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 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You under the Apache License, Version 2.0
 *  (the "License"); you may not use this file except in compliance with
 *  the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
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package org.apache.commons.collections.comparators;

import java.io.Serializable;
import java.util.Comparator;

A Comparator that compares Comparable objects. 

This Comparator is useful, for example,
for enforcing the natural order in custom implementations
of SortedSet and SortedMap.
 Note: In the 2.0 and 2.1 releases of Commons Collections, this class would throw a ClassCastException if either of the arguments to compare were null, not Comparable, or for which compareTo gave inconsistent results. This is no longer the case. See compare for details. 
Author:
Henri Yandell
See Also:
Collections.reverseOrder()
Since:
Commons Collections 2.0
Version:
$Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $/**
 * A {@link Comparator Comparator} that compares 
 * {@link Comparable Comparable} objects.
 * <p />
 * This Comparator is useful, for example,
 * for enforcing the natural order in custom implementations
 * of SortedSet and SortedMap.
 * <p />
 * Note: In the 2.0 and 2.1 releases of Commons Collections, 
 * this class would throw a {@link ClassCastException} if
 * either of the arguments to {@link #compare(Object, Object) compare}
 * were <code>null</code>, not {@link Comparable Comparable},
 * or for which {@link Comparable#compareTo(Object) compareTo} gave
 * inconsistent results.  This is no longer the case.  See
 * {@link #compare(Object, Object) compare} for details.
 *
 * @since Commons Collections 2.0
 * @version $Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $
 *
 * @author Henri Yandell
 *
 * @see java.util.Collections#reverseOrder()
 */
public class ComparableComparator implements Comparator, Serializable {

    
Serialization version. /** Serialization version. */
    private static final long serialVersionUID=-291439688585137865L;

    
The singleton instance. /** The singleton instance. */
    private static final ComparableComparator instance = new ComparableComparator();

    //-----------------------------------------------------------------------
    
Gets the singleton instance of a ComparableComparator.

Developers are encouraged to use the comparator returned from this method
instead of constructing a new instance to reduce allocation and GC overhead
when multiple comparable comparators may be used in the same VM.
Returns:
the singleton ComparableComparator/**
     * Gets the singleton instance of a ComparableComparator.
     * <p>
     * Developers are encouraged to use the comparator returned from this method
     * instead of constructing a new instance to reduce allocation and GC overhead
     * when multiple comparable comparators may be used in the same VM.
     * 
     * @return the singleton ComparableComparator
     */
    public static ComparableComparator getInstance() {
        return instance;
    }

    //-----------------------------------------------------------------------
    
Constructor whose use should be avoided.
 Please use the getInstance() method whenever possible. /**
     * Constructor whose use should be avoided.
     * <p>
     * Please use the {@link #getInstance()} method whenever possible.
     */
    public ComparableComparator() {
        super();
    }

    //-----------------------------------------------------------------------
    
Compare the two Comparable arguments. This method is equivalent to: 
((Comparable)obj1).compareTo(obj2)
Params:
obj1 –  the first object to compare
obj2 –  the second object to compare
Throws:
NullPointerException – when obj1 is null, 
        or when ((Comparable)obj1).compareTo(obj2) does
ClassCastException – when obj1 is not a Comparable,
        or when ((Comparable)obj1).compareTo(obj2) does
Returns:
negative if obj1 is less, positive if greater, zero if equal/**
     * Compare the two {@link Comparable Comparable} arguments.
     * This method is equivalent to:
     * <pre>((Comparable)obj1).compareTo(obj2)</pre>
     * 
     * @param obj1  the first object to compare
     * @param obj2  the second object to compare
     * @return negative if obj1 is less, positive if greater, zero if equal
     * @throws NullPointerException when <i>obj1</i> is <code>null</code>, 
     *         or when <code>((Comparable)obj1).compareTo(obj2)</code> does
     * @throws ClassCastException when <i>obj1</i> is not a <code>Comparable</code>,
     *         or when <code>((Comparable)obj1).compareTo(obj2)</code> does
     */
    public int compare(Object obj1, Object obj2) {
        return ((Comparable)obj1).compareTo(obj2);
    }

    //-----------------------------------------------------------------------
    
Implement a hash code for this comparator that is consistent with equals. 
Returns:
a hash code for this comparator.
Since:
Commons Collections 3.0/**
     * Implement a hash code for this comparator that is consistent with
     * {@link #equals(Object) equals}.
     *
     * @return a hash code for this comparator.
     * @since Commons Collections 3.0
     */
    public int hashCode() {
        return "ComparableComparator".hashCode();
    }

    
Returns true iff that Object is is a Comparator whose ordering is known to be equivalent to mine. 

This implementation returns true
iff object.getClass()
equals this.getClass(). Subclasses may want to override this behavior to remain consistent with the Comparator.equals(Object) contract. 
Params:
object –  the object to compare with
Returns:
true if equal
Since:
Commons Collections 3.0/**
     * Returns <code>true</code> iff <i>that</i> Object is 
     * is a {@link Comparator Comparator} whose ordering is 
     * known to be equivalent to mine.
     * <p>
     * This implementation returns <code>true</code>
     * iff <code><i>object</i>.{@link Object#getClass() getClass()}</code>
     * equals <code>this.getClass()</code>.
     * Subclasses may want to override this behavior to remain consistent
     * with the {@link Comparator#equals(Object)} contract.
     * 
     * @param object  the object to compare with
     * @return true if equal
     * @since Commons Collections 3.0
     */
    public boolean equals(Object object) {
        return (this == object) || 
               ((null != object) && (object.getClass().equals(this.getClass())));
    }

}