001/* 002 * Copyright (C) 2008 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.DoNotCall; 024import com.google.errorprone.annotations.DoNotMock; 025import java.io.Serializable; 026import java.util.AbstractCollection; 027import java.util.Collection; 028import java.util.Collections; 029import java.util.HashSet; 030import java.util.Iterator; 031import java.util.List; 032import java.util.Spliterator; 033import java.util.Spliterators; 034import java.util.function.Predicate; 035import javax.annotation.CheckForNull; 036import org.checkerframework.checker.nullness.qual.Nullable; 037 038/** 039 * A {@link Collection} whose contents will never change, and which offers a few additional 040 * guarantees detailed below. 041 * 042 * <p><b>Warning:</b> avoid <i>direct</i> usage of {@link ImmutableCollection} as a type (just as 043 * with {@link Collection} itself). Prefer subtypes such as {@link ImmutableSet} or {@link 044 * ImmutableList}, which have well-defined {@link #equals} semantics, thus avoiding a common source 045 * of bugs and confusion. 046 * 047 * <h3>About <i>all</i> {@code Immutable-} collections</h3> 048 * 049 * <p>The remainder of this documentation applies to every public {@code Immutable-} type in this 050 * package, whether it is a subtype of {@code ImmutableCollection} or not. 051 * 052 * <h4>Guarantees</h4> 053 * 054 * <p>Each makes the following guarantees: 055 * 056 * <ul> 057 * <li><b>Shallow immutability.</b> Elements can never be added, removed or replaced in this 058 * collection. This is a stronger guarantee than that of {@link 059 * Collections#unmodifiableCollection}, whose contents change whenever the wrapped collection 060 * is modified. 061 * <li><b>Null-hostility.</b> This collection will never contain a null element. 062 * <li><b>Deterministic iteration.</b> The iteration order is always well-defined, depending on 063 * how the collection was created. Typically this is insertion order unless an explicit 064 * ordering is otherwise specified (e.g. {@link ImmutableSortedSet#naturalOrder}). See the 065 * appropriate factory method for details. View collections such as {@link 066 * ImmutableMultiset#elementSet} iterate in the same order as the parent, except as noted. 067 * <li><b>Thread safety.</b> It is safe to access this collection concurrently from multiple 068 * threads. 069 * <li><b>Integrity.</b> This type cannot be subclassed outside this package (which would allow 070 * these guarantees to be violated). 071 * </ul> 072 * 073 * <h4>"Interfaces", not implementations</h4> 074 * 075 * <p>These are classes instead of interfaces to prevent external subtyping, but should be thought 076 * of as interfaces in every important sense. Each public class such as {@link ImmutableSet} is a 077 * <i>type</i> offering meaningful behavioral guarantees. This is substantially different from the 078 * case of (say) {@link HashSet}, which is an <i>implementation</i>, with semantics that were 079 * largely defined by its supertype. 080 * 081 * <p>For field types and method return types, you should generally use the immutable type (such as 082 * {@link ImmutableList}) instead of the general collection interface type (such as {@link List}). 083 * This communicates to your callers all of the semantic guarantees listed above, which is almost 084 * always very useful information. 085 * 086 * <p>On the other hand, a <i>parameter</i> type of {@link ImmutableList} is generally a nuisance to 087 * callers. Instead, accept {@link Iterable} and have your method or constructor body pass it to the 088 * appropriate {@code copyOf} method itself. 089 * 090 * <p>Expressing the immutability guarantee directly in the type that user code references is a 091 * powerful advantage. Although Java offers certain immutable collection factory methods, such as 092 * {@link Collections#singleton(Object)} and <a 093 * href="https://docs.oracle.com/javase/9/docs/api/java/util/Set.html#immutable">{@code Set.of}</a>, 094 * we recommend using <i>these</i> classes instead for this reason (as well as for consistency). 095 * 096 * <h4>Creation</h4> 097 * 098 * <p>Except for logically "abstract" types like {@code ImmutableCollection} itself, each {@code 099 * Immutable} type provides the static operations you need to obtain instances of that type. These 100 * usually include: 101 * 102 * <ul> 103 * <li>Static methods named {@code of}, accepting an explicit list of elements or entries. 104 * <li>Static methods named {@code copyOf} (or {@code copyOfSorted}), accepting an existing 105 * collection whose contents should be copied. 