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 com.google.common.annotations.GwtCompatible; 020import com.google.errorprone.annotations.CanIgnoreReturnValue; 021import com.google.errorprone.annotations.CompatibleWith; 022import java.util.Collection; 023import java.util.Collections; 024import java.util.Iterator; 025import java.util.List; 026import java.util.Set; 027import javax.annotation.CheckForNull; 028import org.checkerframework.checker.nullness.qual.Nullable; 029 030/** 031 * A collection that supports order-independent equality, like {@link Set}, but may have duplicate 032 * elements. A multiset is also sometimes called a <i>bag</i>. 033 * 034 * <p>Elements of a multiset that are equal to one another are referred to as <i>occurrences</i> of 035 * the same single element. The total number of occurrences of an element in a multiset is called 036 * the <i>count</i> of that element (the terms "frequency" and "multiplicity" are equivalent, but 037 * not used in this API). Since the count of an element is represented as an {@code int}, a multiset 038 * may never contain more than {@link Integer#MAX_VALUE} occurrences of any one element. 039 * 040 * <p>{@code Multiset} refines the specifications of several methods from {@code Collection}. It 041 * also defines an additional query operation, {@link #count}, which returns the count of an 042 * element. There are five new bulk-modification operations, for example {@link #add(Object, int)}, 043 * to add or remove multiple occurrences of an element at once, or to set the count of an element to 044 * a specific value. These modification operations are optional, but implementations which support 045 * the standard collection operations {@link #add(Object)} or {@link #remove(Object)} are encouraged 046 * to implement the related methods as well. Finally, two collection views are provided: {@link 047 * #elementSet} contains the distinct elements of the multiset "with duplicates collapsed", and 048 * {@link #entrySet} is similar but contains {@link Entry Multiset.Entry} instances, each providing 049 * both a distinct element and the count of that element. 050 * 051 * <p>In addition to these required methods, implementations of {@code Multiset} are expected to 052 * provide two {@code static} creation methods: {@code create()}, returning an empty multiset, and 053 * {@code create(Iterable<? extends E>)}, returning a multiset containing the given initial 054 * elements. This is simply a refinement of {@code Collection}'s constructor recommendations, 055 * reflecting the new developments of Java 5. 056 * 057 * <p>As with other collection types, the modification operations are optional, and should throw 058 * {@link UnsupportedOperationException} when they are not implemented. Most implementations should 059 * support either all add operations or none of them, all removal operations or none of them, and if 060 * and only if all of these are supported, the {@code setCount} methods as well. 061 * 062 * <p>A multiset uses {@link Object#equals} to determine whether two instances should be considered 063 * "the same," <i>unless specified otherwise</i> by the implementation. 064 * 065 * <p><b>Warning:</b> as with normal {@link Set}s, it is almost always a bad idea to modify an 066 * element (in a way that affects its {@link Object#equals} behavior) while it is contained in a 067 * multiset. Undefined behavior and bugs will result. 068 * 069 * <h3>Implementations</h3> 070 * 071 * <ul> 072 * <li>{@link ImmutableMultiset} 073 * <li>{@link ImmutableSortedMultiset} 074 * <li>{@link HashMultiset} 075 * <li>{@link LinkedHashMultiset} 076 * <li>{@link TreeMultiset} 077 * <li>{@link EnumMultiset} 078 * <li>{@link ConcurrentHashMultiset} 079 * </ul> 080 * 081 * <p>If your values may be zero, negative, or outside the range of an int, you may wish to use 082 * {@link com.google.common.util.concurrent.AtomicLongMap} instead. Note, however, that unlike 083 * {@code Multiset}, {@code AtomicLongMap} does not automatically remove zeros. 084 * 085 * <p>See the Guava User Guide article on <a href= 086 * "https://github.com/google/guava/wiki/NewCollectionTypesExplained#multiset">{@code Multiset}</a>. 087 * 088 * @author Kevin Bourrillion 089 * @since 2.0 090 */ 091@GwtCompatible 092@ElementTypesAreNonnullByDefault 093public interface Multiset<E extends @Nullable Object> extends Collection<E> { 094 // Query Operations 095 096 /** 097 * Returns the total number of all occurrences of all elements in this multiset. 098 * 099 * <p><b>Note:</b> this method does not return the number of <i>distinct elements</i> in the 100 * multiset, which is given by {@code entrySet().size()}. 101 */ 102 @Override 103 int size(); 104 105 /** 106 * Returns the number of occurrences of an element in this multiset (the <i>count</i> of the 107 * element). Note that for an {@link Object#equals}-based multiset, this gives the same result as 108 * {@link Collections#frequency} (which would presumably perform more poorly). 