Returns a Collection containing the intersection of the given Collection s. - Android java.util

Description

Returns a Collection containing the intersection of the given Collection s.

Demo Code

/*//from   w  ww .jav  a  2  s. co  m
 * Copyright 1999-2004 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 com.java2s;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;

import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;

public class Main {
    /**
     * 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.
     *
     * @see Collection#retainAll
     * @see #containsAny
     */
    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 occurances of that element
     * in the {@link Collection}.
     * An entry that maps to <tt>null</tt> indicates that the
     * element does not appear in the given {@link Collection}.
     */
    public static Map getCardinalityMap(final Collection col) {
        HashMap count = new HashMap();
        Iterator it = col.iterator();
        while (it.hasNext()) {
            Object obj = it.next();
            Integer c = (Integer) (count.get(obj));
            if (null == c) {
                count.put(obj, new Integer(1));
            } else {
                count.put(obj, new Integer(c.intValue() + 1));
            }
        }
        return count;
    }

    /**
     * Adds all elements in the iteration to the given collection.
     * 
     * @param collection  the collection to add to
     * @param iterator  the iterator of elements to add, may not be null
     * @throws NullPointerException if the collection or iterator is null
     */
    public static void addAll(Collection collection, Iterator iterator) {
        while (iterator.hasNext()) {
            collection.add(iterator.next());
        }
    }

    /**
     * Adds all elements in the enumeration to the given collection.
     * 
     * @param collection  the collection to add to
     * @param enumeration  the enumeration of elements to add, may not be null
     * @throws NullPointerException if the collection or enumeration is null
     */
    public static void addAll(Collection collection, Enumeration enumeration) {
        while (enumeration.hasMoreElements()) {
            collection.add(enumeration.nextElement());
        }
    }

    /** 
     * Adds all elements in the array to the given collection.
     * 
     * @param collection  the collection to add to
     * @param elements  the array of elements to add, may be null
     * @throws NullPointerException if the collection or array is null
     */
    public static void addAll(Collection collection, Object[] elements) {
        for (int i = 0, size = elements.length; i < size; i++) {
            collection.add(elements[i]);
        }
    }

    private static final int getFreq(final Object obj, final Map freqMap) {
        try {
            return ((Integer) (freqMap.get(obj))).intValue();
        } catch (NullPointerException e) {
            // ignored
        } catch (NoSuchElementException e) {
            // ignored
        }
        return 0;
    }
}

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