001 /*
002 * Copyright (C) 2007 Google Inc.
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 (see "Note on
035 * element equivalence", below) are referred to as <i>occurrences</i> of the
036 * same single element. The total number of occurrences of an element in a
037 * multiset is called the <i>count</i> of that element (the terms "frequency"
038 * and "multiplicity" are equivalent, but not used in this API). Since the count
039 * of an element is represented as an {@code int}, a multiset may never contain
040 * more than {@link Integer#MAX_VALUE} 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 * @author Kevin Bourrillion
074 * @since 2 (imported from Google Collections Library)
075 */
076 @GwtCompatible
077 public interface Multiset<E> extends Collection<E> {
078 // Query Operations
079
080 /**
081 * Returns the number of occurrences of an element in this multiset (the
082 * <i>count</i> of the element). Note that for an {@link Object#equals}-based
083 * multiset, this gives the same result as {@link Collections#frequency}
084 * (which would presumably perform more poorly).
085 *
086 * <p><b>Note:</b> the utility method {@link Iterables#frequency} generalizes
087 * this operation; it correctly delegates to this method when dealing with a
088 * multiset, but it can also accept any other iterable type.
089 *
090 * @param element the element to count occurrences of
091 * @return the number of occurrences of the element in this multiset; possibly
092 * zero but never negative
093 */
094 int count(@Nullable Object element);
095
096 // Bulk Operations
097
098 /**
099 * Adds a number of occurrences of an element to this multiset. Note that if
100 * {@code occurrences == 1}, this method has the identical effect to {@link
101 * #add(Object)}. This method is functionally equivalent (except in the case
102 * of overflow) to the call {@code addAll(Collections.nCopies(element,
103 * occurrences))}, which would presumably perform much more poorly.
104 *
105 * @param element the element to add occurrences of; may be {@code null} only
106 * if explicitly allowed by the implementation
107 * @param occurrences the number of occurrences of the element to add. May be
108 * zero, in which case no change will be made.
109 * @return the count of the element before the operation; possibly zero
110 * @throws IllegalArgumentException if {@code occurrences} is negative, or if
111 * this operation would result in more than {@link Integer#MAX_VALUE}
112 * occurrences of the element
113 * @throws NullPointerException if {@code element} is null and this
114 * implementation does not permit null elements. Note that if {@code
115 * occurrences} is zero, the implementation may opt to return normally.
116 */
117 int add(@Nullable E element, int occurrences);
118
119 /**
120 * Removes a number of occurrences of the specified element from this
121 * multiset. If the multiset contains fewer than this number of occurrences to
122 * begin with, all occurrences will be removed. Note that if
123 * {@code occurrences == 1}, this is functionally equivalent to the call
124 * {@code remove(element)}.
125 *
126 * @param element the element to conditionally remove occurrences of
127 * @param occurrences the number of occurrences of the element to remove. May
128 * be zero, in which case 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
131 */
132 int remove(@Nullable Object element, int occurrences);
133
134 /**
135 * Adds or removes the necessary occurrences of an element such that the
136 * element attains the desired count.
137 *
138 * @param element the element to add or remove occurrences of; may be null
139 * only if explicitly allowed by the implementation
140 * @param count the desired count of the element in this multiset
141 * @return the count of the element before the operation; possibly zero
142 * @throws IllegalArgumentException if {@code count} is negative
143 * @throws NullPointerException if {@code element} is null and this
144 * implementation does not permit null elements. Note that if {@code
145 * count} is zero, the implementor may optionally return zero instead.
146 */
147 int setCount(E element, int count);
148
149 /**
150 * Conditionally sets the count of an element to a new value, as described in
151 * {@link #setCount(Object, int)}, provided that the element has the expected
152 * current count. If the current count is not {@code oldCount}, no change is
153 * made.
154 *
155 * @param element the element to conditionally set the count of; may be null
156 * only if explicitly allowed by the implementation
157 * @param oldCount the expected present count of the element in this multiset
158 * @param newCount the desired count of the element in this multiset
159 * @return {@code true} if the condition for modification was met. This
160 * implies that the multiset was indeed modified, unless
161 * {@code oldCount == newCount}.
