    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     * </p>
    058     *
    059     * @param <E> the element type
    060     * @param multiset the multiset to predicate, must not be null
    061     * @param predicate the predicate for the multiset, must not be null
    062     * @return a predicated multiset backed by the given multiset
    063     * @throws NullPointerException if the MultiSet or Predicate is null
    064     */
    065    public static <E> MultiSet<E> predicatedMultiSet(final MultiSet<E> multiset,
    066            final Predicate<? super E> predicate) {
    067        return PredicatedMultiSet.predicatedMultiSet(multiset, predicate);
    068    }
    069
    070    /**
    071     * Returns a synchronized (thread-safe) multiset backed by the given multiset.
    072     * In order to guarantee serial access, it is critical that all access to the
    073     * backing multiset is accomplished through the returned multiset.
    074     * <p>
    075     * It is imperative that the user manually synchronize on the returned multiset
    076     * when iterating over it:
    077     * </p>
    078     * <pre>
    079     * MultiSet multiset = MultiSetUtils.synchronizedMultiSet(new HashMultiSet());
    080     * ...
    081     * synchronized(multiset) {
    082     *     Iterator i = multiset.iterator(); // Must be in synchronized block
    083     *     while (i.hasNext())
    084     *         foo(i.next());
    085     *     }
    086     * }
    087     * </pre>
    088     *
    089     * Failure to follow this advice may result in non-deterministic behavior.
    090     *
    091     * @param <E> the element type
    092     * @param multiset the multiset to synchronize, must not be null
    093     * @return a synchronized multiset backed by that multiset
    094     * @throws NullPointerException if the MultiSet is null
    095     */
    096    public static <E> MultiSet<E> synchronizedMultiSet(final MultiSet<E> multiset) {
    097        return SynchronizedMultiSet.synchronizedMultiSet(multiset);
    098    }
    099
    100    /**
    101     * Returns an unmodifiable view of the given multiset. Any modification attempts
    102     * to the returned multiset will raise an {@link UnsupportedOperationException}.
    103     *
    104     * @param <E> the element type
    105     * @param multiset the multiset whose unmodifiable view is to be returned, must not be null
    106     * @return an unmodifiable view of that multiset
    107     * @throws NullPointerException if the MultiSet is null
    108     */
    109    public static <E> MultiSet<E> unmodifiableMultiSet(final MultiSet<? extends E> multiset) {
    110        return UnmodifiableMultiSet.unmodifiableMultiSet(multiset);
    111    }
    112
    113    /**
    114     * Don't allow instances.
    115     */
    116    private MultiSetUtils() {
    117        // empty
    118    }
    119
    120}