- 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 TypeMethodDescriptionintCompares its two arguments for order.static <T,U extends Comparable<? super U>>
Comparator<T> Accepts a function that extracts aComparablesort 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 adoublesort 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 anintsort 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 alongsort key from a typeT, and returns aComparator<T>that compares by that sort key.booleanIndicates whether some other object is "equal to" this comparator.static <T extends Comparable<? super T>>
Comparator<T> Returns a comparator that comparesComparableobjects in natural order.static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator) Returns a null-friendly comparator that considersnullto be less than non-null.static <T> Comparator<T> nullsLast(Comparator<? super T> comparator) Returns a null-friendly comparator that considersnullto 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>>
Comparator<T> 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>>
Comparator<T> thenComparing(Function<? super T, ? extends U> keyExtractor) Returns a lexicographic-order comparator with a function that extracts aComparablesort 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 givenComparator.default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor) Returns a lexicographic-order comparator with a function that extracts adoublesort key.default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor) Returns a lexicographic-order comparator with a function that extracts anintsort key.default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor) Returns a lexicographic-order comparator with a function that extracts alongsort 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 allxandy. (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)==0implies 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 returntrueonly 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 referenceo1ando2.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 lexicographic-order comparator with another comparator. If thisComparatorconsiders two elements equal, i.e.compare(a, b) == 0,otheris 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
Stringbased 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); - Parameters:
other- the other comparator to be used when this comparator compares two objects that are equal.- Returns:
- a lexicographic-order 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 lexicographic-order 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- theComparatorused to compare the sort key- Returns:
- a lexicographic-order 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 lexicographic-order comparator with a function that extracts aComparablesort key.- Implementation Requirements:
- This default implementation behaves as if
thenComparing(comparing(keyExtractor)). - Type Parameters:
U- the type of theComparablesort key- Parameters:
keyExtractor- the function used to extract theComparablesort key- Returns:
- a lexicographic-order comparator composed of this and then the
Comparablesort key. - Throws:
NullPointerException- if the argument is null.- Since:
- 1.8
- See Also:
-
thenComparingInt
Returns a lexicographic-order comparator with a function that extracts anintsort 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 lexicographic-order comparator composed of this and then the
intsort key - Throws:
NullPointerException- if the argument is null.- Since:
- 1.8
- See Also:
-
thenComparingLong
Returns a lexicographic-order comparator with a function that extracts alongsort 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 lexicographic-order comparator composed of this and then the
longsort key - Throws:
NullPointerException- if the argument is null.- Since:
- 1.8
- See Also:
-
thenComparingDouble
Returns a lexicographic-order comparator with a function that extracts adoublesort 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 lexicographic-order comparator composed of this and then the
doublesort 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
NullPointerExceptionwhen comparingnull.- Type Parameters:
T- theComparabletype of element to be compared- Returns:
- a comparator that imposes the reverse of the natural
ordering on
Comparableobjects. - Since:
- 1.8
- See Also:
-
naturalOrder
Returns a comparator that comparesComparableobjects in natural order.The returned comparator is serializable and throws
NullPointerExceptionwhen comparingnull.- Type Parameters:
T- theComparabletype of element to be compared- Returns:
- a comparator that imposes the natural ordering on
Comparableobjects. - Since:
- 1.8
- See Also:
-
nullsFirst
Returns a null-friendly comparator that considersnullto be less than non-null. When both arenull, they are considered equal. If both are non-null, the specifiedComparatoris used to determine the order. If the specified comparator isnull, then the returned comparator considers all non-null 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- aComparatorfor comparing non-null values- Returns:
- a comparator that considers
nullto be less than non-null, and compares non-null objects with the suppliedComparator. - Since:
- 1.8
-
nullsLast
Returns a null-friendly comparator that considersnullto be greater than non-null. When both arenull, they are considered equal. If both are non-null, the specifiedComparatoris used to determine the order. If the specified comparator isnull, then the returned comparator considers all non-null 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- aComparatorfor comparing non-null values- Returns:
- a comparator that considers
nullto be greater than non-null, and compares non-null 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
Comparatorthat comparesPersonobjects 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- theComparatorused 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 aComparablesort 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
Comparatorthat comparesPersonobjects by their last name,Comparator<Person> byLastName = Comparator.comparing(Person::getLastName); - Type Parameters:
T- the type of element to be comparedU- the type of theComparablesort key- Parameters:
keyExtractor- the function used to extract theComparablesort 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 anintsort 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 alongsort 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 adoublesort 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:
-