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 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}</code>.
028 * Calling {@link #getCount(Object)} on <code>a</code> would return 2, while
029 * calling {@link #uniqueSet()} would return <code>{a, b, c}</code>.
030 * </p>
031 * <p>
032 * <i>NOTE: This interface violates the {@link Collection} contract.</i>
033 * The behavior specified in many of these methods is <i>not</i> the same
034 * as the behavior specified by <code>Collection</code>.
035 * The noncompliant methods are clearly marked with "(Violation)".
036 * Exercise caution when using a bag as a <code>Collection</code>.
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 *
044 * @param <E> the type of elements in this bag
045 * @since 2.0
046 */
047public interface Bag<E> extends Collection<E> {
048
049    /**
050     * Returns the number of occurrences (cardinality) of the given
051     * object currently in the bag. If the object does not exist in the
052     * bag, return 0.
053     *
054     * @param object  the object to search for
055     * @return the number of occurrences of the object, zero if not found
056     */
057    int getCount(Object object);
058
059    /**
060     * <i>(Violation)</i>
061     * Adds one copy of the specified object to the Bag.
062     * <p>
063     * If the object is already in the {@link #uniqueSet()} then increment its
064     * count as reported by {@link #getCount(Object)}. Otherwise add it to the
065     * {@link #uniqueSet()} and report its count as 1.
066     * </p>
067     * <p>
068     * Since this method always increases the size of the bag,
069     * according to the {@link Collection#add(Object)} contract, it
070     * should always return <code>true</code>.  Since it sometimes returns
071     * <code>false</code>, this method violates the contract.
072     * </p>
073     *
074     * @param object  the object to add
075     * @return <code>true</code> if the object was not already in the <code>uniqueSet</code>
076     */
077    @Override
078    boolean add(E object);
079
080    /**
081     * Adds <code>nCopies</code> copies of the specified object to the Bag.
082     * <p>
083     * If the object is already in the {@link #uniqueSet()} then increment its
084     * count as reported by {@link #getCount(Object)}. Otherwise add it to the
085     * {@link #uniqueSet()} and report its count as <code>nCopies</code>.
086     * </p>
087     *
088     * @param object  the object to add
089     * @param nCopies  the number of copies to add
090     * @return <code>true</code> if the object was not already in the <code>uniqueSet</code>
091     */
092    boolean add(E object, int nCopies);
093
094    /**
095     * <i>(Violation)</i>
096     * Removes all occurrences of the given object from the bag.
097     * <p>
098     * This will also remove the object from the {@link #uniqueSet()}.
099     * </p>
100     * <p>
101     * According to the {@link Collection#remove(Object)} method,
102     * this method should only remove the <i>first</i> occurrence of the
103     * given object, not <i>all</i> occurrences.
104     * </p>
105     *
106     * @param object  the object to remove
107     * @return <code>true</code> if this call changed the collection
108     */
109    @Override
110    boolean remove(Object object);
111
112    /**
113     * Removes <code>nCopies</code> copies of the specified object from the Bag.
114     * <p>
115     * If the number of copies to remove is greater than the actual number of
116     * copies in the Bag, no error is thrown.
117     * </p>
118     *
119     * @param object  the object to remove
120     * @param nCopies  the number of copies to remove
121     * @return <code>true</code> if this call changed the collection
122     */
123    boolean remove(Object object, int nCopies);
124
125    /**
126     * Returns a {@link Set} of unique elements in the Bag.
127     * <p>
128     * Uniqueness constraints are the same as those in {@link java.util.Set}.
129     * </p>
130     *
131     * @return the Set of unique Bag elements
132     */
133    Set<E> uniqueSet();
134
135    /**
136     * Returns the total number of items in the bag across all types.
137     *
138     * @return the total size of the Bag
139     */
140    @Override
141    int size();
142
143    /**
144     * <i>(Violation)</i>
145     * Returns <code>true</code> if the bag contains all elements in
146     * the given collection, respecting cardinality.  That is, if the
147     * given collection <code>coll</code> contains <code>n</code> copies
148     * of a given object, calling {@link #getCount(Object)} on that object must
149     * be <code>&gt;= n</code> for all <code>n</code> in <code>coll</code>.
150     *
151     * <p>
152     * The {@link Collection#containsAll(Collection)} method specifies
153     * that cardinality should <i>not</i> be respected; this method should
154     * return true if the bag contains at least one of every object contained
155     * in the given collection.
156     * </p>
157     *
158     * @param coll  the collection to check against
159     * @return <code>true</code> if the Bag contains all the collection
160     */
161    @Override
162    boolean containsAll(Collection<?> coll);
163
164    /**
165     * <i>(Violation)</i>
166     * Remove all elements represented in the given collection,
167     * respecting cardinality.  That is, if the given collection
168     * <code>coll</code> contains <code>n</code> copies of a given object,
169     * the bag will have <code>n</code> fewer copies, assuming the bag
170     * had at least <code>n</code> copies to begin with.
171     *
172     * <p>
173     * The {@link Collection#removeAll(Collection)} method specifies
174     * that cardinality should <i>not</i> be respected; this method should
175     * remove <i>all</i> occurrences of every object contained in the
176     * given collection.
177     * </p>
178     *
179     * @param coll  the collection to remove
180     * @return <code>true</code> if this call changed the collection
181     */
182    @Override
183    boolean removeAll(Collection<?> coll);
184
185    /**
186     * <i>(Violation)</i>
187     * Remove any members of the bag that are not in the given
188     * collection, respecting cardinality.  That is, if the given
189     * collection <code>coll</code> contains <code>n</code> copies of a
190     * given object and the bag has <code>m &gt; n</code> copies, then
191     * delete <code>m - n</code> copies from the bag.  In addition, if
192     * <code>e</code> is an object in the bag but
193     * <code>!coll.contains(e)</code>, then remove <code>e</code> and any
194     * of its copies.
195     *
196     * <p>
197     * The {@link Collection#retainAll(Collection)} method specifies
198     * that cardinality should <i>not</i> be respected; this method should
199     * keep <i>all</i> occurrences of every object contained in the
200     * given collection.
201     * </p>
202     *
203     * @param coll  the collection to retain
204     * @return <code>true</code> if this call changed the collection
205     */
206    @Override
207    boolean retainAll(Collection<?> coll);
208
209    /**
210     * Returns an {@link Iterator} over the entire set of members,
211     * including copies due to cardinality. This iterator is fail-fast
212     * and will not tolerate concurrent modifications.
213     *
214     * @return iterator over all elements in the Bag
215     */
216    @Override
217    Iterator<E> iterator();
218
219    // The following is not part of the formal Bag interface, however where possible
220    // Bag implementations should follow these comments.
221//    /**
222//     * Compares this Bag to another.
223//     * This Bag equals another Bag if it contains the same number of occurrences of
224//     * the same elements.
225//     * This equals definition is compatible with the Set interface.
226//     *
227//     * @param obj  the Bag to compare to
228//     * @return true if equal
229//     */
230//    boolean equals(Object obj);
231//
232//    /**
233//     * Gets a hash code for the Bag compatible with the definition of equals.
234//     * The hash code is defined as the sum total of a hash code for each element.
235//     * The per element hash code is defined as
236//     * <code>(e==null ? 0 : e.hashCode()) ^ noOccurances)</code>.
237//     * This hash code definition is compatible with the Set interface.
238//     *
239//     * @return the hash code of the Bag
240//     */
241//    int hashCode();
242
243}