109 * 110 * <p><b>Note:</b> the utility method {@link Iterables#frequency} generalizes this operation; it 111 * correctly delegates to this method when dealing with a multiset, but it can also accept any 112 * other iterable type. 113 * 114 * @param element the element to count occurrences of 115 * @return the number of occurrences of the element in this multiset; possibly zero but never 116 * negative 117 */ 118 int count(@CompatibleWith("E") @CheckForNull Object element); 119 120 // Bulk Operations 121 122 /** 123 * Adds a number of occurrences of an element to this multiset. Note that if {@code occurrences == 124 * 1}, this method has the identical effect to {@link #add(Object)}. This method is functionally 125 * equivalent (except in the case of overflow) to the call {@code 126 * addAll(Collections.nCopies(element, occurrences))}, which would presumably perform much more 127 * poorly. 128 * 129 * @param element the element to add occurrences of; may be null only if explicitly allowed by the 130 * implementation 131 * @param occurrences the number of occurrences of the element to add. May be zero, in which case 132 * no change will be made. 133 * @return the count of the element before the operation; possibly zero 134 * @throws IllegalArgumentException if {@code occurrences} is negative, or if this operation would 135 * result in more than {@link Integer#MAX_VALUE} occurrences of the element 136 * @throws NullPointerException if {@code element} is null and this implementation does not permit 137 * null elements. Note that if {@code occurrences} is zero, the implementation may opt to 138 * return normally. 139 */ 140 @CanIgnoreReturnValue 141 int add(@ParametricNullness E element, int occurrences); 142 143 /** 144 * Adds a single occurrence of the specified element to this multiset. 145 * 146 * <p>This method refines {@link Collection#add}, which only <i>ensures</i> the presence of the 147 * element, to further specify that a successful call must always increment the count of the 148 * element, and the overall size of the collection, by one. 149 * 150 * <p>To both add the element and obtain the previous count of that element, use {@link 151 * #add(Object, int) add}{@code (element, 1)} instead. 152 * 153 * @param element the element to add one occurrence of; may be null only if explicitly allowed by 154 * the implementation 155 * @return {@code true} always, since this call is required to modify the multiset, unlike other 156 * {@link Collection} types 157 * @throws NullPointerException if {@code element} is null and this implementation does not permit 158 * null elements 159 * @throws IllegalArgumentException if {@link Integer#MAX_VALUE} occurrences of {@code element} 160 * are already contained in this multiset 161 */ 162 @CanIgnoreReturnValue 163 @Override 164 boolean add(@ParametricNullness E element); 165 166 /** 167 * Removes a number of occurrences of the specified element from this multiset. If the multiset 168 * contains fewer than this number of occurrences to begin with, all occurrences will be removed. 169 * Note that if {@code occurrences == 1}, this is functionally equivalent to the call {@code 170 * remove(element)}. 171 * 172 * @param element the element to conditionally remove occurrences of 173 * @param occurrences the number of occurrences of the element to remove. May be zero, in which 174 * case no change will be made. 175 * @return the count of the element before the operation; possibly zero 176 * @throws IllegalArgumentException if {@code occurrences} is negative 177 */ 178 @CanIgnoreReturnValue 179 int remove(@CompatibleWith("E") @CheckForNull Object element, int occurrences); 180 181 /** 182 * Removes a <i>single</i> occurrence of the specified element from this multiset, if present. 183 * 184 * <p>This method refines {@link Collection#remove} to further specify that it <b>may not</b> 185 * throw an exception in response to {@code element} being null or of the wrong type. 186 * 187 * <p>To both remove the element and obtain the previous count of that element, use {@link 188 * #remove(Object, int) remove}{@code (element, 1)} instead. 189 * 190 * @param element the element to remove one occurrence of 191 * @return {@code true} if an occurrence was found and removed 192 */ 193 @CanIgnoreReturnValue 194 @Override 195 boolean remove(@CheckForNull Object element); 196 197 /** 198 * Adds or removes the necessary occurrences of an element such that the element attains the 199 * desired count. 200 * 201 * @param element the element to add or remove occurrences of; may be null only if explicitly 202 * allowed by the implementation 203 * @param count the desired count of the element in this multiset 204 * @return the count of the element before the operation; possibly zero 205 * @throws IllegalArgumentException if {@code count} is negative 206 * @throws NullPointerException if {@code element} is null and this implementation does not permit 207 * null elements. Note that if {@code count} is zero, the implementor may optionally return 208 * zero instead. 