001/* Comparable.java -- Interface for comparaing objects to obtain an ordering
002   Copyright (C) 1998, 1999, 2001, 2002, 2004, 2005 Free Software Foundation, Inc.
003
004This file is part of GNU Classpath.
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021Linking this library statically or dynamically with other modules is
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037
038
039package java.lang;
040
041/**
042 * Interface for objects that can be ordering among other objects. The
043 * ordering can be <em>total</em>, such that two objects only compare equal
044 * if they are also equal by the equals method, or <em>partial</em> such
045 * that this is not necessarily true. For example, a case-sensitive
046 * dictionary order comparison of Strings is total, but if it is
047 * case-insensitive it is partial, because "abc" and "ABC" compare as
048 * equal even though "abc".equals("ABC") returns false. However, if you use
049 * a partial ordering, it is a good idea to document your class as
050 * "inconsistent with equals", because the behavior of your class in a
051 * SortedMap will be different than in a HashMap.
052 *
053 * <p>Lists, arrays, and sets of objects that implement this interface can
054 * be sorted automatically, without the need for an explicit
055 * {@link java.util.Comparator}. Note that <code>e1.compareTo(null)</code> 
056 * should throw an Exception; as should comparison between incompatible 
057 * classes.
058 *
059 * @author Geoff Berry
060 * @author Warren Levy (warrenl@cygnus.com)
061 * @see java.util.Comparator
062 * @see java.util.Collections#sort(java.util.List)
063 * @see java.util.Arrays#sort(Object[])
064 * @see java.util.SortedSet
065 * @see java.util.SortedMap
066 * @see java.util.TreeSet
067 * @see java.util.TreeMap
068 * @since 1.2
069 * @status updated to 1.5
070 */
071public interface Comparable<T>
072{
073  /**
074   * Compares this object with another, and returns a numerical result based
075   * on the comparison.  If the result is negative, this object sorts less
076   * than the other; if 0, the two are equal, and if positive, this object
077   * sorts greater than the other.  To translate this into boolean, simply
078   * perform <code>o1.compareTo(o2) <em>&lt;op&gt;</em> 0</code>, where op
079   * is one of &lt;, &lt;=, =, !=, &gt;, or &gt;=.
080   *
081   * <p>You must make sure that the comparison is mutual, ie.
082   * <code>sgn(x.compareTo(y)) == -sgn(y.compareTo(x))</code> (where sgn() is
083   * defined as -1, 0, or 1 based on the sign).  This includes throwing an
084   * exception in either direction if the two are not comparable; hence,
085   * <code>compareTo(null)</code> should always throw an Exception.
086   *
087   * <p>You should also ensure transitivity, in two forms:
088   * <code>x.compareTo(y) &gt; 0 && y.compareTo(z) &gt; 0</code> implies
089   * <code>x.compareTo(z) &gt; 0</code>; and <code>x.compareTo(y) == 0</code>
090   * implies <code>x.compareTo(z) == y.compareTo(z)</code>.
091   *
092   * @param o the object to be compared
093   * @return an integer describing the comparison
094   * @throws NullPointerException if o is null
095   * @throws ClassCastException if o cannot be compared
096   */
097  int compareTo(T o);
098}