/*
* @(#)Comparator.java 1.22 03/12/19
*
* Copyright 2004 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.util;
/**
* A comparison function, which imposes a <i>total ordering</i> on some
* collection of objects. Comparators can be passed to a sort method (such as
* <tt>Collections.sort</tt>) to allow precise control over the sort order.
* Comparators can also be used to control the order of certain data
* structures (such as <tt>TreeSet</tt> or <tt>TreeMap</tt>).<p>
*
* The ordering imposed by a Comparator <tt>c</tt> on a set of elements
* <tt>S</tt> is said to be <i>consistent with equals</i> if and only if
* <tt>(compare((Object)e1, (Object)e2)==0)</tt> has the same boolean value as
* <tt>e1.equals((Object)e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in
* <tt>S</tt>.<p>
*
* 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 <tt>c</tt>
* is used with elements (or keys) drawn from a set <tt>S</tt>. If the
* ordering imposed by <tt>c</tt> on <tt>S</tt> 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 <tt>equals</tt>.<p>
*
* For example, if one adds two keys <tt>a</tt> and <tt>b</tt> such that
* <tt>(a.equals((Object)b) && c.compare((Object)a, (Object)b) != 0)</tt> to a
* sorted set with comparator <tt>c</tt>, the second <tt>add</tt> operation
* will return false (and the size of the sorted set will not increase)
* because <tt>a</tt> and <tt>b</tt> are equivalent from the sorted set's
* perspective.<p>
*
* Note: It is generally a good idea for comparators to implement
* <tt>java.io.Serializable</tt>, as they may be used as ordering methods in
* serializable data structures (like <tt>TreeSet</tt>, <tt>TreeMap</tt>). In
* order for the data structure to serialize successfully, the comparator (if
* provided) must implement <tt>Serializable</tt>.<p>
*
* For the mathematically inclined, the <i>relation</i> that defines
* the <i>total order</i> that a given comparator <tt>c</tt> imposes on a
* given set of objects <tt>S</tt> is:<pre>
* {(x, y) such that c.compare((Object)x, (Object)y) <= 0}.
* </pre> The <i>quotient</i> for this total order is:<pre>
* {(x, y) such that c.compare((Object)x, (Object)y) == 0}.
* </pre>
*
* It follows immediately from the contract for <tt>compare</tt> that the
* quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the
* natural ordering is a <i>total order</i> on <tt>S</tt>. When we say that
* the ordering imposed by <tt>c</tt> on <tt>S</tt> is <i>consistent with
* equals</i>, we mean that the quotient for the natural ordering is the
* equivalence relation defined by the objects' <tt>equals(Object)</tt>
* method(s):<pre>
* {(x, y) such that x.equals((Object)y)}.
* </pre><p>
*
* This interface is a member of the
* <a href="{@docRoot}/../guide/collections/index.html">
* Java Collections Framework</a>.
*
* @author Josh Bloch
* @author Neal Gafter
* @version 1.22, 12/19/03
* @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<T> {
/**
* 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.<p>
*
* The implementor must ensure that <tt>sgn(compare(x, y)) ==
* -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This
* implies that <tt>compare(x, y)</tt> must throw an exception if and only
* if <tt>compare(y, x)</tt> throws an exception.)<p>
*
* The implementor must also ensure that the relation is transitive:
* <tt>((compare(x, y)>0) && (compare(y, z)>0))</tt> implies
* <tt>compare(x, z)>0</tt>.<p>
*
* Finally, the implementer must ensure that <tt>compare(x, y)==0</tt>
* implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all
* <tt>z</tt>.<p>
*
* It is generally the case, but <i>not</i> strictly required that
* <tt>(compare(x, y)==0) == (x.equals(y))</tt>. 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.
*/
int compare(T o1, T o2);
/**
*
* Indicates whether some other object is "equal to" this
* Comparator. This method must obey the general contract of
* <tt>Object.equals(Object)</tt>. Additionally, this method can return
* <tt>true</tt> <i>only</i> if the specified Object is also a comparator
* and it imposes the same ordering as this comparator. Thus,
* <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1,
* o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference
* <tt>o1</tt> and <tt>o2</tt>.<p>
*
* Note that it is <i>always</i> safe <i>not</i> to override
* <tt>Object.equals(Object)</tt>. 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 <code>true</code> 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()
*/
boolean equals(Object obj);
}
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