001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import static com.google.common.base.Preconditions.checkNotNull;
020
021import com.google.common.annotations.Beta;
022import com.google.common.annotations.GwtCompatible;
023import com.google.errorprone.annotations.CanIgnoreReturnValue;
024import com.google.errorprone.annotations.CompatibleWith;
025import java.util.Collection;
026import java.util.Collections;
027import java.util.Iterator;
028import java.util.List;
029import java.util.Set;
030import java.util.Spliterator;
031import java.util.function.Consumer;
032import java.util.function.ObjIntConsumer;
033import javax.annotation.CheckForNull;
034import org.checkerframework.checker.nullness.qual.Nullable;
035
036/**
037 * A collection that supports order-independent equality, like {@link Set}, but may have duplicate
038 * elements. A multiset is also sometimes called a <i>bag</i>.
039 *
040 * <p>Elements of a multiset that are equal to one another are referred to as <i>occurrences</i> of
041 * the same single element. The total number of occurrences of an element in a multiset is called
042 * the <i>count</i> of that element (the terms "frequency" and "multiplicity" are equivalent, but
043 * not used in this API). Since the count of an element is represented as an {@code int}, a multiset
044 * may never contain more than {@link Integer#MAX_VALUE} occurrences of any one element.
045 *
046 * <p>{@code Multiset} refines the specifications of several methods from {@code Collection}. It
047 * also defines an additional query operation, {@link #count}, which returns the count of an
048 * element. There are five new bulk-modification operations, for example {@link #add(Object, int)},
049 * to add or remove multiple occurrences of an element at once, or to set the count of an element to
050 * a specific value. These modification operations are optional, but implementations which support
051 * the standard collection operations {@link #add(Object)} or {@link #remove(Object)} are encouraged
052 * to implement the related methods as well. Finally, two collection views are provided: {@link
053 * #elementSet} contains the distinct elements of the multiset "with duplicates collapsed", and
054 * {@link #entrySet} is similar but contains {@link Entry Multiset.Entry} instances, each providing
055 * both a distinct element and the count of that element.
056 *
057 * <p>In addition to these required methods, implementations of {@code Multiset} are expected to
058 * provide two {@code static} creation methods: {@code create()}, returning an empty multiset, and
059 * {@code create(Iterable<? extends E>)}, returning a multiset containing the given initial
060 * elements. This is simply a refinement of {@code Collection}'s constructor recommendations,
061 * reflecting the new developments of Java 5.
062 *
063 * <p>As with other collection types, the modification operations are optional, and should throw
064 * {@link UnsupportedOperationException} when they are not implemented. Most implementations should
065 * support either all add operations or none of them, all removal operations or none of them, and if
066 * and only if all of these are supported, the {@code setCount} methods as well.
067 *
068 * <p>A multiset uses {@link Object#equals} to determine whether two instances should be considered
069 * "the same," <i>unless specified otherwise</i> by the implementation.
070 *
071 * <p><b>Warning:</b> as with normal {@link Set}s, it is almost always a bad idea to modify an
072 * element (in a way that affects its {@link Object#equals} behavior) while it is contained in a
073 * multiset. Undefined behavior and bugs will result.
074 *
075 * <p>Common implementations include {@link ImmutableMultiset}, {@link HashMultiset}, and {@link
076 * ConcurrentHashMultiset}.
077 *
078 * <p>If your values may be zero, negative, or outside the range of an int, you may wish to use
079 * {@link com.google.common.util.concurrent.AtomicLongMap} instead. Note, however, that unlike
080 * {@code Multiset}, {@code AtomicLongMap} does not automatically remove zeros.
081 *
082 * <p>See the Guava User Guide article on <a href=
083 * "https://github.com/google/guava/wiki/NewCollectionTypesExplained#multiset">{@code Multiset}</a>.
