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