    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 MultiSet 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 *
    032 * @param <E> the type held in the multiset
    033 * @since 4.1
    034 */
    035public interface MultiSet<E> extends Collection<E> {
    036
    037    /**
    038     * An unmodifiable entry for an element and its occurrence as contained in a MultiSet.
    039     * <p>
    040     * The {@link MultiSet#entrySet()} method returns a view of the multiset whose elements
    041     * implement this interface.
    042     *
    043     * @param <E>  the element type
    044     */
    045    interface Entry<E> {
    046
    047        /**
    048         * Compares the specified object with this entry for equality.
    049         * Returns true if the given object is also a multiset entry
    050         * and the two entries represent the same element with the same
    051         * number of occurrences.
    052         * <p>
    053         * More formally, two entries {@code e1} and {@code e2} represent
    054         * the same mapping if
    055         * <pre>
    056         *     (e1.getElement()==null ? e2.getElement()==null
    057         *                            : e1.getElement().equals(e2.getElement())) &amp;&amp;
    058         *     (e1.getCount()==e2.getCount())
    059         * </pre>
    060         *
    061         * @param o object to be compared for equality with this multiset entry
    062         * @return true if the specified object is equal to this multiset entry
    063         */
    064        @Override
    065        boolean equals(Object o);
    066
    067        /**
    068         * Returns the number of occurrences for the element of this entry.
    069         *
    070         * @return the number of occurrences of the element
    071         */
    072        int getCount();
    073
    074        /**
    075         * Returns the element corresponding to this entry.
    076         *
    077         * @return the element corresponding to this entry
    078         */
    079        E getElement();
    080
    081        /**
    082         * Returns the hash code value for this multiset entry.
    083         * <p>
    084         * The hash code of a multiset entry {@code e} is defined to be:
    085         * <pre>
    086         *      (e==null ? 0 : e.hashCode()) ^ noOccurrences)
    087         * </pre>
    088         *
    089         * @return the hash code value for this multiset entry
    090         */
    091        @Override
    092        int hashCode();
    093    }
    094
    095    /**
    096     * Adds one copy of the specified object to the MultiSet.
    097     * <p>
    098     * If the object is already in the {@link #uniqueSet()} then increment its
    099     * count as reported by {@link #getCount(Object)}. Otherwise, add it to the
    100     * {@link #uniqueSet()} and report its count as 1.
    101     *
    102     * @param object  the object to add
    103     * @return {@code true} always, as the size of the MultiSet is increased
    104     *   in any case
    105     */
    106    @Override
    107    boolean add(E object);
    108
    109    /**
    110     * Adds a number of occurrences of the specified object to the MultiSet.
    111     * <p>
    112     * If the object is already in the {@link #uniqueSet()} then increment its
    113     * count as reported by {@link #getCount(Object)}. Otherwise, add it to the
    114     * {@link #uniqueSet()} and report its count as {@code occurrences}.
    115     *
    116     * @param object  the object to add
    117     * @param occurrences  the number of occurrences to add, may be zero,
    118     *   in which case no change is made to the multiset
    119     * @return the number of occurrences of the object in the multiset before
    120     *   this operation; possibly zero
    121     * @throws IllegalArgumentException if occurrences is negative
    122     */
    123    int add(E object, int occurrences);
    124
    125    /**
    126     * Returns {@code true} if the MultiSet contains at least one
    127     * occurrence for each element contained in the given collection.
    128     *
    129     * @param coll  the collection to check against
    130     * @return {@code true} if the MultiSet contains all the collection
    131     */
    132    @Override
    133    boolean containsAll(Collection<?> coll);
    134
    135    /**
    136     * Returns a {@link Set} of all entries contained in the MultiSet.
    137     * <p>
    138     * The returned set is backed by this multiset, so any change to either
    139     * is immediately reflected in the other.
    140     *
    141     * @return the Set of MultiSet entries
    142     */
    143    Set<Entry<E>> entrySet();
    144
    145    /**
    146     * Compares this MultiSet to another object.
    147     * <p>
    148     * This MultiSet equals another object if it is also a MultiSet
    149     * that contains the same number of occurrences of the same elements.
