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Interface Comparator<T>
Type Parameters:
T - the type of objects that may be compared by this comparator
Functional Interface:
This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.
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)}.
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.
Since:
1.2
Author:
Josh Bloch, Neal Gafter
See Also:
Method Summary |
int |
Compares its two arguments for order.
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Accepts a function that extracts a Comparable sort key from a type T, and returns a
Comparator<T> that compares by that sort key.
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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.
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Accepts a function that extracts a double sort key from a type
T, and returns a Comparator<T> that compares by that
sort key.
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Accepts a function that extracts an int sort key from a type
T, and returns a Comparator<T> that compares by that
sort key.
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Accepts a function that extracts a long sort key from a type
T, and returns a Comparator<T> that compares by that
sort key.
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boolean |
Indicates whether some other object is "equal to"
this comparator.
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Returns a comparator that compares Comparable objects in natural
order.
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Returns a null-friendly comparator that considers null to be
less than non-null.
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Returns a null-friendly comparator that considers null to be
greater than non-null.
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Returns a comparator that imposes the reverse ordering of this
comparator.
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Returns a comparator that imposes the reverse of the natural
ordering.
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Returns a lexicographic-order comparator with another comparator.
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Returns a lexicographic-order comparator with a function that
extracts a Comparable sort key.
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Returns a lexicographic-order comparator with a function that
extracts a key to be compared with the given Comparator.
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Returns a lexicographic-order comparator with a function that
extracts a double sort key.
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Returns a lexicographic-order comparator with a function that
extracts an int sort key.
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Returns a lexicographic-order comparator with a function that
extracts a long sort key.
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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.
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.
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.
Overrides:
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:
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
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.
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:
Since:
1.8
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 - the Comparator 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:
Since:
1.8
See Also:
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:
Parameters:
keyExtractor - the function used to extract the
Comparable sort key
Returns:
a lexicographic-order comparator composed of this and then the
Comparable sort key.
Throws:
Since:
1.8
See Also:
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:
Since:
1.8
See Also:
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:
Since:
1.8
See Also:
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:
Since:
1.8
See Also:
Returns a comparator that imposes the reverse of the
natural
ordering.
The returned comparator is serializable and throws NullPointerException when comparing null.
Type Parameters:
Returns:
a comparator that imposes the reverse of the natural
ordering on Comparable objects.
Since:
1.8
See Also:
Returns a comparator that compares
Comparable objects in natural
order.
The returned comparator is serializable and throws NullPointerException when comparing null.
Type Parameters:
Returns:
a comparator that imposes the natural ordering on
Comparable objects.
Since:
1.8
See Also:
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 - a Comparator for comparing non-null values
Returns:
a comparator that considers null to be less than
non-null, and compares non-null objects with the supplied
Comparator.
Since:
1.8
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 - a Comparator for comparing non-null values
Returns:
a comparator that considers null to be greater than
non-null, and compares non-null objects with the supplied
Comparator.
Since:
1.8
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.
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 - the Comparator used to compare the sort key
Returns:
a comparator that compares by an extracted key using the
specified Comparator
Throws:
Since:
1.8
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.
Type Parameters:
T - the type of element to be compared
U - the type of the Comparable sort key
Parameters:
keyExtractor - the function used to extract the
Comparable sort key
Returns:
a comparator that compares by an extracted key
Throws:
Since:
1.8
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:
Since:
1.8
See Also:
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:
Since:
1.8
See Also:
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:
Since:
1.8
See Also:
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