java.util.Comparator Maven / Gradle / Ivy
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package java.util;
/**
* 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) to allow precise control over the sort order.
* Comparators can also be used to control the order of certain data
* structures (such as TreeSet or TreeMap).
*
* The ordering imposed by a Comparator c on a set of elements
* S is said to be consistent with equals if and only if
* (compare((Object)e1, (Object)e2)==0) has the same boolean value as
* e1.equals((Object)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, if one adds two keys a and b such that
* (a.equals((Object)b) && c.compare((Object)a, (Object)b) != 0) to a
* sorted set with comparator c, the second add operation
* will return false (and the size of the sorted set will not increase)
* because a and b are equivalent from the sorted set's
* perspective.
*
* Note: It is generally a good idea for comparators to 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 total order that a given comparator c imposes on a
* given set of objects S is:
* {(x, y) such that c.compare((Object)x, (Object)y) <= 0}.
* The quotient for this total order is:
* {(x, y) such that c.compare((Object)x, (Object)y) == 0}.
*
*
* It follows immediately from the contract for compare that the
* quotient is an equivalence relation on S, and that the
* natural 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 natural ordering is the
* equivalence relation defined by the objects' equals(Object)
* method(s):
* {(x, y) such that x.equals((Object)y)}.
*
*
* This interface is a member of the
*
* Java Collections Framework.
*
* @author Josh Bloch
* @version 1.15, 02/02/00
* @see Comparable
* @see Arrays#sort(Object[], Comparator)
* @see TreeMap
* @see TreeSet
* @see SortedMap
* @see SortedSet
* @see java.io.Serializable
* @since 1.2
*/
public interface Comparator
{
/**
* 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 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 implementer 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."
*
* @param o1 the first object to be compared.
* @param o2 the second object to be compared.
* @return a negative integer, zero, or a positive integer as the
* first argument is less than, equal to, or greater than the
* second.
* @throws ClassCastException if the arguments' types prevent them from
* being compared by this Comparator.
*/
public int compare(Object o1, Object o2);
/**
*
* 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 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.
*
* @param obj the reference object with which to compare.
* @return true only if the specified object is also
* a comparator and it imposes the same ordering as this
* comparator.
* @see java.lang.Object#equals(java.lang.Object)
* @see java.lang.Object#hashCode()
*/
public boolean equals(Object obj);
}