001    /* Comparator.java -- Interface for objects that specify an ordering
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003    
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021    Linking this library statically or dynamically with other modules is
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037    
038    
039    package java.util;
040    
041    /**
042     * Interface for objects that specify an ordering between objects. The ordering
043     * should be <em>total</em>, such that any two objects of the correct type
044     * can be compared, and the comparison is reflexive, anti-symmetric, and
045     * transitive.  It is also recommended that the comparator be <em>consistent
046     * with equals</em>, although this is not a strict requirement. A relation
047     * is consistent with equals if these two statements always have the same
048     * results (if no exceptions occur):<br>
049     * <code>compare((Object) e1, (Object) e2) == 0</code> and
050     * <code>e1.equals((Object) e2)</code><br>
051     * Comparators that violate consistency with equals may cause strange behavior
052     * in sorted lists and sets.  For example, a case-sensitive dictionary order
053     * comparison of Strings is consistent with equals, but if it is
054     * case-insensitive it is not, because "abc" and "ABC" compare as equal even
055     * though "abc".equals("ABC") returns false.
056     * <P>
057     * In general, Comparators should be Serializable, because when they are passed
058     * to Serializable data structures such as SortedMap or SortedSet, the entire
059     * data structure will only serialize correctly if the comparator is
060     * Serializable.
061     *
062     * @author Original author unknown
063     * @author Eric Blake (ebb9@email.byu.edu)
064     * @see Comparable
065     * @see TreeMap
066     * @see TreeSet
067     * @see SortedMap
068     * @see SortedSet
069     * @see Arrays#sort(Object[], Comparator)
070     * @see java.io.Serializable
071     * @since 1.2
072     * @status updated to 1.4
073     */
074    public interface Comparator<T>
075    {
076      /**
077       * Return an integer that is negative, zero or positive depending on whether
078       * the first argument is less than, equal to or greater than the second
079       * according to this ordering. This method should obey the following
080       * contract:
081       * <ul>
082       *   <li>if compare(a, b) &lt; 0 then compare(b, a) &gt; 0</li>
083       *   <li>if compare(a, b) throws an exception, so does compare(b, a)</li>
084       *   <li>if compare(a, b) &lt; 0 and compare(b, c) &lt; 0 then compare(a, c)
085       *       &lt; 0</li>
086       *   <li>if compare(a, b) == 0 then compare(a, c) and compare(b, c) must
087       *       have the same sign</li>
088       * </ul>
089       * To be consistent with equals, the following additional constraint is
090       * in place:
091       * <ul>
092       *   <li>if a.equals(b) or both a and b are null, then
093       *       compare(a, b) == 0.</li>
094       * </ul><p>
095       *
096       * Although it is permissible for a comparator to provide an order
097       * inconsistent with equals, that should be documented.
098       *
099       * @param o1 the first object
100       * @param o2 the second object
101       * @return the comparison
102       * @throws ClassCastException if the elements are not of types that can be
103       *         compared by this ordering.
104       */
105      int compare(T o1, T o2);
106    
107      /**
108       * Return true if the object is equal to this object.  To be
109       * considered equal, the argument object must satisfy the constraints
110       * of <code>Object.equals()</code>, be a Comparator, and impose the
111       * same ordering as this Comparator. The default implementation
112       * inherited from Object is usually adequate.
113       *
114       * @param obj The object
115       * @return true if it is a Comparator that imposes the same order
116       * @see Object#equals(Object)
117       */
118      boolean equals(Object obj);
119    }