/*
* 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.util.Comparator;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
/**
* A Comparator which imposes a specific order on a specific set of Objects.
* Objects are presented to the FixedOrderComparator in a specified order and
* subsequent calls to {@link #compare(Object, Object) compare} yield that order.
* For example:
* <pre>
* String[] planets = {"Mercury", "Venus", "Earth", "Mars"};
* FixedOrderComparator distanceFromSun = new FixedOrderComparator(planets);
* Arrays.sort(planets); // Sort to alphabetical order
* Arrays.sort(planets, distanceFromSun); // Back to original order
* </pre>
* <p>
* Once <code>compare</code> has been called, the FixedOrderComparator is locked
* and attempts to modify it yield an UnsupportedOperationException.
* <p>
* Instances of FixedOrderComparator are not synchronized. The class is not
* thread-safe at construction time, but it is thread-safe to perform
* multiple comparisons after all the setup operations are complete.
*
* @since Commons Collections 3.0
* @version $Revision: 155406 $ $Date: 2005-02-26 12:55:26 +0000 (Sat, 26 Feb 2005) $
*
* @author David Leppik
* @author Stephen Colebourne
* @author Janek Bogucki
*/
public class FixedOrderComparator implements Comparator {
/**
* Behavior when comparing unknown Objects:
* unknown objects compare as before known Objects.
*/
public static final int UNKNOWN_BEFORE = 0;
/**
* Behavior when comparing unknown Objects:
* unknown objects compare as after known Objects.
*/
public static final int UNKNOWN_AFTER = 1;
/**
* Behavior when comparing unknown Objects:
* unknown objects cause a IllegalArgumentException to be thrown.
* This is the default behavior.
*/
public static final int UNKNOWN_THROW_EXCEPTION = 2;
/** Internal map of object to position */
private final Map map = new HashMap();
/** Counter used in determining the position in the map */
private int counter = 0;
/** Is the comparator locked against further change */
private boolean isLocked = false;
/** The behaviour in the case of an unknown object */
private int unknownObjectBehavior = UNKNOWN_THROW_EXCEPTION;
// Constructors
//-----------------------------------------------------------------------
/**
* Constructs an empty FixedOrderComparator.
*/
public FixedOrderComparator() {
super();
}
/**
* Constructs a FixedOrderComparator which uses the order of the given array
* to compare the objects.
* <p>
* The array is copied, so later changes will not affect the comparator.
*
* @param items the items that the comparator can compare in order
* @throws IllegalArgumentException if the array is null
*/
public FixedOrderComparator(Object[] items) {
super();
if (items == null) {
throw new IllegalArgumentException("The list of items must not be null");
}
for (int i = 0; i < items.length; i++) {
add(items[i]);
}
}
/**
* Constructs a FixedOrderComparator which uses the order of the given list
* to compare the objects.
* <p>
* The list is copied, so later changes will not affect the comparator.
*
* @param items the items that the comparator can compare in order
* @throws IllegalArgumentException if the list is null
*/
public FixedOrderComparator(List items) {
super();
if (items == null) {
throw new IllegalArgumentException("The list of items must not be null");
}
for (Iterator it = items.iterator(); it.hasNext();) {
add(it.next());
}
}
// Bean methods / state querying methods
//-----------------------------------------------------------------------
/**
* Returns true if modifications cannot be made to the FixedOrderComparator.
* FixedOrderComparators cannot be modified once they have performed a comparison.
*
* @return true if attempts to change the FixedOrderComparator yield an
* UnsupportedOperationException, false if it can be changed.
*/
public boolean isLocked() {
return isLocked;
}
/**
* Checks to see whether the comparator is now locked against further changes.
*
* @throws UnsupportedOperationException if the comparator is locked
*/
protected void checkLocked() {
if (isLocked()) {
throw new UnsupportedOperationException("Cannot modify a FixedOrderComparator after a comparison");
}
}
/**
* Gets the behavior for comparing unknown objects.
*
* @return the flag for unknown behaviour - UNKNOWN_AFTER,
* UNKNOWN_BEFORE or UNKNOWN_THROW_EXCEPTION
*/
public int getUnknownObjectBehavior() {
return unknownObjectBehavior;
}
/**
* Sets the behavior for comparing unknown objects.
*
* @param unknownObjectBehavior the flag for unknown behaviour -
* UNKNOWN_AFTER, UNKNOWN_BEFORE or UNKNOWN_THROW_EXCEPTION
* @throws UnsupportedOperationException if a comparison has been performed
* @throws IllegalArgumentException if the unknown flag is not valid
*/
public void setUnknownObjectBehavior(int unknownObjectBehavior) {
checkLocked();
if (unknownObjectBehavior != UNKNOWN_AFTER
&& unknownObjectBehavior != UNKNOWN_BEFORE
&& unknownObjectBehavior != UNKNOWN_THROW_EXCEPTION) {
throw new IllegalArgumentException("Unrecognised value for unknown behaviour flag");
}
this.unknownObjectBehavior = unknownObjectBehavior;
}
// Methods for adding items
//-----------------------------------------------------------------------
/**
* Adds an item, which compares as after all items known to the Comparator.
* If the item is already known to the Comparator, its old position is
* replaced with the new position.
*
* @param obj the item to be added to the Comparator.
* @return true if obj has been added for the first time, false if
* it was already known to the Comparator.
* @throws UnsupportedOperationException if a comparison has already been made
*/
public boolean add(Object obj) {
checkLocked();
Object position = map.put(obj, new Integer(counter++));
return (position == null);
}
/**
* Adds a new item, which compares as equal to the given existing item.
*
* @param existingObj an item already in the Comparator's set of
* known objects
* @param newObj an item to be added to the Comparator's set of
* known objects
* @return true if newObj has been added for the first time, false if
* it was already known to the Comparator.
* @throws IllegalArgumentException if existingObject is not in the
* Comparator's set of known objects.
* @throws UnsupportedOperationException if a comparison has already been made
*/
public boolean addAsEqual(Object existingObj, Object newObj) {
checkLocked();
Integer position = (Integer) map.get(existingObj);
if (position == null) {
throw new IllegalArgumentException(existingObj + " not known to " + this);
}
Object result = map.put(newObj, position);
return (result == null);
}
// Comparator methods
//-----------------------------------------------------------------------
/**
* Compares two objects according to the order of this Comparator.
* <p>
* It is important to note that this class will throw an IllegalArgumentException
* in the case of an unrecognised object. This is not specified in the
* Comparator interface, but is the most appropriate exception.
*
* @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 IllegalArgumentException if obj1 or obj2 are not known
* to this Comparator and an alternative behavior has not been set
* via {@link #setUnknownObjectBehavior(int)}.
*/
public int compare(Object obj1, Object obj2) {
isLocked = true;
Integer position1 = (Integer) map.get(obj1);
Integer position2 = (Integer) map.get(obj2);
if (position1 == null || position2 == null) {
switch (unknownObjectBehavior) {
case UNKNOWN_BEFORE :
if (position1 == null) {
return (position2 == null) ? 0 : -1;
} else {
return 1;
}
case UNKNOWN_AFTER :
if (position1 == null) {
return (position2 == null) ? 0 : 1;
} else {
return -1;
}
case UNKNOWN_THROW_EXCEPTION :
Object unknownObj = (position1 == null) ? obj1 : obj2;
throw new IllegalArgumentException("Attempting to compare unknown object " + unknownObj);
default :
throw new UnsupportedOperationException("Unknown unknownObjectBehavior: " + unknownObjectBehavior);
}
} else {
return position1.compareTo(position2);
}
}
}