106 * <li>A static nested {@code Builder} class which can be used to populate a new immutable 107 * instance. 108 * </ul> 109 * 110 * <h4>Warnings</h4> 111 * 112 * <ul> 113 * <li><b>Warning:</b> as with any collection, it is almost always a bad idea to modify an element 114 * (in a way that affects its {@link Object#equals} behavior) while it is contained in a 115 * collection. Undefined behavior and bugs will result. It's generally best to avoid using 116 * mutable objects as elements at all, as many users may expect your "immutable" object to be 117 * <i>deeply</i> immutable. 118 * </ul> 119 * 120 * <h4>Performance notes</h4> 121 * 122 * <ul> 123 * <li>Implementations can be generally assumed to prioritize memory efficiency, then speed of 124 * access, and lastly speed of creation. 125 * <li>The {@code copyOf} methods will sometimes recognize that the actual copy operation is 126 * unnecessary; for example, {@code copyOf(copyOf(anArrayList))} should copy the data only 127 * once. This reduces the expense of habitually making defensive copies at API boundaries. 128 * However, the precise conditions for skipping the copy operation are undefined. 129 * <li><b>Warning:</b> a view collection such as {@link ImmutableMap#keySet} or {@link 130 * ImmutableList#subList} may retain a reference to the entire data set, preventing it from 131 * being garbage collected. If some of the data is no longer reachable through other means, 132 * this constitutes a memory leak. Pass the view collection to the appropriate {@code copyOf} 133 * method to obtain a correctly-sized copy. 134 * <li>The performance of using the associated {@code Builder} class can be assumed to be no 135 * worse, and possibly better, than creating a mutable collection and copying it. 136 * <li>Implementations generally do not cache hash codes. If your element or key type has a slow 137 * {@code hashCode} implementation, it should cache it itself. 138 * </ul> 139 * 140 * <h4>Example usage</h4> 141 * 142 * <pre>{@code 143 * class Foo { 144 * private static final ImmutableSet<String> RESERVED_CODES = 145 * ImmutableSet.of("AZ", "CQ", "ZX"); 146 * 147 * private final ImmutableSet<String> codes; 148 * 149 * public Foo(Iterable<String> codes) { 150 * this.codes = ImmutableSet.copyOf(codes); 151 * checkArgument(Collections.disjoint(this.codes, RESERVED_CODES)); 152 * } 153 * } 154 * }</pre> 155 * 156 * <h3>See also</h3> 157 * 158 * <p>See the Guava User Guide article on <a href= 159 * "https://github.com/google/guava/wiki/ImmutableCollectionsExplained"> immutable collections</a>. 160 * 161 * @since 2.0 162 */ 163@DoNotMock("Use ImmutableList.of or another implementation") 164@GwtCompatible(emulated = true) 165@SuppressWarnings("serial") // we're overriding default serialization 166@ElementTypesAreNonnullByDefault 167// TODO(kevinb): I think we should push everything down to "BaseImmutableCollection" or something, 168// just to do everything we can to emphasize the "practically an interface" nature of this class. 169public abstract class ImmutableCollection<E> extends AbstractCollection<E> implements Serializable { 170 /* 171 * We expect SIZED (and SUBSIZED, if applicable) to be added by the spliterator factory methods. 172 * These are properties of the collection as a whole; SIZED and SUBSIZED are more properties of 173 * the spliterator implementation. 174 */ 175 static final int SPLITERATOR_CHARACTERISTICS = 176 Spliterator.IMMUTABLE | Spliterator.NONNULL | Spliterator.ORDERED; 177 178 ImmutableCollection() {} 179 180 /** Returns an unmodifiable iterator across the elements in this collection. */ 181 @Override 182 public abstract UnmodifiableIterator<E> iterator(); 183 184 @Override 185 public Spliterator<E> spliterator() { 186 return Spliterators.spliterator(this, SPLITERATOR_CHARACTERISTICS); 187 } 188 189 private static final Object[] EMPTY_ARRAY = {}; 190 191 @Override 192 public final Object[] toArray() { 193 return toArray(EMPTY_ARRAY); 194 } 195 196 @CanIgnoreReturnValue 197 @Override 198 /* 199 * This suppression is here for two reasons: 200 * 201 * 1. b/192354773 in our checker affects toArray declarations. 202 * 203 * 2. `other[size] = null` is unsound. We could "fix" this by requiring callers to pass in an 204 * array with a nullable element type. But probably they usually want an array with a non-nullable 205 * type. That said, we could *accept* a `@Nullable T[]` (which, given that we treat arrays as 206 * covariant, would still permit a plain `T[]`) and return a plain `T[]`. But of course that would 207 * require its own suppression, since it is also unsound. toArray(T[]) is just a mess from a 208 * nullness perspective. The signature below at least has the virtue of being relatively simple. 