084 *
085 * @author Kevin Bourrillion
086 * @since 2.0
087 */
088@GwtCompatible
089@ElementTypesAreNonnullByDefault
090public interface Multiset<E extends @Nullable Object> extends Collection<E> {
091  // Query Operations
092
093  /**
094   * Returns the total number of all occurrences of all elements in this multiset.
095   *
096   * <p><b>Note:</b> this method does not return the number of <i>distinct elements</i> in the
097   * multiset, which is given by {@code entrySet().size()}.
098   */
099  @Override
100  int size();
101
102  /**
103   * Returns the number of occurrences of an element in this multiset (the <i>count</i> of the
104   * element). Note that for an {@link Object#equals}-based multiset, this gives the same result as
105   * {@link Collections#frequency} (which would presumably perform more poorly).
106   *
107   * <p><b>Note:</b> the utility method {@link Iterables#frequency} generalizes this operation; it
108   * correctly delegates to this method when dealing with a multiset, but it can also accept any
109   * other iterable type.
110   *
111   * @param element the element to count occurrences of
112   * @return the number of occurrences of the element in this multiset; possibly zero but never
113   *     negative
114   */
115  int count(@CompatibleWith("E") @CheckForNull Object element);
116
117  // Bulk Operations
118
119  /**
120   * Adds a number of occurrences of an element to this multiset. Note that if {@code occurrences ==
121   * 1}, this method has the identical effect to {@link #add(Object)}. This method is functionally
122   * equivalent (except in the case of overflow) to the call {@code
123   * addAll(Collections.nCopies(element, occurrences))}, which would presumably perform much more
124   * poorly.
125   *
126   * @param element the element to add occurrences of; may be null only if explicitly allowed by the
127   *     implementation
128   * @param occurrences the number of occurrences of the element to add. May be zero, in which case
129   *     no change will be made.
130   * @return the count of the element before the operation; possibly zero
131   * @throws IllegalArgumentException if {@code occurrences} is negative, or if this operation would
132   *     result in more than {@link Integer#MAX_VALUE} occurrences of the element
133   * @throws NullPointerException if {@code element} is null and this implementation does not permit
134   *     null elements. Note that if {@code occurrences} is zero, the implementation may opt to
135   *     return normally.
136   */
137  @CanIgnoreReturnValue
138  int add(@ParametricNullness E element, int occurrences);
139
140  /**
141   * Adds a single occurrence of the specified element to this multiset.
142   *
143   * <p>This method refines {@link Collection#add}, which only <i>ensures</i> the presence of the
144   * element, to further specify that a successful call must always increment the count of the
145   * element, and the overall size of the collection, by one.
146   *
147   * <p>To both add the element and obtain the previous count of that element, use {@link
148   * #add(Object, int) add}{@code (element, 1)} instead.
149   *
150   * @param element the element to add one occurrence of; may be null only if explicitly allowed by
151   *     the implementation
152   * @return {@code true} always, since this call is required to modify the multiset, unlike other
153   *     {@link Collection} types
154   * @throws NullPointerException if {@code element} is null and this implementation does not permit
155   *     null elements
156   * @throws IllegalArgumentException if {@link Integer#MAX_VALUE} occurrences of {@code element}
157   *     are already contained in this multiset
158   */
159  @CanIgnoreReturnValue
160  @Override
161  boolean add(@ParametricNullness E element);
162
163  /**
164   * Removes a number of occurrences of the specified element from this multiset. If the multiset
165   * contains fewer than this number of occurrences to begin with, all occurrences will be removed.
166   * Note that if {@code occurrences == 1}, this is functionally equivalent to the call {@code
167   * remove(element)}.
168   *
169   * @param element the element to conditionally remove occurrences of
170   * @param occurrences the number of occurrences of the element to remove. May be zero, in which
171   *     case no change will be made.
172   * @return the count of the element before the operation; possibly zero
173   * @throws IllegalArgumentException if {@code occurrences} is negative
174   */
175  @CanIgnoreReturnValue
176  int remove(@CompatibleWith("E") @CheckForNull Object element, int occurrences);
177
178  /**
179   * Removes a <i>single</i> occurrence of the specified element from this multiset, if present.