    150     *
    151     * @param obj  the object to compare to
    152     * @return true if equal
    153     */
    154    @Override
    155    boolean equals(Object obj);
    156
    157    /**
    158     * Returns the number of occurrences of the given object currently
    159     * in the MultiSet. If the object does not exist in the multiset,
    160     * return 0.
    161     *
    162     * @param object  the object to search for
    163     * @return the number of occurrences of the object, zero if not found
    164     */
    165    int getCount(Object object);
    166
    167    /**
    168     * Gets a hash code for the MultiSet compatible with the definition of equals.
    169     * The hash code is defined as the sum total of a hash code for each element.
    170     * The per element hash code is defined as
    171     * {@code (e==null ? 0 : e.hashCode()) ^ noOccurrences)}.
    172     *
    173     * @return the hash code of the MultiSet
    174     */
    175    @Override
    176    int hashCode();
    177
    178    /**
    179     * Returns an {@link Iterator} over the entire set of members,
    180     * including copies due to cardinality. This iterator is fail-fast
    181     * and will not tolerate concurrent modifications.
    182     *
    183     * @return iterator over all elements in the MultiSet
    184     */
    185    @Override
    186    Iterator<E> iterator();
    187
    188    /**
    189     * Removes one occurrence of the given object from the MultiSet.
    190     * <p>
    191     * If the number of occurrences after this operation is reduced
    192     * to zero, the object will be removed from the {@link #uniqueSet()}.
    193     *
    194     * @param object  the object to remove
    195     * @return {@code true} if this call changed the collection
    196     */
    197    @Override
    198    boolean remove(Object object);
    199
    200    /**
    201     * Removes a number of occurrences of the specified object from the MultiSet.
    202     * <p>
    203     * If the number of occurrences to remove is greater than the actual number of
    204     * occurrences in the multiset, the object will be removed from the multiset.
    205     *
    206     * @param object  the object to remove
    207     * @param occurrences  the number of occurrences to remove, may be zero,
    208     *   in which case no change is made to the multiset
    209     * @return the number of occurrences of the object in the multiset
    210     *   before the operation; possibly zero
    211     * @throws IllegalArgumentException if occurrences is negative
    212     */
    213    int remove(Object object, int occurrences);
    214
    215    /**
    216     * Remove all occurrences of all elements from this MultiSet represented
    217     * in the given collection.
    218     *
    219     * @param coll  the collection of elements to remove
    220     * @return {@code true} if this call changed the multiset
    221     */
    222    @Override
    223    boolean removeAll(Collection<?> coll);
    224
    225    /**
    226     * Remove any elements of this MultiSet that are not contained in the
    227     * given collection.
    228     *
    229     * @param coll  the collection of elements to retain
    230     * @return {@code true} if this call changed the multiset
    231     */
    232    @Override
    233    boolean retainAll(Collection<?> coll);
    234
    235    /**
    236     * Sets the number of occurrences of the specified object in the MultiSet
    237     * to the given count.
    238     * <p>
    239     * If the provided count is zero, the object will be removed from the
    240     * {@link #uniqueSet()}.
    241     *
    242     * @param object  the object to update
    243     * @param count  the number of occurrences of the object
    244     * @return the number of occurrences of the object before this operation, zero
    245     *   if the object was not contained in the multiset
    246     * @throws IllegalArgumentException if count is negative
    247     */
    248    int setCount(E object, int count);
    249
    250    /**
    251     * Returns the total number of items in the MultiSet.
    252     *
    253     * @return the total size of the multiset
    254     */
    255    @Override
    256    int size();
    257
    258    /**
    259     * Returns a {@link Set} of unique elements in the MultiSet.
    260     * <p>
    261     * Uniqueness constraints are the same as those in {@link java.util.Set}.
    262     * <p>
    263     * The returned set is backed by this multiset, so any change to either
    264     * is immediately reflected in the other. Only removal operations are
    265     * supported, in which case all occurrences of the element are removed
    266     * from the backing multiset.
    267     *
    268     * @return the Set of unique MultiSet elements
    269     */
    270    Set<E> uniqueSet();
    271
    272}