T - the type of objects that may be compared by this comparator@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 and 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 null-friendly comparator that considers
null to be
less than non-null. |
static <T> Comparator<T> |
nullsLast(Comparator<? super T> comparator)
Returns a null-friendly comparator that considers
null to be
greater than non-null. |
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 lexicographic-order comparator with another comparator.
|
default <U extends Comparable<? super U>> |
thenComparing(Function<? super T,? extends U> keyExtractor)
Returns a lexicographic-order 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 lexicographic-order 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 lexicographic-order comparator with a function that
extracts a
double sort key. |
default Comparator<T> |
thenComparingInt(ToIntFunction<? super T> keyExtractor)
Returns a lexicographic-order comparator with a function that
extracts a
int sort key. |
default Comparator<T> |
thenComparingLong(ToLongFunction<? super T> keyExtractor)
Returns a lexicographic-order 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 Objectobj - 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 case-insensitive 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.Comparablestatic <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 comparedComparable objects.Comparablestatic <T> Comparator<T> nullsFirst(Comparator<? super 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.
The returned comparator is serializable if the specified comparator is serializable.
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> Comparator<T> nullsLast(Comparator<? super 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.
The returned comparator is serializable if the specified comparator is serializable.
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> 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 keyComparatorNullPointerException - 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
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