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 *      http://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;
018
019import org.apache.commons.collections4.multiset.HashMultiSet;
020import org.apache.commons.collections4.multiset.PredicatedMultiSet;
021import org.apache.commons.collections4.multiset.SynchronizedMultiSet;
022import org.apache.commons.collections4.multiset.UnmodifiableMultiSet;
023
024/**
025 * Provides utility methods and decorators for {@link MultiSet} instances.
026 *
027 * @since 4.1
028 */
029public class MultiSetUtils {
030
031    /**
032     * An empty unmodifiable multiset.
033     */
034    @SuppressWarnings("rawtypes") // OK, empty multiset is compatible with any type
035    public static final MultiSet EMPTY_MULTISET =
036        UnmodifiableMultiSet.unmodifiableMultiSet(new HashMultiSet<>());
037
038    /**
039     * Gets an empty {@code MultiSet}.
040     *
041     * @param <E> the element type
042     * @return an empty MultiSet
043     */
044    @SuppressWarnings("unchecked") // OK, empty multiset is compatible with any type
045    public static <E> MultiSet<E> emptyMultiSet() {
046        return EMPTY_MULTISET;
047    }
048
049    /**
050     * Returns a predicated (validating) multiset backed by the given multiset.
051     * <p>
052     * Only objects that pass the test in the given predicate can be added to
053     * the multiset. Trying to add an invalid object results in an
054     * IllegalArgumentException. It is important not to use the original multiset
055     * after invoking this method, as it is a backdoor for adding invalid
056     * objects.
057     *
058     * @param <E> the element type
059     * @param multiset the multiset to predicate, must not be null
060     * @param predicate the predicate for the multiset, must not be null
061     * @return a predicated multiset backed by the given multiset
062     * @throws NullPointerException if the MultiSet or Predicate is null
063     */
064    public static <E> MultiSet<E> predicatedMultiSet(final MultiSet<E> multiset,
065            final Predicate<? super E> predicate) {
066        return PredicatedMultiSet.predicatedMultiSet(multiset, predicate);
067    }
068
069    /**
070     * Returns a synchronized (thread-safe) multiset backed by the given multiset.
071     * In order to guarantee serial access, it is critical that all access to the
072     * backing multiset is accomplished through the returned multiset.
073     * <p>
074     * It is imperative that the user manually synchronize on the returned multiset
075     * when iterating over it:
076     *
077     * <pre>
078     * MultiSet multiset = MultiSetUtils.synchronizedMultiSet(new HashMultiSet());
079     * ...
080     * synchronized(multiset) {
081     *     Iterator i = multiset.iterator(); // Must be in synchronized block
082     *     while (i.hasNext())
083     *         foo(i.next());
084     *     }
085     * }
086     * </pre>
087     *
088     * Failure to follow this advice may result in non-deterministic behavior.
089     *
090     * @param <E> the element type
091     * @param multiset the multiset to synchronize, must not be null
092     * @return a synchronized multiset backed by that multiset
093     * @throws NullPointerException if the MultiSet is null
094     */
095    public static <E> MultiSet<E> synchronizedMultiSet(final MultiSet<E> multiset) {
096        return SynchronizedMultiSet.synchronizedMultiSet(multiset);
097    }
098
099    /**
100     * Returns an unmodifiable view of the given multiset. Any modification attempts
101     * to the returned multiset will raise an {@link UnsupportedOperationException}.
102     *
103     * @param <E> the element type
104     * @param multiset the multiset whose unmodifiable view is to be returned, must not be null
105     * @return an unmodifiable view of that multiset
106     * @throws NullPointerException if the MultiSet is null
107     */
108    public static <E> MultiSet<E> unmodifiableMultiSet(final MultiSet<? extends E> multiset) {
109        return UnmodifiableMultiSet.unmodifiableMultiSet(multiset);
110    }
111
112    /**
113     * Don't allow instances.
114     */
115    private MultiSetUtils() {}
116
117}