |
thenComparing(Remote.Function<? super T,? extends U> keyExtractor,
Remote.Comparator<? super U> keyComparator)
Returns a lexicographic-order comparator with a function that
extracts a key to be compared with the given
Comparator . |
default Remote.Comparator<T> |
thenComparingDouble(Remote.ToDoubleFunction<? super T> keyExtractor)
Returns a lexicographic-order comparator with a function that
extracts a
double sort key. |
default Remote.Comparator<T> |
thenComparingInt(Remote.ToIntFunction<? super T> keyExtractor)
Returns a lexicographic-order comparator with a function that
extracts a
int sort key. |
default Remote.Comparator<T> |
thenComparingLong(Remote.ToLongFunction<? super T> keyExtractor)
Returns a lexicographic-order comparator with a function that
extracts a
long sort key. |
compare, comparing, comparing, comparingDouble, comparingInt, comparingLong, equals, nullsFirst, nullsLast, thenComparing, thenComparing, thenComparing, thenComparingDouble, thenComparingInt, thenComparingLong
default Remote.Comparator<T> reversed()
reversed
in interface Comparator<T>
default Remote.Comparator<T> thenComparing(Remote.Comparator<? super T> other)
Comparator
considers two elements equal, i.e. compare(a, b) == 0
, other
is used to determine the order.other
- the other comparator to be used when this comparator
compares two objects that are equal.NullPointerException
- if the argument is null.String
based on
the length and then case-insensitive natural ordering, the comparator
can be composed using following code,
Comparator<String> cmp = Comparator.comparingInt(String::length)
.thenComparing(String.CASE_INSENSITIVE_ORDER);
default <U> Remote.Comparator<T> thenComparing(Remote.Function<? super T,? extends U> keyExtractor, Remote.Comparator<? super U> keyComparator)
Comparator
.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.Comparator.comparing(Function, Comparator)
,
Comparator.thenComparing(Comparator)
thenComparing(comparing(keyExtractor, cmp))
.default <U extends Comparable<? super U>> Remote.Comparator<T> thenComparing(Remote.Function<? super T,? extends U> keyExtractor)
Comparable
sort key.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.Comparator.comparing(Function)
,
Comparator.thenComparing(Comparator)
thenComparing(comparing(keyExtractor))
.default Remote.Comparator<T> thenComparingInt(Remote.ToIntFunction<? super T> keyExtractor)
int
sort key.keyExtractor
- the function used to extract the integer sort
keyint
sort keyNullPointerException
- if the argument is null.Comparator.comparingInt(ToIntFunction)
,
Comparator.thenComparing(Comparator)
thenComparing(comparingInt(keyExtractor))
.default Remote.Comparator<T> thenComparingLong(Remote.ToLongFunction<? super T> keyExtractor)
long
sort key.keyExtractor
- the function used to extract the long sort keylong
sort keyNullPointerException
- if the argument is null.Comparator.comparingLong(ToLongFunction)
,
Comparator.thenComparing(Comparator)
thenComparing(comparingLong(keyExtractor))
.default Remote.Comparator<T> thenComparingDouble(Remote.ToDoubleFunction<? super T> keyExtractor)
double
sort key.keyExtractor
- the function used to extract the double sort keydouble
sort keyNullPointerException
- if the argument is null.Comparator.comparingDouble(ToDoubleFunction)
,
Comparator.thenComparing(Comparator)
thenComparing(comparingDouble(keyExtractor))
.static <T extends Comparable<? super T>> Remote.Comparator<T> reverseOrder()
reverseOrder
in interface Comparator<T>
T
- the Comparable
type of element to be comparedComparable
objects.Comparable
static <T extends Comparable<? super T>> Remote.Comparator<T> naturalOrder()
Comparable
objects in
natural order.naturalOrder
in interface Comparator<T>
T
- the Comparable
type of element to be comparedComparable
objects.Comparable
static <T> Remote.Comparator<T> nullsFirst(Remote.Comparator<T> comparator)
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.T
- the type of the elements to be comparedcomparator
- a Comparator
for comparing non-null valuesnull
to be less than
non-null, and compares non-null objects with the supplied Comparator
.static <T> Remote.Comparator<T> nullsLast(Remote.Comparator<T> comparator)
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.T
- the type of the elements to be comparedcomparator
- a Comparator
for comparing non-null valuesnull
to be greater than
non-null, and compares non-null objects with the supplied Comparator
.static <T,U> Remote.Comparator<T> comparing(Remote.Function<? super T,? extends U> keyExtractor, Remote.Comparator<? super U> keyComparator)
T
,
and returns a Comparator<T>
that compares by that sort key
using the specified Remote.Comparator
.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 nullComparator
that compares
Person
objects by their last name ignoring case differences,
Comparator<Person> cmp = Comparator.comparing(
Person::getLastName,
String.CASE_INSENSITIVE_ORDER);
static <T,U extends Comparable<? super U>> Remote.Comparator<T> comparing(Remote.Function<? super T,? extends U> keyExtractor)
Comparable
sort key from a type T
, and returns a Comparator<T>
that compares by that sort key.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 nullComparator
that compares
Person
objects by their last name,
Comparator<Person> byLastName = Comparator.comparing(Person::getLastName);
static <T> Remote.Comparator<T> comparingInt(Remote.ToIntFunction<? super T> keyExtractor)
int
sort key from a type
T
, and returns a Comparator<T>
that compares by that
sort key.T
- the type of element to be comparedkeyExtractor
- the function used to extract the integer sort
keyNullPointerException
- if the argument is nullComparator.comparing(Function)
static <T> Remote.Comparator<T> comparingLong(Remote.ToLongFunction<? super T> keyExtractor)
long
sort key from a type
T
, and returns a Comparator<T>
that compares by that
sort key.T
- the type of element to be comparedkeyExtractor
- the function used to extract the long sort keyNullPointerException
- if the argument is nullComparator.comparing(Function)
static <T> Remote.Comparator<T> comparingDouble(Remote.ToDoubleFunction<? super T> keyExtractor)
double
sort key from a
type T
, and returns a Comparator<T>
that compares by
that sort key.T
- the type of element to be comparedkeyExtractor
- the function used to extract the double sort keyNullPointerException
- if the argument is nullComparator.comparing(Function)