Comparatorpublic interface Comparator | A comparison function, which imposes a total ordering on some
collection of objects. Comparators can be passed to a sort method (such
as {@link Collections#sort(List,Comparator) Collections.sort} or {@link
Arrays#sort(Object[],Comparator) 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 {@link SortedSet sorted sets} or {@link
SortedMap sorted maps}), or to provide an ordering for collections of
objects that don't have a {@link Comparable 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 {@code a} and {@code b} such that
{@code (a.equals(b) && c.compare(a, b) != 0)}
to an empty {@code TreeSet} with comparator {@code c}.
The second {@code add} operation will return
true (and the size of the tree set will increase) because {@code a} and
{@code b} are not equivalent from the tree set's perspective, even though
this is contrary to the specification of the
{@link Set#add 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 {@link TreeSet}, {@link 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' {@link Object#equals(Object)
equals(Object)} method(s):
{(x, y) such that x.equals(y)}.
This interface is a member of the
Java Collections Framework. |
| Methods Summary |
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| public 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."
| | public boolean | equals(java.lang.Object obj)Indicates whether some other object is "equal to" this
comparator. This method must obey the general contract of
{@link 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.
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