/*
* Copyright 2001-2004 The Apache Software Foundation
*
* Licensed 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,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.collections.comparators;
import java.io.Serializable;
import java.util.Comparator;
/**
* 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: 155406 $ $Date: 2005-02-26 12:55:26 +0000 (Sat, 26 Feb 2005) $
*
* @author Henri Yandell
*
* @see java.util.Collections#reverseOrder()
*/
public class ComparableComparator implements Comparator, Serializable {
/** Serialization version. */
private static final long serialVersionUID=-291439688585137865L;
/** The singleton instance. */
private static final ComparableComparator instance = new 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.
* <p>
* Please use the {@link #getInstance()} method whenever possible.
*/
public ComparableComparator() {
super();
}
//-----------------------------------------------------------------------
/**
* 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
* {@link #equals(Object) equals}.
*
* @return a hash code for this comparator.
* @since Commons Collections 3.0
*/
public int hashCode() {
return "ComparableComparator".hashCode();
}
/**
* 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())));
}
}