1 /* 2 * @(#)Comparable.java 1.22 03/12/19 3 * 4 * Copyright 2004 Sun Microsystems, Inc. All rights reserved. 5 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. 6 */ 7 8 package java.lang; 9 10 /** 11 * This interface imposes a total ordering on the objects of each class that 12 * implements it. This ordering is referred to as the class's <i>natural 13 * ordering</i>, and the class's <tt>compareTo</tt> method is referred to as 14 * its <i>natural comparison method</i>.<p> 15 * 16 * Lists (and arrays) of objects that implement this interface can be sorted 17 * automatically by <tt>Collections.sort</tt> (and <tt>Arrays.sort</tt>). 18 * Objects that implement this interface can be used as keys in a sorted map 19 * or elements in a sorted set, without the need to specify a comparator.<p> 20 * 21 * The natural ordering for a class <tt>C</tt> is said to be <i>consistent 22 * with equals</i> if and only if <tt>(e1.compareTo((Object)e2) == 0)</tt> has 23 * the same boolean value as <tt>e1.equals((Object)e2)</tt> for every 24 * <tt>e1</tt> and <tt>e2</tt> of class <tt>C</tt>. Note that <tt>null</tt> 25 * is not an instance of any class, and <tt>e.compareTo(null)</tt> should 26 * throw a <tt>NullPointerException</tt> even though <tt>e.equals(null)</tt> 27 * returns <tt>false</tt>.<p> 28 * 29 * It is strongly recommended (though not required) that natural orderings be 30 * consistent with equals. This is so because sorted sets (and sorted maps) 31 * without explicit comparators behave "strangely" when they are used with 32 * elements (or keys) whose natural ordering is inconsistent with equals. In 33 * particular, such a sorted set (or sorted map) violates the general contract 34 * for set (or map), which is defined in terms of the <tt>equals</tt> 35 * method.<p> 36 * 37 * For example, if one adds two keys <tt>a</tt> and <tt>b</tt> such that 38 * <tt>(!a.equals((Object)b) && a.compareTo((Object)b) == 0)</tt> to a sorted 39 * set that does not use an explicit comparator, the second <tt>add</tt> 40 * operation returns false (and the size of the sorted set does not increase) 41 * because <tt>a</tt> and <tt>b</tt> are equivalent from the sorted set's 42 * perspective.<p> 43 * 44 * Virtually all Java core classes that implement comparable have natural 45 * orderings that are consistent with equals. One exception is 46 * <tt>java.math.BigDecimal</tt>, whose natural ordering equates 47 * <tt>BigDecimal</tt> objects with equal values and different precisions 48 * (such as 4.0 and 4.00).<p> 49 * 50 * For the mathematically inclined, the <i>relation</i> that defines 51 * the natural ordering on a given class C is:<pre> 52 * {(x, y) such that x.compareTo((Object)y) <= 0}. 53 * </pre> The <i>quotient</i> for this total order is: <pre> 54 * {(x, y) such that x.compareTo((Object)y) == 0}. 55 * </pre> 56 * 57 * It follows immediately from the contract for <tt>compareTo</tt> that the 58 * quotient is an <i>equivalence relation</i> on <tt>C</tt>, and that the 59 * natural ordering is a <i>total order</i> on <tt>C</tt>. When we say that a 60 * class's natural ordering is <i>consistent with equals</i>, we mean that the 61 * quotient for the natural ordering is the equivalence relation defined by 62 * the class's <tt>equals(Object)</tt> method:<pre> 63 * {(x, y) such that x.equals((Object)y)}. 64 * </pre><p> 65 * 66 * This interface is a member of the 67 * <a HREF="{@docRoot}/../guide/collections/index.html"> 68 * Java Collections Framework</a>. 69 * 70 * @author Josh Bloch 71 * @version 1.22, 12/19/03 72 * @see java.util.Comparator 73 * @see java.util.Collections#sort(java.util.List) 74 * @see java.util.Arrays#sort(Object[]) 75 * @see java.util.SortedSet 76 * @see java.util.SortedMap 77 * @see java.util.TreeSet 78 * @see java.util.TreeMap 79 * @since 1.2 80 */ 81 82 public interface Comparable<T> { 83 /** 84 * Compares this object with the specified object for order. Returns a 85 * negative integer, zero, or a positive integer as this object is less 86 * than, equal to, or greater than the specified object.<p> 87 * 88 * In the foregoing description, the notation 89 * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical 90 * <i>signum</i> function, which is defined to return one of <tt>-1</tt>, 91 * <tt>0</tt>, or <tt>1</tt> according to whether the value of <i>expression</i> 92 * is negative, zero or positive. 93 * 94 * The implementor must ensure <tt>sgn(x.compareTo(y)) == 95 * -sgn(y.compareTo(x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This 96 * implies that <tt>x.compareTo(y)</tt> must throw an exception iff 97 * <tt>y.compareTo(x)</tt> throws an exception.)<p> 98 * 99 * The implementor must also ensure that the relation is transitive: 100 * <tt>(x.compareTo(y)>0 && y.compareTo(z)>0)</tt> implies 101 * <tt>x.compareTo(z)>0</tt>.<p> 102 * 103 * Finally, the implementer must ensure that <tt>x.compareTo(y)==0</tt> 104 * implies that <tt>sgn(x.compareTo(z)) == sgn(y.compareTo(z))</tt>, for 105 * all <tt>z</tt>.<p> 106 * 107 * It is strongly recommended, but <i>not</i> strictly required that 108 * <tt>(x.compareTo(y)==0) == (x.equals(y))</tt>. Generally speaking, any 109 * class that implements the <tt>Comparable</tt> interface and violates 110 * this condition should clearly indicate this fact. The recommended 111 * language is "Note: this class has a natural ordering that is 112 * inconsistent with equals." 113 * 114 * @param o the Object to be compared. 115 * @return a negative integer, zero, or a positive integer as this object 116 * is less than, equal to, or greater than the specified object. 117 * 118 * @throws ClassCastException if the specified object's type prevents it 119 * from being compared to this Object. 120 */ 121 public int compareTo(T o); 122 } 123