Interface Comparator<T>
- 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)}.
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
Method Summary
Modifier and Type | Method | 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 |
static <T,U> | comparing(Function<? super T,? extends U> keyExtractor,
Comparator<? super U> keyComparator) | Accepts a function that extracts a sort key from a type |
static <T> Comparator<T> | comparingDouble(ToDoubleFunction<? super T> keyExtractor) | Accepts a function that extracts a |
static <T> Comparator<T> | comparingInt(ToIntFunction<? super T> keyExtractor) | Accepts a function that extracts an |
static <T> Comparator<T> | comparingLong(ToLongFunction<? super T> keyExtractor) | Accepts a function that extracts a |
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 |
static <T> Comparator<T> | nullsFirst(Comparator<? super T> comparator) | Returns a null-friendly comparator that considers |
static <T> Comparator<T> | nullsLast(Comparator<? super T> comparator) | Returns a null-friendly comparator that considers |
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 |
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 |
default Comparator<T> | thenComparingDouble(ToDoubleFunction<? super T> keyExtractor) | Returns a lexicographic-order comparator with a function that extracts a |
default Comparator<T> | thenComparingInt(ToIntFunction<? super T> keyExtractor) | Returns a lexicographic-order comparator with a function that extracts an |
default Comparator<T> | thenComparingLong(ToLongFunction<? super T> keyExtractor) | Returns a lexicographic-order comparator with a function that extracts a |
Method Detail
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 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."
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, 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 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 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.
- Overrides:
-
equals
in classObject
- 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
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 <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
.
- 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
- theComparator
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 <U extends Comparable<? super U>> 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 theComparable
sort key - Parameters:
-
keyExtractor
- the function used to extract theComparable
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
- theComparable
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
- theComparable
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
- aComparator
for comparing non-null values - Returns:
- a comparator that considers
null
to be less than non-null, and compares non-null objects with the suppliedComparator
. - 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
- aComparator
for comparing non-null values - Returns:
- a comparator that considers
null
to 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 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 comparesPerson
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
- theComparator
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 comparesPerson
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 theComparable
sort key - Parameters:
-
keyExtractor
- the function used to extract theComparable
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)