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| 1 | /* | ||
| 2 | * Copyright (c) 1997, 2007, Oracle and/or its affiliates. All rights reserved. | ||
| 3 | * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. | ||
| 4 | * | ||
| 5 | * This code is free software; you can redistribute it and/or modify it | ||
| 6 | * under the terms of the GNU General Public License version 2 only, as | ||
| 7 | * published by the Free Software Foundation. Oracle designates this | ||
| 8 | * particular file as subject to the "Classpath" exception as provided | ||
| 9 | * by Oracle in the LICENSE file that accompanied this code. | ||
| 10 | * | ||
| 11 | * This code is distributed in the hope that it will be useful, but WITHOUT | ||
| 12 | * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or | ||
| 13 | * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License | ||
| 14 | * version 2 for more details (a copy is included in the LICENSE file that | ||
| 15 | * accompanied this code). | ||
| 16 | * | ||
| 17 | * You should have received a copy of the GNU General Public License version | ||
| 18 | * 2 along with this work; if not, write to the Free Software Foundation, | ||
| 19 | * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. | ||
| 20 | * | ||
| 21 | * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA | ||
| 22 | * or visit www.oracle.com if you need additional information or have any | ||
| 23 | * questions. | ||
| 24 | */ | ||
| 25 | |||
| 26 | package java.util; | ||
| 27 | |||
| 28 | /** | ||
| 29 | * A comparison function, which imposes a <i>total ordering</i> on some | ||
| 30 | * collection of objects. Comparators can be passed to a sort method (such | ||
| 31 | * as {@link Collections#sort(List,Comparator) Collections.sort} or {@link | ||
| 32 | * Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control | ||
| 33 | * over the sort order. Comparators can also be used to control the order of | ||
| 34 | * certain data structures (such as {@link SortedSet sorted sets} or {@link | ||
| 35 | * SortedMap sorted maps}), or to provide an ordering for collections of | ||
| 36 | * objects that don't have a {@link Comparable natural ordering}.<p> | ||
| 37 | * | ||
| 38 | * The ordering imposed by a comparator <tt>c</tt> on a set of elements | ||
| 39 | * <tt>S</tt> is said to be <i>consistent with equals</i> if and only if | ||
| 40 | * <tt>c.compare(e1, e2)==0</tt> has the same boolean value as | ||
| 41 | * <tt>e1.equals(e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in | ||
| 42 | * <tt>S</tt>.<p> | ||
| 43 | * | ||
| 44 | * Caution should be exercised when using a comparator capable of imposing an | ||
| 45 | * ordering inconsistent with equals to order a sorted set (or sorted map). | ||
| 46 | * Suppose a sorted set (or sorted map) with an explicit comparator <tt>c</tt> | ||
| 47 | * is used with elements (or keys) drawn from a set <tt>S</tt>. If the | ||
| 48 | * ordering imposed by <tt>c</tt> on <tt>S</tt> is inconsistent with equals, | ||
| 49 | * the sorted set (or sorted map) will behave "strangely." In particular the | ||
| 50 | * sorted set (or sorted map) will violate the general contract for set (or | ||
| 51 | * map), which is defined in terms of <tt>equals</tt>.<p> | ||
| 52 | * | ||
| 53 | * For example, suppose one adds two elements {@code a} and {@code b} such that | ||
| 54 | * {@code (a.equals(b) && c.compare(a, b) != 0)} | ||
| 55 | * to an empty {@code TreeSet} with comparator {@code c}. | ||
| 56 | * The second {@code add} operation will return | ||
| 57 | * true (and the size of the tree set will increase) because {@code a} and | ||
| 58 | * {@code b} are not equivalent from the tree set's perspective, even though | ||
| 59 | * this is contrary to the specification of the | ||
| 60 | * {@link Set#add Set.add} method.<p> | ||
| 61 | * | ||
| 62 | * Note: It is generally a good idea for comparators to also implement | ||
| 63 | * <tt>java.io.Serializable</tt>, as they may be used as ordering methods in | ||
| 64 | * serializable data structures (like {@link TreeSet}, {@link TreeMap}). In | ||
| 65 | * order for the data structure to serialize successfully, the comparator (if | ||
| 66 | * provided) must implement <tt>Serializable</tt>.<p> | ||
| 67 | * | ||
| 68 | * For the mathematically inclined, the <i>relation</i> that defines the | ||
| 69 | * <i>imposed ordering</i> that a given comparator <tt>c</tt> imposes on a | ||
| 70 | * given set of objects <tt>S</tt> is:<pre> | ||
| 71 | * {(x, y) such that c.compare(x, y) <= 0}. | ||
| 72 | * </pre> The <i>quotient</i> for this total order is:<pre> | ||
| 73 | * {(x, y) such that c.compare(x, y) == 0}. | ||
| 74 | * </pre> | ||
| 75 | * | ||
| 76 | * It follows immediately from the contract for <tt>compare</tt> that the | ||
| 77 | * quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the | ||
