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; 020import static com.google.common.collect.CollectPreconditions.checkNonnegative; 021import static com.google.common.collect.ObjectArrays.checkElementsNotNull; 022 023import com.google.common.annotations.GwtCompatible; 024import com.google.common.annotations.GwtIncompatible; 025import com.google.common.annotations.J2ktIncompatible; 026import com.google.errorprone.annotations.CanIgnoreReturnValue; 027import com.google.errorprone.annotations.DoNotCall; 028import com.google.errorprone.annotations.DoNotMock; 029import java.io.InvalidObjectException; 030import java.io.ObjectInputStream; 031import java.io.Serializable; 032import java.util.AbstractCollection; 033import java.util.Arrays; 034import java.util.Collection; 035import java.util.Collections; 036import java.util.HashSet; 037import java.util.Iterator; 038import java.util.List; 039import java.util.Spliterator; 040import java.util.Spliterators; 041import javax.annotation.CheckForNull; 042import org.checkerframework.checker.nullness.qual.Nullable; 043 044/** 045 * A {@link Collection} whose contents will never change, and which offers a few additional 046 * guarantees detailed below. 047 * 048 * <p><b>Warning:</b> avoid <i>direct</i> usage of {@link ImmutableCollection} as a type (just as 049 * with {@link Collection} itself). Prefer subtypes such as {@link ImmutableSet} or {@link 050 * ImmutableList}, which have well-defined {@link #equals} semantics, thus avoiding a common source 051 * of bugs and confusion. 052 * 053 * <h3>About <i>all</i> {@code Immutable-} collections</h3> 054 * 055 * <p>The remainder of this documentation applies to every public {@code Immutable-} type in this 056 * package, whether it is a subtype of {@code ImmutableCollection} or not. 057 * 058 * <h4>Guarantees</h4> 059 * 060 * <p>Each makes the following guarantees: 061 * 062 * <ul> 063 * <li><b>Shallow immutability.</b> Elements can never be added, removed or replaced in this 064 * collection. This is a stronger guarantee than that of {@link 065 * Collections#unmodifiableCollection}, whose contents change whenever the wrapped collection 066 * is modified. 067 * <li><b>Null-hostility.</b> This collection will never contain a null element. 068 * <li><b>Deterministic iteration.</b> The iteration order is always well-defined, depending on 069 * how the collection was created. Typically this is insertion order unless an explicit 070 * ordering is otherwise specified (e.g. {@link ImmutableSortedSet#naturalOrder}). See the 071 * appropriate factory method for details. View collections such as {@link 072 * ImmutableMultiset#elementSet} iterate in the same order as the parent, except as noted. 073 * <li><b>Thread safety.</b> It is safe to access this collection concurrently from multiple 074 * threads. 075 * <li><b>Integrity.</b> This type cannot be subclassed outside this package (which would allow 076 * these guarantees to be violated). 077 * </ul> 078 * 079 * <h4>"Interfaces", not implementations</h4> 080 * 081 * <p>These are classes instead of interfaces to prevent external subtyping, but should be thought 082 * of as interfaces in every important sense. Each public class such as {@link ImmutableSet} is a 083 * <i>type</i> offering meaningful behavioral guarantees. This is substantially different from the 084 * case of (say) {@link HashSet}, which is an <i>implementation</i>, with semantics that were 085 * largely defined by its supertype. 086 * 087 * <p>For field types and method return types, you should generally use the immutable type (such as 088 * {@link ImmutableList}) instead of the general collection interface type (such as {@link List}). 089 * This communicates to your callers all of the semantic guarantees listed above, which is almost 090 * always very useful information. 091 * 092 * <p>On the other hand, a <i>parameter</i> type of {@link ImmutableList} is generally a nuisance to 093 * callers. Instead, accept {@link Iterable} and have your method or constructor body pass it to the 094 * appropriate {@code copyOf} method itself. 