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