209 */ 210 @CanIgnoreReturnValue 211 int setCount(@ParametricNullness E element, int count); 212 213 /** 214 * Conditionally sets the count of an element to a new value, as described in {@link 215 * #setCount(Object, int)}, provided that the element has the expected current count. If the 216 * current count is not {@code oldCount}, no change is made. 217 * 218 * @param element the element to conditionally set the count of; may be null only if explicitly 219 * allowed by the implementation 220 * @param oldCount the expected present count of the element in this multiset 221 * @param newCount the desired count of the element in this multiset 222 * @return {@code true} if the condition for modification was met. This implies that the multiset 223 * was indeed modified, unless {@code oldCount == newCount}. 224 * @throws IllegalArgumentException if {@code oldCount} or {@code newCount} is negative 225 * @throws NullPointerException if {@code element} is null and the implementation does not permit 226 * null elements. Note that if {@code oldCount} and {@code newCount} are both zero, the 227 * implementor may optionally return {@code true} instead. 228 */ 229 @CanIgnoreReturnValue 230 boolean setCount(@ParametricNullness E element, int oldCount, int newCount); 231 232 // Views 233 234 /** 235 * Returns the set of distinct elements contained in this multiset. The element set is backed by 236 * the same data as the multiset, so any change to either is immediately reflected in the other. 237 * The order of the elements in the element set is unspecified. 238 * 239 * <p>If the element set supports any removal operations, these necessarily cause <b>all</b> 240 * occurrences of the removed element(s) to be removed from the multiset. Implementations are not 241 * expected to support the add operations, although this is possible. 242 * 243 * <p>A common use for the element set is to find the number of distinct elements in the multiset: 244 * {@code elementSet().size()}. 245 * 246 * @return a view of the set of distinct elements in this multiset 247 */ 248 Set<E> elementSet(); 249 250 /** 251 * Returns a view of the contents of this multiset, grouped into {@code Multiset.Entry} instances, 252 * each providing an element of the multiset and the count of that element. This set contains 253 * exactly one entry for each distinct element in the multiset (thus it always has the same size 254 * as the {@link #elementSet}). The order of the elements in the element set is unspecified. 255 * 256 * <p>The entry set is backed by the same data as the multiset, so any change to either is 257 * immediately reflected in the other. However, multiset changes may or may not be reflected in 258 * any {@code Entry} instances already retrieved from the entry set (this is 259 * implementation-dependent). Furthermore, implementations are not required to support 260 * modifications to the entry set at all, and the {@code Entry} instances themselves don't even 261 * have methods for modification. See the specific implementation class for more details on how 262 * its entry set handles modifications. 263 * 264 * @return a set of entries representing the data of this multiset 265 */ 266 Set<Entry<E>> entrySet(); 267 268 /** 269 * An unmodifiable element-count pair for a multiset. The {@link Multiset#entrySet} method returns 270 * a view of the multiset whose elements are of this class. A multiset implementation may return 271 * Entry instances that are either live "read-through" views to the Multiset, or immutable 272 * snapshots. Note that this type is unrelated to the similarly-named type {@code Map.Entry}. 273 * 274 * @since 2.0 275 */ 276 interface Entry<E extends @Nullable Object> { 277 278 /** 279 * Returns the multiset element corresponding to this entry. Multiple calls to this method 280 * always return the same instance. 281 * 282 * @return the element corresponding to this entry 283 */ 284 @ParametricNullness 285 E getElement(); 286 287 /** 288 * Returns the count of the associated element in the underlying multiset. This count may either 289 * be an unchanging snapshot of the count at the time the entry was retrieved, or a live view of 290 * the current count of the element in the multiset, depending on the implementation. Note that 291 * in the former case, this method can never return zero, while in the latter, it will return 292 * zero if all occurrences of the element were since removed from the multiset. 293 * 294 * @return the count of the element; never negative 295 */ 296 int getCount(); 297 298 /** 299 * {@inheritDoc} 300 * 301 * <p>Returns {@code true} if the given object is also a multiset entry and the two entries 302 * represent the same element and count. That is, two entries {@code a} and {@code b} are equal 303 * if: 304 * 305 * <pre>{@code 306 * Objects.equal(a.getElement(), b.getElement()) 307 * && a.getCount() == b.getCount() 308 * }</pre> 309 */ 310 @Override 311 // TODO(kevinb): check this wrt TreeMultiset? 