CollectionUtils.java

/*
 * Copyright 2014 Red Hat, Inc. and/or its affiliates.
 *
 * 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.dashbuilder.common.client;

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class CollectionUtils {

    /** Constant to avoid repeated object creation */
    private static Integer INTEGER_ONE = new Integer(1);

    /**
     * Returns a {@link Collection} containing the intersection
     * of the given {@link Collection}s.
     * <p>
     * The cardinality of each element in the returned {@link Collection}
     * will be equal to the minimum of the cardinality of that element
     * in the two given {@link Collection}s.
     *
     * @param a  the first collection, must not be null
     * @param b  the second collection, must not be null
     * @return the intersection of the two collections
     */
    public static Collection intersection(final Collection a, final Collection b) {
        ArrayList list = new ArrayList();
        Map mapa = getCardinalityMap(a);
        Map mapb = getCardinalityMap(b);
        Set elts = new HashSet(a);
        elts.addAll(b);
        Iterator it = elts.iterator();
        while(it.hasNext()) {
            Object obj = it.next();
            for(int i=0,m=Math.min(getFreq(obj,mapa),getFreq(obj,mapb));i<m;i++) {
                list.add(obj);
            }
        }
        return list;
    }

    /**
     * Returns a {@link Map} mapping each unique element in the given
     * {@link Collection} to an {@link Integer} representing the number
     * of occurrences of that element in the {@link Collection}.
     * <p>
     * Only those elements present in the collection will appear as
     * keys in the map.
     *
     * @param coll  the collection to get the cardinality map for, must not be null
     * @return the populated cardinality map
     */
    public static Map getCardinalityMap(final Collection coll) {
        Map count = new HashMap();
        for (Iterator it = coll.iterator(); it.hasNext();) {
            Object obj = it.next();
            Integer c = (Integer) (count.get(obj));
            if (c == null) {
                count.put(obj,INTEGER_ONE);
            } else {
                count.put(obj,new Integer(c.intValue() + 1));
            }
        }
        return count;
    }

    private static final int getFreq(final Object obj, final Map freqMap) {
        Integer count = (Integer) freqMap.get(obj);
        if (count != null) {
            return count.intValue();
        }
        return 0;
    }

}