180   *
181   * <p>This method refines {@link Collection#remove} to further specify that it <b>may not</b>
182   * throw an exception in response to {@code element} being null or of the wrong type.
183   *
184   * <p>To both remove the element and obtain the previous count of that element, use {@link
185   * #remove(Object, int) remove}{@code (element, 1)} instead.
186   *
187   * @param element the element to remove one occurrence of
188   * @return {@code true} if an occurrence was found and removed
189   */
190  @CanIgnoreReturnValue
191  @Override
192  boolean remove(@CheckForNull Object element);
193
194  /**
195   * Adds or removes the necessary occurrences of an element such that the element attains the
196   * desired count.
197   *
198   * @param element the element to add or remove occurrences of; may be null only if explicitly
199   *     allowed by the implementation
200   * @param count the desired count of the element in this multiset
201   * @return the count of the element before the operation; possibly zero
202   * @throws IllegalArgumentException if {@code count} is negative
203   * @throws NullPointerException if {@code element} is null and this implementation does not permit
204   *     null elements. Note that if {@code count} is zero, the implementor may optionally return
205   *     zero instead.
206   */
207  @CanIgnoreReturnValue
208  int setCount(@ParametricNullness E element, int count);
209
210  /**
211   * Conditionally sets the count of an element to a new value, as described in {@link
212   * #setCount(Object, int)}, provided that the element has the expected current count. If the
213   * current count is not {@code oldCount}, no change is made.
214   *
215   * @param element the element to conditionally set the count of; may be null only if explicitly
216   *     allowed by the implementation
217   * @param oldCount the expected present count of the element in this multiset
218   * @param newCount the desired count of the element in this multiset
219   * @return {@code true} if the condition for modification was met. This implies that the multiset
220   *     was indeed modified, unless {@code oldCount == newCount}.
221   * @throws IllegalArgumentException if {@code oldCount} or {@code newCount} is negative
222   * @throws NullPointerException if {@code element} is null and the implementation does not permit
223   *     null elements. Note that if {@code oldCount} and {@code newCount} are both zero, the
224   *     implementor may optionally return {@code true} instead.
225   */
226  @CanIgnoreReturnValue
227  boolean setCount(@ParametricNullness E element, int oldCount, int newCount);
228
229  // Views
230
231  /**
232   * Returns the set of distinct elements contained in this multiset. The element set is backed by
233   * the same data as the multiset, so any change to either is immediately reflected in the other.
234   * The order of the elements in the element set is unspecified.
235   *
236   * <p>If the element set supports any removal operations, these necessarily cause <b>all</b>
237   * occurrences of the removed element(s) to be removed from the multiset. Implementations are not
238   * expected to support the add operations, although this is possible.
239   *
240   * <p>A common use for the element set is to find the number of distinct elements in the multiset:
241   * {@code elementSet().size()}.
242   *
243   * @return a view of the set of distinct elements in this multiset
244   */
245  Set<E> elementSet();
246
247  /**
248   * Returns a view of the contents of this multiset, grouped into {@code Multiset.Entry} instances,
249   * each providing an element of the multiset and the count of that element. This set contains
250   * exactly one entry for each distinct element in the multiset (thus it always has the same size
251   * as the {@link #elementSet}). The order of the elements in the element set is unspecified.
252   *
253   * <p>The entry set is backed by the same data as the multiset, so any change to either is
254   * immediately reflected in the other. However, multiset changes may or may not be reflected in
255   * any {@code Entry} instances already retrieved from the entry set (this is
256   * implementation-dependent). Furthermore, implementations are not required to support
257   * modifications to the entry set at all, and the {@code Entry} instances themselves don't even
258   * have methods for modification. See the specific implementation class for more details on how
259   * its entry set handles modifications.
