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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 | +} | |