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Commons Collections example source code file (FixedOrderComparator.java)

This example Commons Collections source code file (FixedOrderComparator.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Commons Collections tags/keywords

attempting, fixedordercomparator, fixedordercomparator, illegalargumentexception, illegalargumentexception, integer, integer, object, object, the, unknown_after, unknown_before, unknown_throw_exception, unsupportedoperationexception, util

The Commons Collections FixedOrderComparator.java source code

/*
 *  Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  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,
 *  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 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: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
 *
 * @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);
        }
    }

}

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