260   *
261   * @return a set of entries representing the data of this multiset
262   */
263  Set<Entry<E>> entrySet();
264
265  /**
266   * An unmodifiable element-count pair for a multiset. The {@link Multiset#entrySet} method returns
267   * a view of the multiset whose elements are of this class. A multiset implementation may return
268   * Entry instances that are either live "read-through" views to the Multiset, or immutable
269   * snapshots. Note that this type is unrelated to the similarly-named type {@code Map.Entry}.
270   *
271   * @since 2.0
272   */
273  interface Entry<E extends @Nullable Object> {
274
275    /**
276     * Returns the multiset element corresponding to this entry. Multiple calls to this method
277     * always return the same instance.
278     *
279     * @return the element corresponding to this entry
280     */
281    @ParametricNullness
282    E getElement();
283
284    /**
285     * Returns the count of the associated element in the underlying multiset. This count may either
286     * be an unchanging snapshot of the count at the time the entry was retrieved, or a live view of
287     * the current count of the element in the multiset, depending on the implementation. Note that
288     * in the former case, this method can never return zero, while in the latter, it will return
289     * zero if all occurrences of the element were since removed from the multiset.
290     *
291     * @return the count of the element; never negative
292     */
293    int getCount();
294
295    /**
296     * {@inheritDoc}
297     *
298     * <p>Returns {@code true} if the given object is also a multiset entry and the two entries
299     * represent the same element and count. That is, two entries {@code a} and {@code b} are equal
300     * if:
301     *
302     * <pre>{@code
303     * Objects.equal(a.getElement(), b.getElement())
304     *     && a.getCount() == b.getCount()
305     * }</pre>
306     */
307    @Override
308    // TODO(kevinb): check this wrt TreeMultiset?
309    boolean equals(@CheckForNull Object o);
310
311    /**
312     * {@inheritDoc}
313     *
314     * <p>The hash code of a multiset entry for element {@code element} and count {@code count} is
315     * defined as:
316     *
317     * <pre>{@code
318     * ((element == null) ? 0 : element.hashCode()) ^ count
319     * }</pre>
320     */
321    @Override
322    int hashCode();
323
324    /**
325     * Returns the canonical string representation of this entry, defined as follows. If the count
326     * for this entry is one, this is simply the string representation of the corresponding element.
327     * Otherwise, it is the string representation of the element, followed by the three characters
328     * {@code " x "} (space, letter x, space), followed by the count.
329     */
330    @Override
331    String toString();
332  }
333
334  /**
335   * Runs the specified action for each distinct element in this multiset, and the number of
336   * occurrences of that element. For some {@code Multiset} implementations, this may be more
337   * efficient than iterating over the {@link #entrySet()} either explicitly or with {@code
338   * entrySet().forEach(action)}.
339   *
340   * @since 21.0
341   */
342  @Beta
343  default void forEachEntry(ObjIntConsumer<? super E> action) {
344    checkNotNull(action);
345    entrySet().forEach(entry -> action.accept(entry.getElement(), entry.getCount()));
346  }
347
348  // Comparison and hashing
349
350  /**
351   * Compares the specified object with this multiset for equality. Returns {@code true} if the
352   * given object is also a multiset and contains equal elements with equal counts, regardless of
353   * order.
354   */
355  @Override
356  // TODO(kevinb): caveats about equivalence-relation?
357  boolean equals(@CheckForNull Object object);
358
359  /**
360   * Returns the hash code for this multiset. This is defined as the sum of
361   *
362   * <pre>{@code
363   * ((element == null) ? 0 : element.hashCode()) ^ count(element)
364   * }</pre>
365   *
366   * <p>over all distinct elements in the multiset. It follows that a multiset and its entry set
367   * always have the same hash code.
368   */
369  @Override
370  int hashCode();
371
372  /**
373   * {@inheritDoc}
374   *
375   * <p>It is recommended, though not mandatory, that this method return the result of invoking
376   * {@link #toString} on the {@link #entrySet}, yielding a result such as {@code [a x 3, c, d x 2,
377   * e]}.
