1 /*******************************************************************************

     2  * Copyright (c) 2000, 2005 IBM Corporation and others.

     3  * All rights reserved. This program and the accompanying materials

     4  * are made available under the terms of the Eclipse Public License v1.0

     5  * which accompanies this distribution, and is available at

     6  * http://www.eclipse.org/legal/epl-v10.html

     7  *

     8  * Contributors:

     9  *     IBM Corporation - initial API and implementation

    10  *******************************************************************************/

    11 package org.eclipse.core.internal.watson;

    12 

    13 /**

    14  * This is what you would expect for an element tree comparator.

    15  * Clients of the element tree that want specific comparison behaviour

    16  * must define their own element comparator (without subclassing or

    17  * otherwise extending this comparator).  Internal element tree operations 

    18  * rely on the behaviour of this type, and the ElementTree maintainer 

    19  * reserves the right to change its behaviour as necessary.

    20  */

    21 public final class DefaultElementComparator implements IElementComparator {

    22 	private static DefaultElementComparator singleton;

    23 

    24 	/**

    25 	 * Force clients to use the singleton

    26 	 */

    27 	protected DefaultElementComparator() {

    28 		super();

    29 	}

    30 

    31 	/**

    32 	 * Returns the type of change.

    33 	 */

    34 	public int compare(Object oldInfo, Object newInfo) {

    35 		if (oldInfo == null && newInfo == null)

    36 			return 0;

    37 		if (oldInfo == null || newInfo == null)

    38 			return 1;

    39 		return testEquality(oldInfo, newInfo) ? 0 : 1;

    40 	}

    41 

    42 	/**

    43 	 * Returns the singleton instance

    44 	 */

    45 	public static IElementComparator getComparator() {

    46 		if (singleton == null) {

    47 			singleton = new DefaultElementComparator();

    48 		}

    49 		return singleton;

    50 	}

    51 

    52 	/**

    53 	 * Makes a comparison based on equality

    54 	 */

    55 	protected boolean testEquality(Object oldInfo, Object newInfo) {

    56 		if (oldInfo == null && newInfo == null)

    57 			return true;

    58 		if (oldInfo == null || newInfo == null)

    59 			return false;

    60 

    61 		return oldInfo.equals(newInfo);

    62 	}

    63 }