T - the type of objects that may be compared by this comparator
Collator, RuleBasedCollator
A comparison function, which imposes a
total orderingon 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 setsor
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
quotientfor 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 relationon
S
, and that the imposed ordering is a
total orderon
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.
int
Compares its two arguments for order.
Accepts a function that extracts a
Comparable
sort key from a type
T
, and returns a
Comparator<T>
that compares by that sort key.
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
.
Accepts a function that extracts a double sort key from a type T, and returns a Comparator<T> that compares by that sort key.
Accepts a function that extracts an int sort key from a type T, and returns a Comparator<T> that compares by that sort key.
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
Indicates whether some other object is "equal to" this comparator.
Returns a comparator that compares
Comparable
objects in natural order.
Returns a null-friendly comparator that considers null to be less than non-null.
Returns a null-friendly comparator that considers null to be greater than non-null.
Returns a comparator that imposes the reverse ordering of this comparator.
Returns a comparator that imposes the reverse of the natural ordering.
Returns a lexicographic-order comparator with another comparator.
Returns a lexicographic-order comparator with a function that extracts a Comparable sort key.
Returns a lexicographic-order comparator with a function that extracts a key to be compared with the given Comparator.
Returns a lexicographic-order comparator with a function that extracts a double sort key.
Returns a lexicographic-order comparator with a function that extracts an int sort key.
Returns a lexicographic-order comparator with a function that extracts a long sort key.
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.
(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."
o1 - the first object to be compared.
o2 - the second object to be compared.
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.
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.
Returns a comparator that imposes the reverse ordering of this comparator.
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);
other - the other comparator to be used when this comparator compares two objects that are equal.
NullPointerException - if the argument is null.
Returns a lexicographic-order comparator with a function that extracts a key to be compared with the given Comparator.
thenComparing(comparing(keyExtractor, cmp)).
U - the type of the sort key
keyExtractor - the function used to extract the sort key
keyComparator - the Comparator used to compare the sort key
NullPointerException - if either argument is null.
Returns a lexicographic-order comparator with a function that extracts a Comparable sort key.
thenComparing(comparing(keyExtractor)).
U - the type of the Comparable sort key
keyExtractor - the function used to extract the Comparable sort key
Comparable sort key.
NullPointerException - if the argument is null.
Returns a lexicographic-order comparator with a function that extracts an int sort key.
thenComparing(comparingInt(keyExtractor)).
keyExtractor - the function used to extract the integer sort key
int sort key
NullPointerException - if the argument is null.
Returns a lexicographic-order comparator with a function that extracts a long sort key.
thenComparing(comparingLong(keyExtractor)).
keyExtractor - the function used to extract the long sort key
long sort key
NullPointerException - if the argument is null.
Returns a lexicographic-order comparator with a function that extracts a double sort key.
thenComparing(comparingDouble(keyExtractor)).
keyExtractor - the function used to extract the double sort key
double sort key
NullPointerException - if the argument is null.
Returns a comparator that imposes the reverse of the
natural ordering.
The returned comparator is serializable and throws NullPointerException when comparing null.
T - the Comparable type of element to be compared
Comparable objects.
Returns a comparator that compares
Comparable
objects in natural order.
The returned comparator is serializable and throws NullPointerException when comparing null.
T - the Comparable type of element to be compared
Comparable objects.
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.
T - the type of the elements to be compared
comparator - a Comparator for comparing non-null values
null to be less than non-null, and compares non-null objects with the supplied 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.
T - the type of the elements to be compared
comparator - a Comparator for comparing non-null values
null to be greater than non-null, and compares non-null objects with the supplied Comparator.
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);
T - the type of element to be compared
U - the type of the sort key
keyExtractor - the function used to extract the sort key
keyComparator - the Comparator used to compare the sort key
Comparator
NullPointerException - if either argument is null
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);
T - the type of element to be compared
U - the type of the Comparable sort key
keyExtractor - the function used to extract the Comparable sort key
NullPointerException - if the argument is null
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.
T - the type of element to be compared
keyExtractor - the function used to extract the integer sort key
NullPointerException - if the argument is null
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.
T - the type of element to be compared
keyExtractor - the function used to extract the long sort key
NullPointerException - if the argument is null
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.
T - the type of element to be compared
keyExtractor - the function used to extract the double sort key
NullPointerException - if the argument is null
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