001    /* Comparable.java -- Interface for comparaing objects to obtain an ordering
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003    
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021    Linking this library statically or dynamically with other modules is
022    making a combined work based on this library.  Thus, the terms and
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037    
038    
039    package 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     */
071    public 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    }