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