162 * @throws IllegalArgumentException if {@code oldCount} or {@code newCount} is
163 * negative
164 * @throws NullPointerException if {@code element} is null and the
165 * implementation does not permit null elements. Note that if {@code
166 * oldCount} and {@code newCount} are both zero, the implementor may
167 * optionally return {@code true} instead.
168 */
169 boolean setCount(E element, int oldCount, int newCount);
170
171 // Views
172
173 /**
174 * Returns the set of distinct elements contained in this multiset. The
175 * element set is backed by the same data as the multiset, so any change to
176 * either is immediately reflected in the other. The order of the elements in
177 * the element set is unspecified.
178 *
179 * <p>If the element set supports any removal operations, these necessarily
180 * cause <b>all</b> occurrences of the removed element(s) to be removed from
181 * the multiset. Implementations are not expected to support the add
182 * operations, although this is possible.
183 *
184 * <p>A common use for the element set is to find the number of distinct
185 * elements in the multiset: {@code elementSet().size()}.
186 *
187 * @return a view of the set of distinct elements in this multiset
188 */
189 Set<E> elementSet();
190
191 /**
192 * Returns a view of the contents of this multiset, grouped into {@code
193 * Multiset.Entry} instances, each providing an element of the multiset and
194 * the count of that element. This set contains exactly one entry for each
195 * distinct element in the multiset (thus it always has the same size as the
196 * {@link #elementSet}). The order of the elements in the element set is
197 * unspecified.
198 *
199 * <p>The entry set is backed by the same data as the multiset, so any change
200 * to either is immediately reflected in the other. However, multiset changes
201 * may or may not be reflected in any {@code Entry} instances already
202 * retrieved from the entry set (this is implementation-dependent).
203 * Furthermore, implementations are not required to support modifications to
204 * the entry set at all, and the {@code Entry} instances themselves don't
205 * even have methods for modification. See the specific implementation class
206 * for more details on how its entry set handles modifications.
207 *
208 * @return a set of entries representing the data of this multiset
209 */
210 Set<Entry<E>> entrySet();
211
212 /**
213 * An unmodifiable element-count pair for a multiset. The {@link
214 * Multiset#entrySet} method returns a view of the multiset whose elements
215 * are of this class. A multiset implementation may return Entry instances
216 * that are either live "read-through" views to the Multiset, or immutable
217 * snapshots. Note that this type is unrelated to the similarly-named type
218 * {@code Map.Entry}.
219 */
220 interface Entry<E> {
221
222 /**
223 * Returns the multiset element corresponding to this entry. Multiple calls
224 * to this method always return the same instance.
225 *
226 * @return the element corresponding to this entry
227 */
228 E getElement();
229
230 /**
231 * Returns the count of the associated element in the underlying multiset.
232 * This count may either be an unchanging snapshot of the count at the time
233 * the entry was retrieved, or a live view of the current count of the
234 * element in the multiset, depending on the implementation. Note that in
235 * the former case, this method can never return zero, while in the latter,
236 * it will return zero if all occurrences of the element were since removed
237 * from the multiset.
238 *
239 * @return the count of the element; never negative
240 */
241 int getCount();
242
243 /**
244 * {@inheritDoc}
245 *
246 * <p>Returns {@code true} if the given object is also a multiset entry and
247 * the two entries represent the same element and count. More formally, two
248 * entries {@code a} and {@code b} are equal if:
249 *
250 * <pre> ((a.getElement() == null)
251 * ? (b.getElement() == null) : a.getElement().equals(b.getElement()))
252 * && (a.getCount() == b.getCount())</pre>
253 */
254 // TODO: check this wrt TreeMultiset?
255 boolean equals(Object o);
256
257 /**
258 * {@inheritDoc}
259 *
260 * <p>The hash code of a multiset entry for element {@code element} and
261 * count {@code count} is defined as:
262 *
263 * <pre> (element == null ? 0 : element.hashCode()) ^ count</pre>
264 */
265 int hashCode();
266
267 /**
268 * Returns the canonical string representation of this entry, defined as
269 * follows. If the count for this entry is one, this is simply the string
270 * representation of the corresponding element. Otherwise, it is the string
271 * representation of the element, followed by the three characters {@code
272 * " x "} (space, letter x, space), followed by the count.
273 */
274 String toString();
275 }
276
277 // Comparison and hashing
278
279 /**
280 * Compares the specified object with this multiset for equality. Returns
281 * {@code true} if the given object is also a multiset and contains equal
282 * elements with equal counts, regardless of order.
