public interface Comparator
java.util.Comparator<T> 
Known Indirect Subclasses 
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.
See also:
Public methods 


abstract int 
compare(T o1, T o2) Compares its two arguments for order. 
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 
static <T, U extends Comparable<? super U>> Comparator<T> 
comparing(Function<? super T, ? extends U> keyExtractor) Accepts a function that extracts a 
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 
abstract boolean 
equals(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 compares 
static <T> Comparator<T> 
nullsFirst(Comparator<? super T> comparator) Returns a nullfriendly comparator that considers 
static <T> Comparator<T> 
nullsLast(Comparator<? super T> comparator) Returns a nullfriendly comparator that considers 
static <T extends Comparable<? super T>> Comparator<T> 
reverseOrder() Returns a comparator that imposes the reverse of the natural ordering. 
default Comparator<T> 
reversed() Returns a comparator that imposes the reverse ordering of this comparator. 
default <U extends Comparable<? super U>> Comparator<T> 
thenComparing(Function<? super T, ? extends U> keyExtractor) Returns a lexicographicorder comparator with a function that extracts a 
default <U> Comparator<T> 
thenComparing(Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator) Returns a lexicographicorder comparator with a function that extracts a key to be compared with the given 
default Comparator<T> 
thenComparing(Comparator<? super T> other) Returns a lexicographicorder comparator with another comparator. 
default Comparator<T> 
thenComparingDouble(ToDoubleFunction<? super T> keyExtractor) Returns a lexicographicorder comparator with a function that extracts a 
default Comparator<T> 
thenComparingInt(ToIntFunction<? super T> keyExtractor) Returns a lexicographicorder comparator with a function that extracts a 
default Comparator<T> 
thenComparingLong(ToLongFunction<? super T> keyExtractor) Returns a lexicographicorder comparator with a function that extracts a 
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.
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.
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."
Parameters  

o1 
T : the first object to be compared. 
o2 
T : the second object to be compared. 
Returns  

int 
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. 
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.
Comparator
that compares Person
objects by their last name ignoring case differences, Comparator<Person> cmp = Comparator.comparing(
Person::getLastName,
String.CASE_INSENSITIVE_ORDER);
Parameters  

keyExtractor 
Function : the function used to extract the sort key 
keyComparator 
Comparator : the Comparator used to compare the sort key 
Returns  

Comparator<T> 
a comparator that compares by an extracted key using the specified Comparator 
Throws  

NullPointerException 
if either argument is null 
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.
Comparator
that compares Person
objects by their last name, Comparator<Person> byLastName = Comparator.comparing(Person::getLastName);
Parameters  

keyExtractor 
Function : the function used to extract the Comparable sort key 
Returns  

Comparator<T> 
a comparator that compares by an extracted key 
Throws  

NullPointerException 
if the argument is null 
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.
Parameters  

keyExtractor 
ToDoubleFunction : the function used to extract the double sort key 
Returns  

Comparator<T> 
a comparator that compares by an extracted key 
Throws  

NullPointerException 
if the argument is null 
See also:
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.
Parameters  

keyExtractor 
ToIntFunction : the function used to extract the integer sort key 
Returns  

Comparator<T> 
a comparator that compares by an extracted key 
Throws  

NullPointerException 
if the argument is null 
See also:
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.
Parameters  

keyExtractor 
ToLongFunction : the function used to extract the long sort key 
Returns  

Comparator<T> 
a comparator that compares by an extracted key 
Throws  

NullPointerException 
if the argument is null 
See also:
boolean equals (Object obj)
Indicates whether some other object is "equal to" this comparator. This method must obey the general contract of 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.
Parameters  

obj 
Object : the reference object with which to compare. 
Returns  

boolean 
true only if the specified object is also a comparator and it imposes the same ordering as this comparator. 
See also:
Comparator<T> naturalOrder ()
Returns a comparator that compares Comparable
objects in natural order.
The returned comparator is serializable and throws NullPointerException
when comparing null
.
Returns  