378   */
379  @Override
380  String toString();
381
382  // Refined Collection Methods
383
384  /**
385   * {@inheritDoc}
386   *
387   * <p>Elements that occur multiple times in the multiset will appear multiple times in this
388   * iterator, though not necessarily sequentially.
389   */
390  @Override
391  Iterator<E> iterator();
392
393  /**
394   * Determines whether this multiset contains the specified element.
395   *
396   * <p>This method refines {@link Collection#contains} to further specify that it <b>may not</b>
397   * throw an exception in response to {@code element} being null or of the wrong type.
398   *
399   * @param element the element to check for
400   * @return {@code true} if this multiset contains at least one occurrence of the element
401   */
402  @Override
403  boolean contains(@CheckForNull Object element);
404
405  /**
406   * Returns {@code true} if this multiset contains at least one occurrence of each element in the
407   * specified collection.
408   *
409   * <p>This method refines {@link Collection#containsAll} to further specify that it <b>may not</b>
410   * throw an exception in response to any of {@code elements} being null or of the wrong type.
411   *
412   * <p><b>Note:</b> this method does not take into account the occurrence count of an element in
413   * the two collections; it may still return {@code true} even if {@code elements} contains several
414   * occurrences of an element and this multiset contains only one. This is no different than any
415   * other collection type like {@link List}, but it may be unexpected to the user of a multiset.
416   *
417   * @param elements the collection of elements to be checked for containment in this multiset
418   * @return {@code true} if this multiset contains at least one occurrence of each element
419   *     contained in {@code elements}
420   * @throws NullPointerException if {@code elements} is null
421   */
422  @Override
423  boolean containsAll(Collection<?> elements);
424
425  /**
426   * {@inheritDoc}
427   *
428   * <p><b>Note:</b> This method ignores how often any element might appear in {@code c}, and only
429   * cares whether or not an element appears at all. If you wish to remove one occurrence in this
430   * multiset for every occurrence in {@code c}, see {@link Multisets#removeOccurrences(Multiset,
431   * Multiset)}.
432   *
433   * <p>This method refines {@link Collection#removeAll} to further specify that it <b>may not</b>
434   * throw an exception in response to any of {@code elements} being null or of the wrong type.
435   */
436  @CanIgnoreReturnValue
437  @Override
438  boolean removeAll(Collection<?> c);
439
440  /**
441   * {@inheritDoc}
442   *
443   * <p><b>Note:</b> This method ignores how often any element might appear in {@code c}, and only
444   * cares whether or not an element appears at all. If you wish to remove one occurrence in this
445   * multiset for every occurrence in {@code c}, see {@link Multisets#retainOccurrences(Multiset,
446   * Multiset)}.
447   *
448   * <p>This method refines {@link Collection#retainAll} to further specify that it <b>may not</b>
449   * throw an exception in response to any of {@code elements} being null or of the wrong type.
450   *
451   * @see Multisets#retainOccurrences(Multiset, Multiset)
452   */
453  @CanIgnoreReturnValue
454  @Override
455  boolean retainAll(Collection<?> c);
456
457  /**
458   * {@inheritDoc}
459   *
460   * <p>Elements that occur multiple times in the multiset will be passed to the {@code Consumer}
461   * correspondingly many times, though not necessarily sequentially.
462   */
463  @Override
464  default void forEach(Consumer<? super E> action) {
465    checkNotNull(action);
466    entrySet()
467        .forEach(
468            entry -> {
469              E elem = entry.getElement();
470              int count = entry.getCount();
471              for (int i = 0; i < count; i++) {
472                action.accept(elem);
473              }
474            });
475  }
476
477  @Override
478  default Spliterator<E> spliterator() {
479    return Multisets.spliteratorImpl(this);
480  }
481}