001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      https://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.collections4.multiset;
018
019import java.io.IOException;
020import java.io.ObjectInputStream;
021import java.io.ObjectOutputStream;
022import java.io.Serializable;
023import java.util.Collection;
024import java.util.HashMap;
025
026/**
027 * Implements {@code MultiSet}, using a {@link HashMap} to provide the
028 * data storage. This is the standard implementation of a multiset.
029 * <p>
030 * A {@code MultiSet} stores each object in the collection together with a
031 * count of occurrences. Extra methods on the interface allow multiple copies
032 * of an object to be added or removed at once.
033 * </p>
034 * <p>
035 * <strong>Note that HashMultiSet is not synchronized and is not thread-safe.</strong>
036 * If you wish to use this multiset from multiple threads concurrently, you must use
037 * appropriate synchronization. The simplest approach is to wrap this multiset using
038 * {@link org.apache.commons.collections4.MultiSetUtils#synchronizedMultiSet(org.apache.commons.collections4.MultiSet)
039 * MultiSetUtils.synchronizedMultiSet(MultiSet)}.
040 * Unsynchronized concurrent modification can corrupt the structure of the backing
041 * {@link HashMap}, which may cause subsequent operations to throw exceptions,
042 * return incorrect results, or loop indefinitely.
043 * </p>
044 *
045 * @param <E> The type held in the multiset
046 * @since 4.1
047 */
048public class HashMultiSet<E> extends AbstractMapMultiSet<E> implements Serializable {
049
050    /** Serial version lock */
051    private static final long serialVersionUID = 20150610L;
052
053    /**
054     * Constructs an empty {@link HashMultiSet}.
055     */
056    public HashMultiSet() {
057        super(new HashMap<>());
058    }
059
060    /**
061     * Constructs a multiset containing all the members of the given collection.
062     *
063     * @param coll  A collection to copy into this multiset
064     */
065    public HashMultiSet(final Collection<? extends E> coll) {
066        this();
067        addAll(coll);
068    }
069
070    /**
071     * Constructs a multiset containing all the members of the given Iterable.
072     *
073     * @param iterable An iterable to copy into this multiset.
074     * @since 4.6.0
075     */
076    public HashMultiSet(final Iterable<? extends E> iterable) {
077        super(new HashMap<>(), iterable);
078    }
079
080    /**
081     * Deserializes the multiset in using a custom routine.
082     *
083     * @param in The input stream
084     * @throws IOException Thrown if an error occurs while reading from the stream
085     * @throws ClassNotFoundException if an object read from the stream cannot be loaded
086     */
087    private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
088        in.defaultReadObject();
089        setMap(new HashMap<>());
090        super.doReadObject(in);
091    }
092
093    /**
094     * Serializes this object to an ObjectOutputStream.
095     *
096     * @param out The target ObjectOutputStream.
097     * @throws IOException thrown when an I/O errors occur writing to the target stream.
098     */
099    private void writeObject(final ObjectOutputStream out) throws IOException {
100        out.defaultWriteObject();
101        super.doWriteObject(out);
102    }
103
104}