    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}}.
    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 * <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}.
    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 *
    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     * <i>(Violation)</i>
    051     * Adds one copy of the specified object to the Bag.
    052     * <p>
    053     * If the object is already in the {@link #uniqueSet()} then increment its
    054     * count as reported by {@link #getCount(Object)}. Otherwise add it to the
    055     * {@link #uniqueSet()} and report its count as 1.
    056     * </p>
    057     * <p>
    058     * Since this method always increases the size of the bag,
    059     * according to the {@link Collection#add(Object)} contract, it
    060     * should always return {@code true}.  Since it sometimes returns
    061     * {@code false}, this method violates the contract.
    062     * </p>
    063     *
    064     * @param object  the object to add
    065     * @return {@code true} if the object was not already in the {@code uniqueSet}
    066     */
    067    @Override
    068    boolean add(E object);
    069
    070    /**
    071     * Adds {@code nCopies} copies of the specified object to the Bag.
    072     * <p>
    073     * If the object is already in the {@link #uniqueSet()} then increment its
    074     * count as reported by {@link #getCount(Object)}. Otherwise add it to the
    075     * {@link #uniqueSet()} and report its count as {@code nCopies}.
    076     * </p>
    077     *
    078     * @param object  the object to add
    079     * @param nCopies  the number of copies to add
    080     * @return {@code true} if the object was not already in the {@code uniqueSet}
    081     */
    082    boolean add(E object, int nCopies);
    083
    084    /**
    085     * <i>(Violation)</i>
    086     * Returns {@code true} if the bag contains all elements in
    087     * the given collection, respecting cardinality.  That is, if the
    088     * given collection {@code coll} contains {@code n} copies
    089     * of a given object, calling {@link #getCount(Object)} on that object must
    090     * be {@code &gt;= n} for all {@code n} in {@code coll}.
    091     *
    092     * <p>
    093     * The {@link Collection#containsAll(Collection)} method specifies
    094     * that cardinality should <i>not</i> be respected; this method should
    095     * return true if the bag contains at least one of every object contained
    096     * in the given collection.
    097     * </p>
    098     *
    099     * @param coll  the collection to check against
    100     * @return {@code true} if the Bag contains all the collection
    101     */
    102    @Override
    103    boolean containsAll(Collection<?> coll);
    104
    105    /**
    106     * Returns the number of occurrences (cardinality) of the given
    107     * object currently in the bag. If the object does not exist in the
    108     * bag, return 0.
    109     *
    110     * @param object  the object to search for
    111     * @return the number of occurrences of the object, zero if not found
    112     */
    113    int getCount(Object object);
    114
    115    /**
    116     * Returns an {@link Iterator} over the entire set of members,
    117     * including copies due to cardinality. This iterator is fail-fast
    118     * and will not tolerate concurrent modifications.
    119     *
    120     * @return iterator over all elements in the Bag
    121     */
    122    @Override
    123    Iterator<E> iterator();
    124
    125    /**
    126     * <i>(Violation)</i>
    127     * Removes all occurrences of the given object from the bag.
    128     * <p>
    129     * This will also remove the object from the {@link #uniqueSet()}.
    130     * </p>
    131     * <p>
    132     * According to the {@link Collection#remove(Object)} method,
    133     * this method should only remove the <i>first</i> occurrence of the
    134     * given object, not <i>all</i> occurrences.
    135     * </p>
    136     *
    137     * @param object  the object to remove
    138     * @return {@code true} if this call changed the collection
    139     */
    140    @Override
    141    boolean remove(Object object);
    142
    143    /**
    144     * Removes {@code nCopies} copies of the specified object from the Bag.
    145     * <p>
    146     * If the number of copies to remove is greater than the actual number of
    147     * copies in the Bag, no error is thrown.
    148     * </p>
    149     *
    150     * @param object  the object to remove
    151     * @param nCopies  the number of copies to remove
    152     * @return {@code true} if this call changed the collection
    153     */
    154    boolean remove(Object object, int nCopies);
    155
    156    /**
    157     * <i>(Violation)</i>
    158     * Remove all elements represented in the given collection,
    159     * respecting cardinality.  That is, if the given collection
    160     * {@code coll} contains {@code n} copies of a given object,
    161     * the bag will have {@code n} fewer copies, assuming the bag
    162     * had at least {@code n} copies to begin with.
    163     *
    164     * <p>
    165     * The {@link Collection#removeAll(Collection)} method specifies
    166     * that cardinality should <i>not</i> be respected; this method should
    167     * remove <i>all</i> occurrences of every object contained in the
    168     * given collection.
    169     * </p>
    170     *
    171     * @param coll  the collection to remove
    172     * @return {@code true} if this call changed the collection
    173     */
    174    @Override
    175    boolean removeAll(Collection<?> coll);
    176
    177    /**
    178     * <i>(Violation)</i>
    179     * Remove any members of the bag that are not in the given
    180     * collection, respecting cardinality.  That is, if the given
    181     * collection {@code coll} contains {@code n} copies of a
    182     * given object and the bag has {@code m &gt; n} copies, then
    183     * delete {@code m - n} copies from the bag.  In addition, if
    184     * {@code e} is an object in the bag but
    185     * {@code !coll.contains(e)}, then remove {@code e} and any
    186     * of its copies.
    187     *
    188     * <p>
    189     * The {@link Collection#retainAll(Collection)} method specifies
    190     * that cardinality should <i>not</i> be respected; this method should
    191     * keep <i>all</i> occurrences of every object contained in the
    192     * given collection.
    193     * </p>
    194     *
    195     * @param coll  the collection to retain
    196     * @return {@code true} if this call changed the collection
    197     */
    198    @Override
    199    boolean retainAll(Collection<?> coll);
    200
    201    /**
    202     * Returns the total number of items in the bag across all types.
    203     *
    204     * @return the total size of the Bag
    205     */
    206    @Override
    207    int size();
    208
    209    /**
    210     * Returns a {@link Set} of unique elements in the Bag.
    211     * <p>
    212     * Uniqueness constraints are the same as those in {@link java.util.Set}.
    213     * </p>
    214     *
    215     * @return the Set of unique Bag elements
    216     */
    217    Set<E> uniqueSet();
    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()) ^ noOccurrences)}.
    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}