/*

 * 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;

/**

 * 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) <= 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);

}