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;
018
019import java.util.Collection;
020import java.util.Iterator;
021import java.util.Set;
022
023/**
024 * Defines a collection that counts the number of times an object appears in
025 * the collection.
026 * <p>
027 * Suppose you have a Bag that contains {@code {a, a, b, c}}.
028 * Calling {@link #getCount(Object)} on {@code a} would return 2, while
029 * calling {@link #uniqueSet()} would return {@code {a, b, c}}.
030 * </p>
031 * <p>
032 * <em>NOTE: This interface violates the {@link Collection} contract.</em>
033 * The behavior specified in many of these methods is <em>not</em> the same
034 * as the behavior specified by {@code Collection}.
035 * The non-compliant methods are clearly marked with "(Violation)".
036 * Exercise caution when using a bag as a {@code Collection}.
037 * </p>
038 * <p>
039 * This violation resulted from the original specification of this interface.
040 * In an ideal world, the interface would be changed to fix the problems, however
041 * it has been decided to maintain backwards compatibility instead.
042 * </p>
043 * <p>
044 * The {@link MultiSet} interface, added in version 4.1, provides the same
045 * functionality while complying with the {@link Collection} contract, and
046 * should be preferred for new code. Existing code can migrate as follows,
047 * with the cardinality-respecting behavior preserved through explicitly
048 * named methods:
049 * </p>
050 * <ul>
051 *   <li>{@code bag.add(e)} becomes {@code multiSet.add(e)}
052 *       (which always returns {@code true}, per the {@code MultiSet} contract)</li>
053 *   <li>{@code bag.getCount(e)} becomes {@code multiSet.getCount(e)}</li>
054 *   <li>{@code bag.remove(e)}, which removes all copies, becomes
055 *       {@code multiSet.setCount(e, 0)}</li>
056 *   <li>{@code bag.containsAll(coll)} becomes
057 *       {@code MultiSetUtils.containsOccurrences(multiSet, new HashMultiSet<>(coll))}</li>
058 *   <li>{@code bag.removeAll(coll)} becomes
059 *       {@code MultiSetUtils.removeOccurrences(multiSet, new HashMultiSet<>(coll))}</li>
060 *   <li>{@code bag.retainAll(coll)} becomes
061 *       {@code MultiSetUtils.retainOccurrences(multiSet, new HashMultiSet<>(coll))}</li>
062 *   <li>{@link SortedBag} and {@code TreeBag} become {@link SortedMultiSet}
063 *       and {@code TreeMultiSet}</li>
064 *   <li>the {@code CollectionBag} and {@code CollectionSortedBag} wrappers are
065 *       not needed, as a {@code MultiSet} already complies with the
066 *       {@code Collection} contract</li>
067 * </ul>
068 *
069 * @param <E> The type of elements in this bag
070 * @see MultiSet
071 * @since 2.0
072 * @deprecated Since 4.6.0, use {@link MultiSet} instead; see the migration notes above.
073 */
074@Deprecated
075public interface Bag<E> extends Collection<E> {
076
077    /**
078     * <em>(Violation)</em>
079     * Adds one copy of the specified object to the Bag.
080     * <p>
081     * If the object is already in the {@link #uniqueSet()} then increment its
082     * count as reported by {@link #getCount(Object)}. Otherwise add it to the
083     * {@link #uniqueSet()} and report its count as 1.
084     * </p>
085     * <p>
086     * Since this method always increases the size of the bag,
087     * according to the {@link Collection#add(Object)} contract, it
088     * should always return {@code true}. Since it sometimes returns
089     * {@code false}, this method violates the contract.
090     * </p>
091     *
092     * @param object  The object to add.
093     * @return {@code true} if the object was not already in the {@code uniqueSet}.
094     */
095    @Override
096    boolean add(E object);
097
098    /**
099     * Adds {@code nCopies} copies of the specified object to the Bag.
100     * <p>
101     * If the object is already in the {@link #uniqueSet()} then increment its
102     * count as reported by {@link #getCount(Object)}. Otherwise add it to the
103     * {@link #uniqueSet()} and report its count as {@code nCopies}.
104     * </p>
105     *
106     * @param object  The object to add.
107     * @param nCopies  The number of copies to add.
108     * @return {@code true} if the object was not already in the {@code uniqueSet}.
109     * @throws ClassCastException if the class of the specified element prevents it from being added to this collection.
110     */
111    boolean add(E object, int nCopies);
112
113    /**
114     * <em>(Violation)</em>
115     * Returns {@code true} if the bag contains all elements in
116     * the given collection, respecting cardinality. That is, if the
117     * given collection {@code coll} contains {@code n} copies
118     * of a given object, calling {@link #getCount(Object)} on that object must
119     * be {@code &gt;= n} for all {@code n} in {@code coll}.
120     *
121     * <p>
122     * The {@link Collection#containsAll(Collection)} method specifies
123     * that cardinality should <em>not</em> be respected; this method should
124     * return true if the bag contains at least one of every object contained
125     * in the given collection.