209 */ 210 @SuppressWarnings("nullness") 211 public final <T extends @Nullable Object> T[] toArray(T[] other) { 212 checkNotNull(other); 213 int size = size(); 214 215 if (other.length < size) { 216 Object[] internal = internalArray(); 217 if (internal != null) { 218 return Platform.copy(internal, internalArrayStart(), internalArrayEnd(), other); 219 } 220 other = ObjectArrays.newArray(other, size); 221 } else if (other.length > size) { 222 other[size] = null; 223 } 224 copyIntoArray(other, 0); 225 return other; 226 } 227 228 /** If this collection is backed by an array of its elements in insertion order, returns it. */ 229 @CheckForNull 230 Object[] internalArray() { 231 return null; 232 } 233 234 /** 235 * If this collection is backed by an array of its elements in insertion order, returns the offset 236 * where this collection's elements start. 237 */ 238 int internalArrayStart() { 239 throw new UnsupportedOperationException(); 240 } 241 242 /** 243 * If this collection is backed by an array of its elements in insertion order, returns the offset 244 * where this collection's elements end. 245 */ 246 int internalArrayEnd() { 247 throw new UnsupportedOperationException(); 248 } 249 250 @Override 251 public abstract boolean contains(@CheckForNull Object object); 252 253 /** 254 * Guaranteed to throw an exception and leave the collection unmodified. 255 * 256 * @throws UnsupportedOperationException always 257 * @deprecated Unsupported operation. 258 */ 259 @CanIgnoreReturnValue 260 @Deprecated 261 @Override 262 @DoNotCall("Always throws UnsupportedOperationException") 263 public final boolean add(E e) { 264 throw new UnsupportedOperationException(); 265 } 266 267 /** 268 * Guaranteed to throw an exception and leave the collection unmodified. 269 * 270 * @throws UnsupportedOperationException always 271 * @deprecated Unsupported operation. 272 */ 273 @CanIgnoreReturnValue 274 @Deprecated 275 @Override 276 @DoNotCall("Always throws UnsupportedOperationException") 277 public final boolean remove(@CheckForNull Object object) { 278 throw new UnsupportedOperationException(); 279 } 280 281 /** 282 * Guaranteed to throw an exception and leave the collection unmodified. 283 * 284 * @throws UnsupportedOperationException always 285 * @deprecated Unsupported operation. 286 */ 287 @CanIgnoreReturnValue 288 @Deprecated 289 @Override 290 @DoNotCall("Always throws UnsupportedOperationException") 291 public final boolean addAll(Collection<? extends E> newElements) { 292 throw new UnsupportedOperationException(); 293 } 294 295 /** 296 * Guaranteed to throw an exception and leave the collection unmodified. 297 * 298 * @throws UnsupportedOperationException always 299 * @deprecated Unsupported operation. 300 */ 301 @CanIgnoreReturnValue 302 @Deprecated 303 @Override 304 @DoNotCall("Always throws UnsupportedOperationException") 305 public final boolean removeAll(Collection<?> oldElements) { 306 throw new UnsupportedOperationException(); 307 } 308 309 /** 310 * Guaranteed to throw an exception and leave the collection unmodified. 311 * 312 * @throws UnsupportedOperationException always 313 * @deprecated Unsupported operation. 314 */ 315 @CanIgnoreReturnValue 316 @Deprecated 317 @Override 318 @DoNotCall("Always throws UnsupportedOperationException") 319 public final boolean removeIf(Predicate<? super E> filter) { 320 throw new UnsupportedOperationException(); 321 } 322 323 /** 324 * Guaranteed to throw an exception and leave the collection unmodified. 325 * 326 * @throws UnsupportedOperationException always 327 * @deprecated Unsupported operation. 328 */ 329 @Deprecated 330 @Override 331 @DoNotCall("Always throws UnsupportedOperationException") 332 public final boolean retainAll(Collection<?> elementsToKeep) { 333 throw new UnsupportedOperationException(); 334 } 335 336 /** 337 * Guaranteed to throw an exception and leave the collection unmodified. 338 * 339 * @throws UnsupportedOperationException always 340 * @deprecated Unsupported operation. 341 */ 342 @Deprecated 343 @Override 344 @DoNotCall("Always throws UnsupportedOperationException") 345 public final void clear() { 346 throw new UnsupportedOperationException(); 347 } 348 349 /** 350 * Returns an {@code ImmutableList} containing the same elements, in the same order, as this 351 * collection. 