
Java™ Platform Standard Ed. 6 

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T
 the type of objects that may be compared by this comparatorpublic 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)}.
This interface is a member of the Java Collections Framework.
Comparable
,
Serializable
Method Summary  

int 
compare(T o1,
T o2)
Compares its two arguments for order. 
boolean 
equals(Object obj)
Indicates whether some other object is "equal to" this comparator. 
Method Detail 

int compare(T o1, T o2)
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."
o1
 the first object to be compared.o2
 the second object to be compared.
ClassCastException
 if the arguments' types prevent them from
being compared by this comparator.boolean equals(Object obj)
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.
equals
in class Object
obj
 the reference object with which to compare.
true
only if the specified object is also
a comparator and it imposes the same ordering as this
comparator.Object.equals(Object)
,
Object.hashCode()

Java™ Platform Standard Ed. 6 

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