/* * 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 * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.lang;import java.util.*; /** * This interface imposes a total ordering on the objects of each class that * implements it.  This ordering is referred to as the class's <i>natural * ordering</i>, and the class's <tt>compareTo</tt> method is referred to as * its <i>natural comparison method</i>.<p> * * Lists (and arrays) of objects that implement this interface can be sorted * automatically by {@link Collections#sort(List) Collections.sort} (and * {@link Arrays#sort(Object[]) Arrays.sort}).  Objects that implement this * interface can be used as keys in a {@linkplain SortedMap sorted map} or as * elements in a {@linkplain SortedSet sorted set}, without the need to * specify a {@linkplain Comparator comparator}.<p> * * The natural ordering for a class <tt>C</tt> is said to be <i>consistent * with equals</i> if and only if <tt>e1.compareTo(e2) == 0</tt> has * the same boolean value as <tt>e1.equals(e2)</tt> for every * <tt>e1</tt> and <tt>e2</tt> of class <tt>C</tt>.  Note that <tt>null</tt> * is not an instance of any class, and <tt>e.compareTo(null)</tt> should * throw a <tt>NullPointerException</tt> even though <tt>e.equals(null)</tt> * returns <tt>false</tt>.<p> * * It is strongly recommended (though not required) that natural orderings be * consistent with equals.  This is so because sorted sets (and sorted maps) * without explicit comparators behave "strangely" when they are used with * elements (or keys) whose natural ordering is inconsistent with equals.  In * particular, such a sorted set (or sorted map) violates the general contract * for set (or map), which is defined in terms of the <tt>equals</tt> * method.<p> * * For example, if one adds two keys <tt>a</tt> and <tt>b</tt> such that * <tt>(!a.equals(b) && a.compareTo(b) == 0)</tt> to a sorted * set that does not use an explicit comparator, the second <tt>add</tt> * operation returns false (and the size of the sorted set does not increase) * because <tt>a</tt> and <tt>b</tt> are equivalent from the sorted set's * perspective.<p> * * Virtually all Java core classes that implement <tt>Comparable</tt> have natural * orderings that are consistent with equals.  One exception is * <tt>java.math.BigDecimal</tt>, whose natural ordering equates * <tt>BigDecimal</tt> objects with equal values and different precisions * (such as 4.0 and 4.00).<p> * * For the mathematically inclined, the <i>relation</i> that defines * the natural ordering on a given class C is:<pre> *       {(x, y) such that x.compareTo(y) &lt;= 0}. * </pre> The <i>quotient</i> for this total order is: <pre> *       {(x, y) such that x.compareTo(y) == 0}. * </pre> * * It follows immediately from the contract for <tt>compareTo</tt> that the * quotient is an <i>equivalence relation</i> on <tt>C</tt>, and that the * natural ordering is a <i>total order</i> on <tt>C</tt>.  When we say that a * class's natural ordering is <i>consistent with equals</i>, we mean that the * quotient for the natural ordering is the equivalence relation defined by * the class's {@link Object#equals(Object) equals(Object)} method:<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"> * Java Collections Framework</a>. * * @param <T> the type of objects that this object may be compared to * * @author  Josh Bloch * @see java.util.Comparator * @since 1.2 */ public interface Comparable<T> {    /**     * Compares this object with the specified object for order.  Returns a     * negative integer, zero, or a positive integer as this object is less     * than, equal to, or greater than the specified object.     *     * <p>The implementor must ensure <tt>sgn(x.compareTo(y)) ==     * -sgn(y.compareTo(x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This     * implies that <tt>x.compareTo(y)</tt> must throw an exception iff     * <tt>y.compareTo(x)</tt> throws an exception.)     *     * <p>The implementor must also ensure that the relation is transitive:     * <tt>(x.compareTo(y)&gt;0 &amp;&amp; y.compareTo(z)&gt;0)</tt> implies     * <tt>x.compareTo(z)&gt;0</tt>.     *     * <p>Finally, the implementor must ensure that <tt>x.compareTo(y)==0</tt>     * implies that <tt>sgn(x.compareTo(z)) == sgn(y.compareTo(z))</tt>, for     * all <tt>z</tt>.     *     * <p>It is strongly recommended, but <i>not</i> strictly required that     * <tt>(x.compareTo(y)==0) == (x.equals(y))</tt>.  Generally speaking, any     * class that implements the <tt>Comparable</tt> interface and violates     * this condition should clearly indicate this fact.  The recommended     * language is "Note: this class has a natural ordering that is     * inconsistent with equals."     *     * <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.     *     * @param   o the object to be compared.     * @return  a negative integer, zero, or a positive integer as this object     *          is less than, equal to, or greater than the specified object.     *     * @throws NullPointerException if the specified object is null     * @throws ClassCastException if the specified object's type prevents it     *         from being compared to this object.     */    public int compareTo(T o);}