352 * 353 * <p><b>Performance note:</b> in most cases this method can return quickly without actually 354 * copying anything. The exact circumstances under which the copy is performed are undefined and 355 * subject to change. 356 * 357 * @since 2.0 358 */ 359 public ImmutableList<E> asList() { 360 switch (size()) { 361 case 0: 362 return ImmutableList.of(); 363 case 1: 364 return ImmutableList.of(iterator().next()); 365 default: 366 return new RegularImmutableAsList<E>(this, toArray()); 367 } 368 } 369 370 /** 371 * Returns {@code true} if this immutable collection's implementation contains references to 372 * user-created objects that aren't accessible via this collection's methods. This is generally 373 * used to determine whether {@code copyOf} implementations should make an explicit copy to avoid 374 * memory leaks. 375 */ 376 abstract boolean isPartialView(); 377 378 /** 379 * Copies the contents of this immutable collection into the specified array at the specified 380 * offset. Returns {@code offset + size()}. 381 */ 382 @CanIgnoreReturnValue 383 int copyIntoArray(@Nullable Object[] dst, int offset) { 384 for (E e : this) { 385 dst[offset++] = e; 386 } 387 return offset; 388 } 389 390 Object writeReplace() { 391 // We serialize by default to ImmutableList, the simplest thing that works. 392 return new ImmutableList.SerializedForm(toArray()); 393 } 394 395 /** 396 * Abstract base class for builders of {@link ImmutableCollection} types. 397 * 398 * @since 10.0 399 */ 400 @DoNotMock 401 public abstract static class Builder<E> { 402 static final int DEFAULT_INITIAL_CAPACITY = 4; 403 404 static int expandedCapacity(int oldCapacity, int minCapacity) { 405 if (minCapacity < 0) { 406 throw new AssertionError("cannot store more than MAX_VALUE elements"); 407 } 408 // careful of overflow! 409 int newCapacity = oldCapacity + (oldCapacity >> 1) + 1; 410 if (newCapacity < minCapacity) { 411 newCapacity = Integer.highestOneBit(minCapacity - 1) << 1; 412 } 413 if (newCapacity < 0) { 414 newCapacity = Integer.MAX_VALUE; 415 // guaranteed to be >= newCapacity 416 } 417 return newCapacity; 418 } 419 420 Builder() {} 421 422 /** 423 * Adds {@code element} to the {@code ImmutableCollection} being built. 424 * 425 * <p>Note that each builder class covariantly returns its own type from this method. 426 * 427 * @param element the element to add 428 * @return this {@code Builder} instance 429 * @throws NullPointerException if {@code element} is null 430 */ 431 @CanIgnoreReturnValue 432 public abstract Builder<E> add(E element); 433 434 /** 435 * Adds each element of {@code elements} to the {@code ImmutableCollection} being built. 436 * 437 * <p>Note that each builder class overrides this method in order to covariantly return its own 438 * type. 439 * 440 * @param elements the elements to add 441 * @return this {@code Builder} instance 442 * @throws NullPointerException if {@code elements} is null or contains a null element 443 */ 444 @CanIgnoreReturnValue 445 public Builder<E> add(E... elements) { 446 for (E element : elements) { 447 add(element); 448 } 449 return this; 450 } 451 452 /** 453 * Adds each element of {@code elements} to the {@code ImmutableCollection} being built. 454 * 455 * <p>Note that each builder class overrides this method in order to covariantly return its own 456 * type. 457 * 458 * @param elements the elements to add 459 * @return this {@code Builder} instance 460 * @throws NullPointerException if {@code elements} is null or contains a null element 461 */ 462 @CanIgnoreReturnValue 463 public Builder<E> addAll(Iterable<? extends E> elements) { 464 for (E element : elements) { 465 add(element); 466 } 467 return this; 468 } 469 470 /** 471 * Adds each element of {@code elements} to the {@code ImmutableCollection} being built. 472 * 473 * <p>Note that each builder class overrides this method in order to covariantly return its own 474 * type. 475 * 476 * @param elements the elements to add 477 * @return this {@code Builder} instance 478 * @throws NullPointerException if {@code elements} is null or contains a null element 479 */ 480 @CanIgnoreReturnValue 481 public Builder<E> addAll(Iterator<? extends E> elements) { 482 while (elements.hasNext()) { 483 add(elements.next()); 484 } 485 return this; 486 } 487 488 /** 489 * Returns a newly-created {@code ImmutableCollection} of the appropriate type, containing the 490 * elements provided to this builder. 491 * 492 * <p>Note that each builder class covariantly returns the appropriate type of {@code 493 * ImmutableCollection} from this method. 494 */ 495 public abstract ImmutableCollection<E> build(); 496 } 497}