| /* |
| * Copyright (c) 1997, 2007, Oracle and/or its affiliates. All rights reserved. |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. Oracle designates this |
| * particular file as subject to the "Classpath" exception as provided |
| * by Oracle in the LICENSE file that accompanied this code. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
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| * |
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| */ |
| |
| 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>Unlike {@code Comparable}, a comparator may optionally permit |
| * comparison of null arguments, while maintaining the requirements for |
| * an equivalence relation. |
| * |
| * <p>This interface is a member of the |
| * <a href="{@docRoot}/../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 |
| * @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 NullPointerException if an argument is null and this |
| * comparator does not permit null arguments |
| * @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); |
| } |
