java.util
interface Comparator

All Known Implementing Classes:

Collator, RuleBasedCollator

public 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}.
 

       {(x, y) such that c.compare(x, y) == 0}.
 

     {(x, y) such that x.equals(y)}. 

This interface is a member of the Java Collections Framework.

compare

public int compare(Object o1,
                   Object 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 - 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.

equals

public boolean equals(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.

Overrides:

equals in class Object

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.