283 */
284 // TODO: caveats about equivalence-relation?
285 boolean equals(@Nullable Object object);
286
287 /**
288 * Returns the hash code for this multiset. This is defined as the sum of
289 *
290 * <pre> (element == null ? 0 : element.hashCode()) ^ count(element)</pre>
291 *
292 * over all distinct elements in the multiset. It follows that a multiset and
293 * its entry set always have the same hash code.
294 */
295 int hashCode();
296
297 /**
298 * {@inheritDoc}
299 *
300 * <p>It is recommended, though not mandatory, that this method return the
301 * result of invoking {@link #toString} on the {@link #entrySet}, yielding a
302 * result such as
303 * <pre>
304 * [a x 3, c, d x 2, e]
305 * </pre>
306 */
307 String toString();
308
309 // Refined Collection Methods
310
311 /**
312 * {@inheritDoc}
313 *
314 * <p>Elements that occur multiple times in the multiset will appear
315 * multiple times in this iterator, though not necessarily sequentially.
316 */
317 Iterator<E> iterator();
318
319 /**
320 * Determines whether this multiset contains the specified element.
321 *
322 * <p>This method refines {@link Collection#contains} to further specify that
323 * it <b>may not</b> throw an exception in response to {@code element} being
324 * null or of the wrong type.
325 *
326 * @param element the element to check for
327 * @return {@code true} if this multiset contains at least one occurrence of
328 * the element
329 */
330 boolean contains(@Nullable Object element);
331
332 /**
333 * Returns {@code true} if this multiset contains at least one occurrence of
334 * each element in the specified collection.
335 *
336 * <p>This method refines {@link Collection#containsAll} to further specify
337 * that it <b>may not</b> throw an exception in response to any of {@code
338 * elements} being null or of the wrong type.
339 *
340 * <p><b>Note:</b> this method does not take into account the occurrence
341 * count of an element in the two collections; it may still return {@code
342 * true} even if {@code elements} contains several occurrences of an element
343 * and this multiset contains only one. This is no different than any other
344 * collection type like {@link List}, but it may be unexpected to the user of
345 * a multiset.
346 *
347 * @param elements the collection of elements to be checked for containment in
348 * this multiset
349 * @return {@code true} if this multiset contains at least one occurrence of
350 * each element contained in {@code elements}
351 * @throws NullPointerException if {@code elements} is null
352 */
353 boolean containsAll(Collection<?> elements);
354
355 /**
356 * Adds a single occurrence of the specified element to this multiset.
357 *
358 * <p>This method refines {@link Collection#add}, which only <i>ensures</i>
359 * the presence of the element, to further specify that a successful call must
360 * always increment the count of the element, and the overall size of the
361 * collection, by one.
362 *
363 * @param element the element to add one occurrence of; may be null only if
364 * explicitly allowed by the implementation
365 * @return {@code true} always, since this call is required to modify the
366 * multiset, unlike other {@link Collection} types
367 * @throws NullPointerException if {@code element} is null and this
368 * implementation does not permit null elements
369 * @throws IllegalArgumentException if {@link Integer#MAX_VALUE} occurrences
370 * of {@code element} are already contained in this multiset
371 */
372 boolean add(E element);
373
374 /**
375 * Removes a <i>single</i> occurrence of the specified element from this
376 * multiset, if present.
377 *
378 * <p>This method refines {@link Collection#remove} to further specify that it
379 * <b>may not</b> throw an exception in response to {@code element} being null
380 * or of the wrong type.
381 *
382 * @param element the element to remove one occurrence of
383 * @return {@code true} if an occurrence was found and removed
384 */
385 boolean remove(@Nullable Object element);
386
387 /**
388 * {@inheritDoc}
389 *
390 * <p>This method refines {@link Collection#removeAll} to further specify that
391 * it <b>may not</b> throw an exception in response to any of {@code elements}
392 * being null or of the wrong type.
393 */
394 boolean removeAll(Collection<?> c);
395
396 /**
397 * {@inheritDoc}
398 *
399 * <p>This method refines {@link Collection#retainAll} to further specify that
400 * it <b>may not</b> throw an exception in response to any of {@code elements}
401 * being null or of the wrong type.
402 */
403 boolean retainAll(Collection<?> c);
404 }