095 * 096 * <p>Expressing the immutability guarantee directly in the type that user code references is a 097 * powerful advantage. Although Java offers certain immutable collection factory methods, such as 098 * {@link Collections#singleton(Object)} and <a 099 * href="https://docs.oracle.com/javase/9/docs/api/java/util/Set.html#immutable">{@code Set.of}</a>, 100 * we recommend using <i>these</i> classes instead for this reason (as well as for consistency). 101 * 102 * <h4>Creation</h4> 103 * 104 * <p>Except for logically "abstract" types like {@code ImmutableCollection} itself, each {@code 105 * Immutable} type provides the static operations you need to obtain instances of that type. These 106 * usually include: 107 * 108 * <ul> 109 * <li>Static methods named {@code of}, accepting an explicit list of elements or entries. 110 * <li>Static methods named {@code copyOf} (or {@code copyOfSorted}), accepting an existing 111 * collection whose contents should be copied. 112 * <li>A static nested {@code Builder} class which can be used to populate a new immutable 113 * instance. 114 * </ul> 115 * 116 * <h4>Warnings</h4> 117 * 118 * <ul> 119 * <li><b>Warning:</b> as with any collection, it is almost always a bad idea to modify an element 120 * (in a way that affects its {@link Object#equals} behavior) while it is contained in a 121 * collection. Undefined behavior and bugs will result. It's generally best to avoid using 122 * mutable objects as elements at all, as many users may expect your "immutable" object to be 123 * <i>deeply</i> immutable. 124 * </ul> 125 * 126 * <h4>Performance notes</h4> 127 * 128 * <ul> 129 * <li>Implementations can be generally assumed to prioritize memory efficiency, then speed of 130 * access, and lastly speed of creation. 131 * <li>The {@code copyOf} methods will sometimes recognize that the actual copy operation is 132 * unnecessary; for example, {@code copyOf(copyOf(anArrayList))} should copy the data only 133 * once. This reduces the expense of habitually making defensive copies at API boundaries. 134 * However, the precise conditions for skipping the copy operation are undefined. 135 * <li><b>Warning:</b> a view collection such as {@link ImmutableMap#keySet} or {@link 136 * ImmutableList#subList} may retain a reference to the entire data set, preventing it from 137 * being garbage collected. If some of the data is no longer reachable through other means, 138 * this constitutes a memory leak. Pass the view collection to the appropriate {@code copyOf} 139 * method to obtain a correctly-sized copy. 140 * <li>The performance of using the associated {@code Builder} class can be assumed to be no 141 * worse, and possibly better, than creating a mutable collection and copying it. 142 * <li>Implementations generally do not cache hash codes. If your element or key type has a slow 143 * {@code hashCode} implementation, it should cache it itself. 144 * </ul> 145 * 146 * <h4>Example usage</h4> 147 * 148 * <pre>{@code 149 * class Foo { 150 * private static final ImmutableSet<String> RESERVED_CODES = 151 * ImmutableSet.of("AZ", "CQ", "ZX"); 152 * 153 * private final ImmutableSet<String> codes; 154 * 155 * public Foo(Iterable<String> codes) { 156 * this.codes = ImmutableSet.copyOf(codes); 157 * checkArgument(Collections.disjoint(this.codes, RESERVED_CODES)); 158 * } 159 * } 160 * }</pre> 161 * 162 * <h3>See also</h3> 163 * 164 * <p>See the Guava User Guide article on <a href= 165 * "https://github.com/google/guava/wiki/ImmutableCollectionsExplained">immutable collections</a>. 166 * 167 * @since 2.0 168 */ 169@DoNotMock("Use ImmutableList.of or another implementation") 170@GwtCompatible(emulated = true) 171@SuppressWarnings("serial") // we're overriding default serialization 172// TODO(kevinb): I think we should push everything down to "BaseImmutableCollection" or something, 173// just to do everything we can to emphasize the "practically an interface" nature of this class. 