Comparator<T> 
a comparator that imposes the natural ordering on Comparable objects. 
See also:
Comparator<T> nullsFirst (Comparator<? super T> comparator)
Returns a nullfriendly comparator that considers null
to be less than nonnull. When both are null
, they are considered equal. If both are nonnull, the specified Comparator
is used to determine the order. If the specified comparator is null
, then the returned comparator considers all nonnull values to be equal.
The returned comparator is serializable if the specified comparator is serializable.
Parameters  

comparator 
Comparator : a Comparator for comparing nonnull values 
Returns  

Comparator<T> 
a comparator that considers null to be less than nonnull, and compares nonnull objects with the supplied Comparator . 
Comparator<T> nullsLast (Comparator<? super T> comparator)
Returns a nullfriendly comparator that considers null
to be greater than nonnull. When both are null
, they are considered equal. If both are nonnull, the specified Comparator
is used to determine the order. If the specified comparator is null
, then the returned comparator considers all nonnull values to be equal.
The returned comparator is serializable if the specified comparator is serializable.
Parameters  

comparator 
Comparator : a Comparator for comparing nonnull values 
Returns  

Comparator<T> 
a comparator that considers null to be greater than nonnull, and compares nonnull objects with the supplied Comparator . 
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
.
Returns  

Comparator<T> 
a comparator that imposes the reverse of the natural ordering on Comparable objects. 
See also:
Comparator<T> reversed ()
Returns a comparator that imposes the reverse ordering of this comparator.
Returns  

Comparator<T> 
a comparator that imposes the reverse ordering of this comparator. 
Comparator<T> thenComparing (Function<? super T, ? extends U> keyExtractor)
Returns a lexicographicorder comparator with a function that extracts a Comparable
sort key.
thenComparing(comparing(keyExtractor))
.Parameters  

keyExtractor 
Function : the function used to extract the Comparable sort key 
Returns  

Comparator<T> 
a lexicographicorder comparator composed of this and then the Comparable sort key. 
Throws  

NullPointerException 
if the argument is null. 
See also:
Comparator<T> thenComparing (Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator)
Returns a lexicographicorder comparator with a function that extracts a key to be compared with the given Comparator
.
thenComparing(comparing(keyExtractor, cmp))
.Parameters  

keyExtractor 
Function : the function used to extract the sort key 
keyComparator 
Comparator : the Comparator used to compare the sort key 
Returns  

Comparator<T> 
a lexicographicorder comparator composed of this comparator and then comparing on the key extracted by the keyExtractor function 
Throws  

NullPointerException 
if either argument is null. 
Comparator<T> thenComparing (Comparator<? super T> other)
Returns a lexicographicorder 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.
String
based on the length and then caseinsensitive natural ordering, the comparator can be composed using following code, Comparator<String> cmp = Comparator.comparingInt(String::length)
.thenComparing(String.CASE_INSENSITIVE_ORDER);
Parameters  

other 
Comparator : the other comparator to be used when this comparator compares two objects that are equal. 
Returns  

Comparator<T> 
a lexicographicorder comparator composed of this and then the other comparator 
Throws  

NullPointerException 
if the argument is null. 
Comparator<T> thenComparingDouble (ToDoubleFunction<? super T> keyExtractor)
Returns a lexicographicorder comparator with a function that extracts a double
sort key.
thenComparing(comparingDouble(keyExtractor))
.Parameters  

keyExtractor 
ToDoubleFunction : the function used to extract the double sort key 
Returns  

Comparator<T> 
a lexicographicorder comparator composed of this and then the double sort key 
Throws  

NullPointerException 
if the argument is null. 
Comparator<T> thenComparingInt (ToIntFunction<? super T> keyExtractor)
Returns a lexicographicorder comparator with a function that extracts a int
sort key.
thenComparing(comparingInt(keyExtractor))
.Parameters  

keyExtractor 
ToIntFunction : the function used to extract the integer sort key 
Returns  

Comparator<T> 
a lexicographicorder comparator composed of this and then the int sort key 
Throws  

NullPointerException 
if the argument is null. 
Comparator<T> thenComparingLong (ToLongFunction<? super T> keyExtractor)
Returns a lexicographicorder comparator with a function that extracts a long
sort key.
thenComparing(comparingLong(keyExtractor))
.Parameters  

keyExtractor 
ToLongFunction : the function used to extract the long sort key 
Returns  

Comparator<T> 
a lexicographicorder comparator composed of this and then the long sort key 
Throws  

NullPointerException 
if the argument is null. 