/*
* Copyright (c) 1997, 2006, 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) <= 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)>0 && y.compareTo(z)>0)</tt> implies
* <tt>x.compareTo(z)>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 ClassCastException if the specified object's type prevents it
* from being compared to this object.
*/
public int compareTo(T o);
}