174public abstract class ImmutableCollection<E> extends AbstractCollection<E> implements Serializable { 175 /* 176 * We expect SIZED (and SUBSIZED, if applicable) to be added by the spliterator factory methods. 177 * These are properties of the collection as a whole; SIZED and SUBSIZED are more properties of 178 * the spliterator implementation. 179 */ 180 @SuppressWarnings("Java7ApiChecker") 181 // @IgnoreJRERequirement is not necessary because this compiles down to a constant. 182 // (which is fortunate because Animal Sniffer doesn't look for @IgnoreJRERequirement on fields) 183 static final int SPLITERATOR_CHARACTERISTICS = 184 Spliterator.IMMUTABLE | Spliterator.NONNULL | Spliterator.ORDERED; 185 186 ImmutableCollection() {} 187 188 /** Returns an unmodifiable iterator across the elements in this collection. */ 189 @Override 190 public abstract UnmodifiableIterator<E> iterator(); 191 192 @Override 193 @SuppressWarnings("Java7ApiChecker") 194 @IgnoreJRERequirement // used only from APIs with Java 8 types in them 195 // (not used within guava-android as of this writing, but we include it in the jar as a test) 196 public Spliterator<E> spliterator() { 197 return Spliterators.spliterator(this, SPLITERATOR_CHARACTERISTICS); 198 } 199 200 private static final Object[] EMPTY_ARRAY = {}; 201 202 @Override 203 @J2ktIncompatible // Incompatible return type change. Use inherited (unoptimized) implementation 204 public final Object[] toArray() { 205 return toArray(EMPTY_ARRAY); 206 } 207 208 @CanIgnoreReturnValue 209 @Override 210 /* 211 * This suppression is here for two reasons: 212 * 213 * 1. b/192354773 in our checker affects toArray declarations. 214 * 215 * 2. `other[size] = null` is unsound. We could "fix" this by requiring callers to pass in an 216 * array with a nullable element type. But probably they usually want an array with a non-nullable 217 * type. That said, we could *accept* a `@Nullable T[]` (which, given that we treat arrays as 218 * covariant, would still permit a plain `T[]`) and return a plain `T[]`. But of course that would 219 * require its own suppression, since it is also unsound. toArray(T[]) is just a mess from a 220 * nullness perspective. The signature below at least has the virtue of being relatively simple. 221 */ 222 @SuppressWarnings("nullness") 223 public final <T extends @Nullable Object> T[] toArray(T[] other) { 224 checkNotNull(other); 225 int size = size(); 226 227 if (other.length < size) { 228 Object[] internal = internalArray(); 229 if (internal != null) { 230 return Platform.copy(internal, internalArrayStart(), internalArrayEnd(), other); 231 } 232 other = ObjectArrays.newArray(other, size); 233 } else if (other.length > size) { 234 other[size] = null; 235 } 236 copyIntoArray(other, 0); 237 return other; 238 } 239 240 /** If this collection is backed by an array of its elements in insertion order, returns it. */ 241 @CheckForNull 242 @Nullable 243 Object[] internalArray() { 244 return null; 245 } 246 247 /** 248 * If this collection is backed by an array of its elements in insertion order, returns the offset 249 * where this collection's elements start. 250 */ 251 int internalArrayStart() { 252 throw new UnsupportedOperationException(); 253 } 254 255 /** 256 * If this collection is backed by an array of its elements in insertion order, returns the offset 257 * where this collection's elements end. 258 */ 259 int internalArrayEnd() { 260 throw new UnsupportedOperationException(); 261 } 262 263 @Override 264 public abstract boolean contains(@CheckForNull Object object); 265 266 /** 267 * Guaranteed to throw an exception and leave the collection unmodified. 268 * 269 * @throws UnsupportedOperationException always 270 * @deprecated Unsupported operation. 