comparable.java

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/* Comparable.java -- Interface for comparaing objects to obtain an ordering
   Copyright (C) 1998, 1999, 2001, 2002 Free Software Foundation, Inc.

This file is part of GNU Classpath.

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package java.lang;

/**
 * Interface for objects that can be ordering among other objects. The
 * ordering can be <em>total</em>, such that two objects only compare equal
 * if they are also equal by the equals method, or <em>partial</em> such
 * that this is not necessarily true. For example, a case-sensitive
 * dictionary order comparison of Strings is total, but if it is
 * case-insensitive it is partial, because "abc" and "ABC" compare as
 * equal even though "abc".equals("ABC") returns false. However, if you use
 * a partial ordering, it is a good idea to document your class as
 * "inconsistent with equals", because the behavior of your class in a
 * SortedMap will be different than in a HashMap.
 *
 * <p>Lists, arrays, and sets of objects that implement this interface can
 * be sorted automatically, without the need for an explicit
 * {@link Comparator}. Note that <code>e1.compareTo(null)</code> should
 * throw an Exception; as should comparison between incompatible classes.
 *
 * @author Geoff Berry
 * @author Warren Levy <[e-mail]>
 * @see Comparator
 * @see Collections#sort(List)
 * @see Arrays#sort(Object[])
 * @see SortedSet
 * @see SortedMap
 * @see TreeSet
 * @see TreeMap
 * @since 1.2
 * @status updated to 1.4
 */
public interface Comparable
{
  /**
   * Compares this object with another, and returns a numerical result based
   * on the comparison.  If the result is negative, this object sorts less
   * than the other; if 0, the two are equal, and if positive, this object
   * sorts greater than the other.  To translate this into boolean, simply
   * perform <code>o1.compareTo(o2) <em>&lt;op&gt;</em> 0</code>, where op
   * is one of &lt;, &lt;=, =, !=, &gt;, or &gt;=.
   *
   * <p>You must make sure that the comparison is mutual, ie.
   * <code>sgn(x.compareTo(y)) == -sgn(y.compareTo(x))</code> (where sgn() is
   * defined as -1, 0, or 1 based on the sign).  This includes throwing an
   * exception in either direction if the two are not comparable; hence,
   * <code>compareTo(null)</code> should always throw an Exception.
   *
   * <p>You should also ensure transitivity, in two forms:
   * <code>x.compareTo(y) &gt; 0 && y.compareTo(z) &gt; 0</code> implies
   * <code>x.compareTo(z) &gt; 0</code>; and <code>x.compareTo(y) == 0</code>
   * implies <code>x.compareTo(z) == y.compareTo(z)</code>.
   *
   * @param o the object to be compared
   * @return an integer describing the comparison
   * @throws NullPointerException if o is null
   * @throws ClassCastException if o cannot be compared
   */
  int compareTo(Object o);
}