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Comparator (Java SE 26 & JDK 26)

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Hide sidebar Show sidebar 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.

@FunctionalInterface public interface Comparator<T>

A comparison function, which imposes a total ordering on some collection of objects. Comparators can be passed to a sort method (such as 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

All MethodsStatic MethodsInstance MethodsAbstract MethodsDefault Methods

Modifier and Type

Method

Description

int

compare(T o1, T o2)

Compares its two arguments for order.

static <T, U extends Comparable<? super U>>
Comparator<T>

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.

default 
U

max(U o1, U o2)

Returns the greater of two values according to this comparator.

default 
U

min(U o1, U o2)

Returns the smaller of two values according to this comparator.

static <T extends Comparable<? super T>>
Comparator<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>>
Comparator<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 >
Comparator<T>

thenComparing(Function<? super T, ? extends U> keyExtractor)

Returns a lexicographic-order comparator with a function that extracts a Comparable sort key.

default 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 an 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.

Method Details
- compare
 
 int compare(T o1, T o2)
 
 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 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.
 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 arguments ClassCastException - if the arguments' types prevent them from being compared by this comparator.
- equals
 boolean equals(Object obj)
 
 Indicates whether some other object is "equal to" this comparator. This method must obey the general contract of 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.
 Overrides: equals in class Object Parameters: obj - the reference object with which to compare. Returns: true only if the specified object is also a comparator and it imposes the same ordering as this comparator. See Also:
 - Object.equals(Object)
 - Object.hashCode()
- reversed
 
 default Comparator<T> 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
- max
 
 default U max(U o1, U o2)
 
 Returns the greater of two values according to this comparator. If the arguments are equal with respect to this comparator, the o1 argument is returned.
 Implementation Requirements: This default implementation behaves as if compare(o1, o2) >= 0 ? o1 : o2. Type Parameters: U - the type of the arguments and the result. Parameters: o1 - an argument. o2 - another argument. Returns: the larger of o1 and o2 according to this comparator. Throws: NullPointerException - if an argument is null and this comparator does not permit null arguments ClassCastException - if the arguments' types prevent them from being compared by this comparator. Since: 26
- min
 
 default U min(U o1, U o2)
 
 Returns the smaller of two values according to this comparator. If the arguments are equal with respect to this comparator, the o1 argument is returned.
 Implementation Requirements: This default implementation behaves as if compare(o1, o2) <= 0 ? o1 : o2. Type Parameters: U - the type of the arguments and the result. Parameters: o1 - an argument. o2 - another argument. Returns: the smaller of o1 and o2 according to this comparator. Throws: NullPointerException - if an argument is null and this comparator does not permit null arguments ClassCastException - if the arguments' types prevent them from being compared by this comparator. Since: 26
- thenComparing
 
 default Comparator<T> thenComparing(Comparator<? super T> other)
 
 Returns a lexicographic-order comparator with another comparator. If this 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.
 API Note: For example, to sort a collection of 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); 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 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.
 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 key keyComparator - the Comparator used 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:
 - comparing(Function, Comparator)
 - thenComparing(Comparator)
- thenComparing
 default > Comparator<T> thenComparing(Function<? super T, ? extends U> keyExtractor)
 
 Returns a lexicographic-order comparator with a function that extracts a Comparable sort key.
 Implementation Requirements: This default implementation behaves as if thenComparing(comparing(keyExtractor)). Type Parameters: U - the type of the Comparable sort key Parameters: keyExtractor - the function used to extract the Comparable sort key Returns: a lexicographic-order comparator composed of this and then the Comparable sort key. Throws: NullPointerException - if the argument is null. Since: 1.8 See Also:
 - comparing(Function)
 - thenComparing(Comparator)
- thenComparingInt
 default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor)
 
 Returns a lexicographic-order comparator with a function that extracts an int 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 lexicographic-order comparator composed of this and then the int sort key Throws: NullPointerException - if the argument is null. Since: 1.8 See Also:
 - comparingInt(ToIntFunction)
 - thenComparing(Comparator)
- thenComparingLong
 default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor)
 
 Returns a lexicographic-order comparator with a function that extracts a long 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 lexicographic-order comparator composed of this and then the long sort key Throws: NullPointerException - if the argument is null. Since: 1.8 See Also:
 - comparingLong(ToLongFunction)
 - thenComparing(Comparator)
- thenComparingDouble
 default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor)
 
 Returns a lexicographic-order comparator with a function that extracts a double 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 lexicographic-order comparator composed of this and then the double sort key Throws: NullPointerException - if the argument is null. Since: 1.8 See Also:
 - comparingDouble(ToDoubleFunction)
 - thenComparing(Comparator)
- reverseOrder
 static <T extends Comparable<? super T>> Comparator<T> reverseOrder()
 
 Returns a comparator that imposes the reverse of the natural ordering.
 The returned comparator is serializable and throws NullPointerException when comparing null.
 Type Parameters: T - the Comparable 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:
 - Comparable
- naturalOrder
 static <T extends Comparable<? super T>> Comparator<T> naturalOrder()
 
 Returns a comparator that compares Comparable objects in natural order.
 The returned comparator is serializable and throws NullPointerException when comparing null.
 Type Parameters: T - the Comparable type of element to be compared Returns: a comparator that imposes the natural ordering on Comparable objects. Since: 1.8 See Also:
 - Comparable
- nullsFirst
 
 static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator)
 
 Returns a null-friendly comparator that considers 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.
 Type Parameters: T - the type of the elements to be compared Parameters: comparator - a Comparator for comparing non-null values Returns: a comparator that considers null to be less than non-null, and compares non-null objects with the supplied Comparator. Since: 1.8
- nullsLast
 
 static <T> Comparator<T> nullsLast(Comparator<? super T> comparator)
 
 Returns a null-friendly comparator that considers 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.
 Type Parameters: T - the type of the elements to be compared Parameters: comparator - a Comparator for comparing non-null values Returns: a comparator that considers null to be greater than non-null, and compares non-null objects with the supplied Comparator. 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 type 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.
 API Note: For example, to obtain a Comparator that compares Person 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 compared U - the type of the sort key Parameters: keyExtractor - the function used to extract the sort key keyComparator - the Comparator 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 a 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.
 API Note: For example, to obtain a Comparator that compares Person objects by their last name, Comparator<Person> byLastName = Comparator.comparing(Person::getLastName); Type Parameters: T - the type of element to be compared U - the type of the Comparable sort key Parameters: keyExtractor - the function used to extract the Comparable sort key Returns: a comparator that compares by an extracted key Throws: NullPointerException - if the argument is null Since: 1.8
- comparingInt
 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.
 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:
 - comparing(Function)
- comparingLong
 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.
 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:
 - comparing(Function)
- comparingDouble
 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.
 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:
 - comparing(Function)

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Interface Comparator<T>

Method Summary

Method Details

compare

equals

reversed

max

min

thenComparing

thenComparing

thenComparing

thenComparingInt

thenComparingLong

thenComparingDouble

reverseOrder

naturalOrder

nullsFirst

nullsLast

comparing

comparing

comparingInt

comparingLong

comparingDouble