271 */ 272 @CanIgnoreReturnValue 273 @Deprecated 274 @Override 275 @DoNotCall("Always throws UnsupportedOperationException") 276 public final boolean add(E e) { 277 throw new UnsupportedOperationException(); 278 } 279 280 /** 281 * Guaranteed to throw an exception and leave the collection unmodified. 282 * 283 * @throws UnsupportedOperationException always 284 * @deprecated Unsupported operation. 285 */ 286 @CanIgnoreReturnValue 287 @Deprecated 288 @Override 289 @DoNotCall("Always throws UnsupportedOperationException") 290 public final boolean remove(@CheckForNull Object object) { 291 throw new UnsupportedOperationException(); 292 } 293 294 /** 295 * Guaranteed to throw an exception and leave the collection unmodified. 296 * 297 * @throws UnsupportedOperationException always 298 * @deprecated Unsupported operation. 299 */ 300 @CanIgnoreReturnValue 301 @Deprecated 302 @Override 303 @DoNotCall("Always throws UnsupportedOperationException") 304 public final boolean addAll(Collection<? extends E> newElements) { 305 throw new UnsupportedOperationException(); 306 } 307 308 /** 309 * Guaranteed to throw an exception and leave the collection unmodified. 310 * 311 * @throws UnsupportedOperationException always 312 * @deprecated Unsupported operation. 313 */ 314 @CanIgnoreReturnValue 315 @Deprecated 316 @Override 317 @DoNotCall("Always throws UnsupportedOperationException") 318 public final boolean removeAll(Collection<?> oldElements) { 319 throw new UnsupportedOperationException(); 320 } 321 322 /** 323 * Guaranteed to throw an exception and leave the collection unmodified. 324 * 325 * @throws UnsupportedOperationException always 326 * @deprecated Unsupported operation. 327 */ 328 @CanIgnoreReturnValue 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 return isEmpty() ? ImmutableList.of() : ImmutableList.asImmutableList(toArray()); 361 } 362 363 /** 364 * Returns {@code true} if this immutable collection's implementation contains references to 365 * user-created objects that aren't accessible via this collection's methods. This is generally 366 * used to determine whether {@code copyOf} implementations should make an explicit copy to avoid 367 * memory leaks. 368 */ 369 abstract boolean isPartialView(); 370 371 /** 372 * Copies the contents of this immutable collection into the specified array at the specified 373 * offset. Returns {@code offset + size()}. 374 */ 375 @CanIgnoreReturnValue 376 int copyIntoArray(@Nullable Object[] dst, int offset) { 377 for (E e : this) { 378 dst[offset++] = e; 379 } 380 return offset; 381 } 382 383 @J2ktIncompatible // serialization 384 @GwtIncompatible // serialization 385 Object writeReplace() { 386 // We serialize by default to ImmutableList, the simplest thing that works. 387 return new ImmutableList.SerializedForm(toArray()); 388 } 389 390 @J2ktIncompatible // serialization 391 private void readObject(ObjectInputStream stream) throws InvalidObjectException { 392 throw new InvalidObjectException("Use SerializedForm"); 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 IllegalArgumentException("cannot store more than Integer.MAX_VALUE elements"); 407 } else if (minCapacity <= oldCapacity) { 408 return oldCapacity; 409 } 410 // careful of overflow! 411 int newCapacity = oldCapacity + (oldCapacity >> 1) + 1; 412 if (newCapacity < minCapacity) { 413 newCapacity = Integer.highestOneBit(minCapacity - 1) << 1; 414 } 415 if (newCapacity < 0) { 416 newCapacity = Integer.MAX_VALUE; 417 // guaranteed to be >= newCapacity 418 } 419 return newCapacity; 420 } 421 422 Builder() {} 423 424 /** 425 * Adds {@code element} to the {@code ImmutableCollection} being built. 426 * 427 * <p>Note that each builder class covariantly returns its own type from this method. 428 * 429 * @param element the element to add 430 * @return this {@code Builder} instance 431 * @throws NullPointerException if {@code element} is null 432 */ 433 @CanIgnoreReturnValue 434 public abstract Builder<E> add(E element); 435 436 /** 437 * Adds each element of {@code elements} to the {@code ImmutableCollection} being built. 