312 boolean equals(@CheckForNull Object o); 313 314 /** 315 * {@inheritDoc} 316 * 317 * <p>The hash code of a multiset entry for element {@code element} and count {@code count} is 318 * defined as: 319 * 320 * <pre>{@code 321 * ((element == null) ? 0 : element.hashCode()) ^ count 322 * }</pre> 323 */ 324 @Override 325 int hashCode(); 326 327 /** 328 * Returns the canonical string representation of this entry, defined as follows. If the count 329 * for this entry is one, this is simply the string representation of the corresponding element. 330 * Otherwise, it is the string representation of the element, followed by the three characters 331 * {@code " x "} (space, letter x, space), followed by the count. 332 */ 333 @Override 334 String toString(); 335 } 336 337 // Comparison and hashing 338 339 /** 340 * Compares the specified object with this multiset for equality. Returns {@code true} if the 341 * given object is also a multiset and contains equal elements with equal counts, regardless of 342 * order. 343 */ 344 @Override 345 // TODO(kevinb): caveats about equivalence-relation? 346 boolean equals(@CheckForNull Object object); 347 348 /** 349 * Returns the hash code for this multiset. This is defined as the sum of 350 * 351 * <pre>{@code 352 * ((element == null) ? 0 : element.hashCode()) ^ count(element) 353 * }</pre> 354 * 355 * <p>over all distinct elements in the multiset. It follows that a multiset and its entry set 356 * always have the same hash code. 357 */ 358 @Override 359 int hashCode(); 360 361 /** 362 * {@inheritDoc} 363 * 364 * <p>It is recommended, though not mandatory, that this method return the result of invoking 365 * {@link #toString} on the {@link #entrySet}, yielding a result such as {@code [a x 3, c, d x 2, 366 * e]}. 367 */ 368 @Override 369 String toString(); 370 371 // Refined Collection Methods 372 373 /** 374 * {@inheritDoc} 375 * 376 * <p>Elements that occur multiple times in the multiset will appear multiple times in this 377 * iterator, though not necessarily sequentially. 378 */ 379 @Override 380 Iterator<E> iterator(); 381 382 /** 383 * Determines whether this multiset contains the specified element. 384 * 385 * <p>This method refines {@link Collection#contains} to further specify that it <b>may not</b> 386 * throw an exception in response to {@code element} being null or of the wrong type. 387 * 388 * @param element the element to check for 389 * @return {@code true} if this multiset contains at least one occurrence of the element 390 */ 391 @Override 392 boolean contains(@CheckForNull Object element); 393 394 /** 395 * Returns {@code true} if this multiset contains at least one occurrence of each element in the 396 * specified collection. 397 * 398 * <p>This method refines {@link Collection#containsAll} to further specify that it <b>may not</b> 399 * throw an exception in response to any of {@code elements} being null or of the wrong type. 400 * 401 * <p><b>Note:</b> this method does not take into account the occurrence count of an element in 402 * the two collections; it may still return {@code true} even if {@code elements} contains several 403 * occurrences of an element and this multiset contains only one. This is no different than any 404 * other collection type like {@link List}, but it may be unexpected to the user of a multiset. 405 * 406 * @param elements the collection of elements to be checked for containment in this multiset 407 * @return {@code true} if this multiset contains at least one occurrence of each element 408 * contained in {@code elements} 409 * @throws NullPointerException if {@code elements} is null 410 */ 411 @Override 412 boolean containsAll(Collection<?> elements); 413 414 /** 415 * {@inheritDoc} 416 * 417 * <p><b>Note:</b> This method ignores how often any element might appear in {@code c}, and only 418 * cares whether or not an element appears at all. If you wish to remove one occurrence in this 419 * multiset for every occurrence in {@code c}, see {@link Multisets#removeOccurrences(Multiset, 420 * Multiset)}. 421 * 422 * <p>This method refines {@link Collection#removeAll} to further specify that it <b>may not</b> 423 * throw an exception in response to any of {@code elements} being null or of the wrong type. 424 */ 425 @CanIgnoreReturnValue 426 @Override 427 boolean removeAll(Collection<?> c); 428 429 /** 430 * {@inheritDoc} 431 * 432 * <p><b>Note:</b> This method ignores how often any element might appear in {@code c}, and only 433 * cares whether or not an element appears at all. If you wish to remove one occurrence in this 434 * multiset for every occurrence in {@code c}, see {@link Multisets#retainOccurrences(Multiset, 435 * Multiset)}. 436 * 437 * <p>This method refines {@link Collection#retainAll} to further specify that it <b>may not</b> 438 * throw an exception in response to any of {@code elements} being null or of the wrong type. 439 * 440 * @see Multisets#retainOccurrences(Multiset, Multiset) 441 */ 442 @CanIgnoreReturnValue 443 @Override 444 boolean retainAll(Collection<?> c); 445}