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)}. 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.
| Modifier and Type | Method | Description |
|---|---|---|
int |
compare |
Compares its two arguments for order. |
static <T, |
comparing |
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, |
comparing |
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 |
comparingDouble |
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 |
comparingInt |
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 |
comparingLong |
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 |
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 |
nullsFirst |
Returns a null-friendly comparator that considers null to be less than non-null. |
static <T> Comparator |
nullsLast |
Returns a null-friendly comparator that considers null to be greater than non-null. |
default Comparator |
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 |
thenComparing |
Returns a lexicographic-order comparator with another comparator. |
default <U extends Comparable<? super U>> |
thenComparing |
Returns a lexicographic-order comparator with a function that extracts a Comparable sort key. |
default <U> Comparator |
thenComparing |
Returns a lexicographic-order comparator with a function that extracts a key to be compared with the given Comparator. |
default Comparator |
thenComparingDouble |
Returns a lexicographic-order comparator with a function that extracts a double sort key. |
default Comparator |
thenComparingInt |
Returns a lexicographic-order comparator with a function that extracts an int sort key. |
default Comparator |
thenComparingLong |
Returns a lexicographic-order comparator with a function that extracts a long sort key. |
int compare(T o1, T o2)
The implementor must ensure that signum(compare(x, y)) == -signum(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 signum(compare(x,
z))==signum(compare(y, z)) for all z.
(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 signum(comp1.compare(o1,
o2))==signum(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.
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.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.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.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.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.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.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.static <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 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 nullstatic <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 nullstatic <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 null
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