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:
Developers
{(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 null-friendly comparator that considers |
static
<T>
Comparator<T>
|
nullsLast(Comparator<? super T> comparator)
Returns a null-friendly 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 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>
|
thenComparing(Comparator<? super T> other)
Returns a lexicographic-order comparator with another comparator. |
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 a |
default
Comparator<T>
|
thenComparingLong(ToLongFunction<? super T> keyExtractor)
Returns a lexicographic-order 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 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.
| Parameters | |
|---|---|
comparator |
Comparator:
a Comparator for comparing non-null values |
| Returns | |
|---|---|
Comparator<T> |
a comparator that considers null to be less than
non-null, and compares non-null objects with the supplied
Comparator. |
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.
| Parameters | |
|---|---|
comparator |
Comparator:
a Comparator for comparing non-null values |
| Returns | |
|---|---|
Comparator<T> |
a comparator that considers null to be greater than
non-null, and compares non-null 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 lexicographic-order 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 lexicographic-order comparator composed of this and then the
Comparable sort key. |
| Throws | |
|---|---|
NullPointerException |
if the argument is null. |
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.
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 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. |
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.
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 |
Comparator:
the other comparator to be used when this comparator
compares two objects that are equal. |
| Returns | |
|---|---|
Comparator<T> |
a lexicographic-order 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 lexicographic-order 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 lexicographic-order 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 lexicographic-order 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 lexicographic-order 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 lexicographic-order 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 lexicographic-order comparator composed of this and then the
long sort key |
| Throws | |
|---|---|
NullPointerException |
if the argument is null. |