Comparator Interface

 

/* * @(#)Comparator.java1.26 06/04/21 * * Copyright 2006 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 {@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}.<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>c.compare(e1, e2)==0</tt> has the same boolean value as * <tt>e1.equals(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, 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.<p> * * Note: It is generally a good idea for comparators to also implement * <tt>java.io.Serializable</tt>, 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 <tt>Serializable</tt>.<p> * * For the mathematically inclined, the <i>relation</i> that defines the * <i>imposed ordering</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(x, y) <= 0}. * </pre> The <i>quotient</i> for this total order is:<pre> * {(x, y) such that c.compare(x, 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 * imposed 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 ordering is the equivalence * relation defined by the objects' {@link Object#equals(Object) * equals(Object)} method(s):<pre> * {(x, y) such that x.equals(y)}. </pre><p> * * This interface is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html" mce_href="technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @param <T> the type of objects that may be compared by this comparator * * @author Josh Bloch * @author Neal Gafter * @version 1.26, 04/21/06 * @see Comparable * @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> * * In the foregoing description, the notation * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical * <i>signum</i> function, which is defined to return one of <tt>-1</tt>, * <tt>0</tt>, or <tt>1</tt> according to whether the value of * <i>expression</i> is negative, zero or positive.<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 implementor 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 * {@link Object#equals(Object)}. 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 Object#equals(Object) * @see Object#hashCode() */ boolean equals(Object obj);}