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- 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 asCollections.sortorArrays.sort) to allow precise control over the sort order. Comparators can also be used to control the order of certain data structures (such assorted setsorsorted maps), or to provide an ordering for collections of objects that don't have anatural ordering.The ordering imposed by a comparator
con a set of elementsSis said to be consistent with equals if and only ifc.compare(e1, e2)==0has the same boolean value ase1.equals(e2)for everye1ande2inS.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
cis used with elements (or keys) drawn from a setS. If the ordering imposed byconSis 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 ofequals.For example, suppose one adds two elements
aandbsuch that(a.equals(b) && c.compare(a, b) != 0)to an emptyTreeSetwith comparatorc. The secondaddoperation will return true (and the size of the tree set will increase) becauseaandbare not equivalent from the tree set's perspective, even though this is contrary to the specification of theSet.addmethod.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 (likeTreeSet,TreeMap). In order for the data structure to serialize successfully, the comparator (if provided) must implementSerializable.For the mathematically inclined, the relation that defines the imposed ordering that a given comparator
cimposes on a given set of objectsSis:{(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 forcomparethat the quotient is an equivalence relation onS, and that the imposed ordering is a total order onS. When we say that the ordering imposed byconSis 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.
- Since:
- 1.2
- See Also:
Comparable,Serializable
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Method Summary
All Methods Static Methods Instance Methods Abstract Methods Default Methods Modifier and Type Method Description intcompare(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 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.booleanequals(Object obj)Indicates whether some other object is "equal to" this comparator.static <T extends Comparable<? super T>>
Comparator<T>naturalOrder()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>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>>
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.
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Method Detail
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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
sgn(compare(x, y)) == -sgn(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 thatsgn(compare(x, z))==sgn(compare(y, z))for allz.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."In the foregoing description, the notation
sgn(expression)designates the mathematical signum function, which is defined to return one of-1,0, or1according to whether the value of expression is negative, zero, or positive, respectively.- 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.
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equals
boolean equals(Object obj)
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 thatsgn(comp1.compare(o1, o2))==sgn(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.- Overrides:
equalsin classObject- Parameters:
obj- the reference object with which to compare.- Returns:
trueonly if the specified object is also a comparator and it imposes the same ordering as this comparator.- See Also:
Object.equals(Object),Object.hashCode()
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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
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thenComparing
default Comparator<T> thenComparing(Comparator<? super T> other)
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
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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:
comparing(Function, Comparator),thenComparing(Comparator)
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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:
comparing(Function),thenComparing(Comparator)
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thenComparingInt
default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor)
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:
comparingInt(ToIntFunction),thenComparing(Comparator)
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thenComparingLong
default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor)
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:
comparingLong(ToLongFunction),thenComparing(Comparator)
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thenComparingDouble
default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor)
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:
comparingDouble(ToDoubleFunction),thenComparing(Comparator)
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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
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:
Comparable
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naturalOrder
static <T extends Comparable<? super T>> Comparator<T> 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:
Comparable
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nullsFirst
static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator)
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
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nullsLast
static <T> Comparator<T> nullsLast(Comparator<? super T> comparator)
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
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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
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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
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comparingInt
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.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)
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comparingLong
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.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)
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comparingDouble
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.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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