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 // empty 117 } 118 119}