| 78 | * imposed ordering is a <i>total order</i> on <tt>S</tt>. When we say that | ||
| 79 | * the ordering imposed by <tt>c</tt> on <tt>S</tt> is <i>consistent with | ||
| 80 | * equals</i>, we mean that the quotient for the ordering is the equivalence | ||
| 81 | * relation defined by the objects' {@link Object#equals(Object) | ||
| 82 | * equals(Object)} method(s):<pre> | ||
| 83 | * {(x, y) such that x.equals(y)}. </pre> | ||
| 84 | * | ||
| 85 | * <p>Unlike {@code Comparable}, a comparator may optionally permit | ||
| 86 | * comparison of null arguments, while maintaining the requirements for | ||
| 87 | * an equivalence relation. | ||
| 88 | * | ||
| 89 | * <p>This interface is a member of the | ||
| 90 | * <a href="{@docRoot}/../technotes/guides/collections/index.html"> | ||
| 91 | * Java Collections Framework</a>. | ||
| 92 | * | ||
| 93 | * @param <T> the type of objects that may be compared by this comparator | ||
| 94 | * | ||
| 95 | * @author Josh Bloch | ||
| 96 | * @author Neal Gafter | ||
| 97 | * @see Comparable | ||
| 98 | * @see java.io.Serializable | ||
| 99 | * @since 1.2 | ||
| 100 | */ | ||
| 101 | |||
| 102 | public interface Comparator<T> { | ||
| 103 | /** | ||
| 104 | * Compares its two arguments for order. Returns a negative integer, | ||
| 105 | * zero, or a positive integer as the first argument is less than, equal | ||
| 106 | * to, or greater than the second.<p> | ||
| 107 | * | ||
| 108 | * In the foregoing description, the notation | ||
| 109 | * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical | ||
| 110 | * <i>signum</i> function, which is defined to return one of <tt>-1</tt>, | ||
| 111 | * <tt>0</tt>, or <tt>1</tt> according to whether the value of | ||
| 112 | * <i>expression</i> is negative, zero or positive.<p> | ||
| 113 | * | ||
| 114 | * The implementor must ensure that <tt>sgn(compare(x, y)) == | ||
| 115 | * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This | ||
| 116 | * implies that <tt>compare(x, y)</tt> must throw an exception if and only | ||
| 117 | * if <tt>compare(y, x)</tt> throws an exception.)<p> | ||
| 118 | * | ||
| 119 | * The implementor must also ensure that the relation is transitive: | ||
| 120 | * <tt>((compare(x, y)>0) && (compare(y, z)>0))</tt> implies | ||
| 121 | * <tt>compare(x, z)>0</tt>.<p> | ||
| 122 | * | ||
| 123 | * Finally, the implementor must ensure that <tt>compare(x, y)==0</tt> | ||
| 124 | * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all | ||
| 125 | * <tt>z</tt>.<p> | ||
| 126 | * | ||
| 127 | * It is generally the case, but <i>not</i> strictly required that | ||
| 128 | * <tt>(compare(x, y)==0) == (x.equals(y))</tt>. Generally speaking, | ||
| 129 | * any comparator that violates this condition should clearly indicate | ||
| 130 | * this fact. The recommended language is "Note: this comparator | ||
| 131 | * imposes orderings that are inconsistent with equals." | ||
| 132 | * | ||
| 133 | * @param o1 the first object to be compared. | ||
| 134 | * @param o2 the second object to be compared. | ||
| 135 | * @return a negative integer, zero, or a positive integer as the | ||
| 136 | * first argument is less than, equal to, or greater than the | ||
| 137 | * second. | ||
| 138 | * @throws NullPointerException if an argument is null and this | ||
| 139 | * comparator does not permit null arguments | ||
| 140 | * @throws ClassCastException if the arguments' types prevent them from | ||
| 141 | * being compared by this comparator. | ||
| 142 | */ | ||
| 143 | int compare(T o1, T o2); | ||
| 144 | |||
| 145 | /** | ||
| 146 | * Indicates whether some other object is "equal to" this | ||
| 147 | * comparator. This method must obey the general contract of | ||
| 148 | * {@link Object#equals(Object)}. Additionally, this method can return | ||
| 149 | * <tt>true</tt> <i>only</i> if the specified object is also a comparator | ||
| 150 | * and it imposes the same ordering as this comparator. Thus, | ||
| 151 | * <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1, | ||
| 152 | * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference | ||
| 153 | * <tt>o1</tt> and <tt>o2</tt>.<p> | ||
| 154 | * | ||
| 155 | * Note that it is <i>always</i> safe <i>not</i> to override | ||
| 156 | * <tt>Object.equals(Object)</tt>. However, overriding this method may, | ||
| 157 | * in some cases, improve performance by allowing programs to determine | ||
| 158 | * that two distinct comparators impose the same order. | ||
| 159 | * | ||
| 160 | * @param obj the reference object with which to compare. | ||
| 161 | * @return <code>true</code> only if the specified object is also | ||
| 162 | * a comparator and it imposes the same ordering as this | ||
| 163 | * comparator. | ||
| 164 | * @see Object#equals(Object) | ||
| 165 | * @see Object#hashCode() | ||
| 166 | */ | ||
| 167 | boolean equals(Object obj); | ||
| 168 | } | ||