Comparator (Java Platform SE 6) (original) (raw)

java.util

Interface Comparator

Type Parameters:

T - the type of objects that may be compared by this comparator

All Known Implementing Classes:

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 [Collections.sort](../../java/util/Collections.html#sort%28java.util.List, java.util.Comparator%29) or [Arrays.sort](../../java/util/Arrays.html#sort%28T[], java.util.Comparator%29)) 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 elementsS is said to be consistent with equals if and only ifc.compare(e1, e2)==0 has the same boolean value ase1.equals(e2) for every e1 and e2 inS.

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 andb are not equivalent from the tree set's perspective, even though this is contrary to the specification of theSet.add method.

Note: It is generally a good idea for comparators to also implementjava.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.

Since:

1.2

See Also:

Comparable, Serializable

Method Summary
int	[compare](../../java/util/Comparator.html#compare%28T, T%29)(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

compare

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 notationsgn(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)) impliescompare(x, z)>0.

Finally, the implementor must ensure that compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for allz.

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.

Throws:

[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the arguments' types prevent them from being compared by this comparator.

equals

boolean equals(Object obj)

Indicates whether some other object is "equal to" this comparator. This method must obey the general contract ofObject.equals(Object). Additionally, this method can returntrue 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 referenceo1 and o2.

Note that it is always safe not to overrideObject.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](../../java/lang/Object.html#equals%28java.lang.Object%29) in class [Object](../../java/lang/Object.html "class in java.lang")

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.

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