126     * </p>
127     *
128     * @param coll  The collection to check against.
129     * @return {@code true} if the Bag contains all the collection.
130     */
131    @Override
132    boolean containsAll(Collection<?> coll);
133
134    /**
135     * Gets the number of occurrences (cardinality) of the given
136     * object currently in the bag. If the object does not exist in the
137     * bag, return 0.
138     *
139     * @param object  The object to search for.
140     * @return The number of occurrences of the object, zero if not found.
141     */
142    int getCount(Object object);
143
144    /**
145     * Returns an {@link Iterator} over the entire set of members,
146     * including copies due to cardinality. This iterator is fail-fast
147     * and will not tolerate concurrent modifications.
148     *
149     * @return iterator over all elements in the Bag.
150     */
151    @Override
152    Iterator<E> iterator();
153
154    /**
155     * <em>(Violation)</em>
156     * Removes all occurrences of the given object from the bag.
157     * <p>
158     * This will also remove the object from the {@link #uniqueSet()}.
159     * </p>
160     * <p>
161     * According to the {@link Collection#remove(Object)} method,
162     * this method should only remove the <em>first</em> occurrence of the
163     * given object, not <em>all</em> occurrences.
164     * </p>
165     *
166     * @param object  The object to remove.
167     * @return {@code true} if this call changed the collection.
168     */
169    @Override
170    boolean remove(Object object);
171
172    /**
173     * Removes {@code nCopies} copies of the specified object from the Bag.
174     * <p>
175     * If the number of copies to remove is greater than the actual number of
176     * copies in the Bag, no error is thrown.
177     * </p>
178     *
179     * @param object  The object to remove.
180     * @param nCopies  The number of copies to remove.
181     * @return {@code true} if this call changed the collection.
182     */
183    boolean remove(Object object, int nCopies);
184
185    /**
186     * <em>(Violation)</em>
187     * Remove all elements represented in the given collection,
188     * respecting cardinality. That is, if the given collection
189     * {@code coll} contains {@code n} copies of a given object,
190     * the bag will have {@code n} fewer copies, assuming the bag
191     * had at least {@code n} copies to begin with.
192     *
193     * <p>
194     * The {@link Collection#removeAll(Collection)} method specifies
195     * that cardinality should <em>not</em> be respected; this method should
196     * remove <em>all</em> occurrences of every object contained in the
197     * given collection.
198     * </p>
199     *
200     * @param coll  The collection to remove.
201     * @return {@code true} if this call changed the collection.
202     */
203    @Override
204    boolean removeAll(Collection<?> coll);
205
206    /**
207     * <em>(Violation)</em>
208     * Remove any members of the bag that are not in the given
209     * collection, respecting cardinality. That is, if the given
210     * collection {@code coll} contains {@code n} copies of a
211     * given object and the bag has {@code m > n} copies, then
212     * delete {@code m - n} copies from the bag. In addition, if
213     * {@code e} is an object in the bag but
214     * {@code !coll.contains(e)}, then remove {@code e} and any
215     * of its copies.
216     *
217     * <p>
218     * The {@link Collection#retainAll(Collection)} method specifies
219     * that cardinality should <em>not</em> be respected; this method should
220     * keep <em>all</em> occurrences of every object contained in the
221     * given collection.
222     * </p>
223     *
224     * @param coll  The collection to retain.
225     * @return {@code true} if this call changed the collection.
226     */
227    @Override
228    boolean retainAll(Collection<?> coll);
229
230    /**
231     * Returns the total number of items in the bag across all types.
232     *
233     * @return The total size of the Bag.
234     */
235    @Override
236    int size();
237
238    /**
239     * Returns a {@link Set} of unique elements in the Bag.
240     * <p>
241     * Uniqueness constraints are the same as those in {@link Set}.
242     * </p>
243     *
244     * @return The Set of unique Bag elements.
245     */
246    Set<E> uniqueSet();
247
248    // The following is not part of the formal Bag interface, however where possible
249    // Bag implementations should follow these comments.
250//    /**
251//     * Compares this Bag to another.
252//     * This Bag equals another Bag if it contains the same number of occurrences of
253//     * the same elements.
254//     * This equals definition is compatible with the Set interface.
255//     *
256//     * @param obj  The Bag to compare to
257//     * @return true if equal
258//     */
259//    boolean equals(Object obj);
260//
261//    /**
262//     * Gets a hash code for the Bag compatible with the definition of equals.
263//     * The hash code is defined as the sum total of a hash code for each element.
264//     * The per element hash code is defined as
265//     * {@code (e==null ? 0 : e.hashCode()) ^ noOccurrences)}.
266//     * This hash code definition is compatible with the Set interface.
267//     *
268//     * @return The hash code of the Bag
269//     */
270//    int hashCode();
271
272}