438 * 439 * <p>Note that each builder class overrides this method in order to covariantly return its own 440 * type. 441 * 442 * @param elements the elements to add 443 * @return this {@code Builder} instance 444 * @throws NullPointerException if {@code elements} is null or contains a null element 445 */ 446 @CanIgnoreReturnValue 447 public Builder<E> add(E... elements) { 448 for (E element : elements) { 449 add(element); 450 } 451 return this; 452 } 453 454 /** 455 * Adds each element of {@code elements} to the {@code ImmutableCollection} being built. 456 * 457 * <p>Note that each builder class overrides this method in order to covariantly return its own 458 * type. 459 * 460 * @param elements the elements to add 461 * @return this {@code Builder} instance 462 * @throws NullPointerException if {@code elements} is null or contains a null element 463 */ 464 @CanIgnoreReturnValue 465 public Builder<E> addAll(Iterable<? extends E> elements) { 466 for (E element : elements) { 467 add(element); 468 } 469 return this; 470 } 471 472 /** 473 * Adds each element of {@code elements} to the {@code ImmutableCollection} being built. 474 * 475 * <p>Note that each builder class overrides this method in order to covariantly return its own 476 * type. 477 * 478 * @param elements the elements to add 479 * @return this {@code Builder} instance 480 * @throws NullPointerException if {@code elements} is null or contains a null element 481 */ 482 @CanIgnoreReturnValue 483 public Builder<E> addAll(Iterator<? extends E> elements) { 484 while (elements.hasNext()) { 485 add(elements.next()); 486 } 487 return this; 488 } 489 490 /** 491 * Returns a newly-created {@code ImmutableCollection} of the appropriate type, containing the 492 * elements provided to this builder. 493 * 494 * <p>Note that each builder class covariantly returns the appropriate type of {@code 495 * ImmutableCollection} from this method. 496 */ 497 public abstract ImmutableCollection<E> build(); 498 } 499 500 abstract static class ArrayBasedBuilder<E> extends ImmutableCollection.Builder<E> { 501 // The first `size` elements are non-null. 502 @Nullable Object[] contents; 503 int size; 504 boolean forceCopy; 505 506 ArrayBasedBuilder(int initialCapacity) { 507 checkNonnegative(initialCapacity, "initialCapacity"); 508 this.contents = new @Nullable Object[initialCapacity]; 509 this.size = 0; 510 } 511 512 /* 513 * Expand the absolute capacity of the builder so it can accept at least the specified number of 514 * elements without being resized. Also, if we've already built a collection backed by the 515 * current array, create a new array. 516 */ 517 private void ensureRoomFor(int newElements) { 518 @Nullable Object[] contents = this.contents; 519 int newCapacity = expandedCapacity(contents.length, size + newElements); 520 // expandedCapacity handles the overflow case 521 if (newCapacity > contents.length || forceCopy) { 522 this.contents = Arrays.copyOf(this.contents, newCapacity); 523 forceCopy = false; 524 } 525 } 526 527 @CanIgnoreReturnValue 528 @Override 529 public ArrayBasedBuilder<E> add(E element) { 530 checkNotNull(element); 531 ensureRoomFor(1); 532 contents[size++] = element; 533 return this; 534 } 535 536 @CanIgnoreReturnValue 537 @Override 538 public Builder<E> add(E... elements) { 539 addAll(elements, elements.length); 540 return this; 541 } 542 543 final void addAll(@Nullable Object[] elements, int n) { 544 checkElementsNotNull(elements, n); 545 ensureRoomFor(n); 546 /* 547 * The following call is not statically checked, since arraycopy accepts plain Object for its 548 * parameters. If it were statically checked, the checker would still be OK with it, since 549 * we're copying into a `contents` array whose type allows it to contain nulls. Still, it's 550 * worth noting that we promise not to put nulls into the array in the first `size` elements. 551 * We uphold that promise here because our callers promise that `elements` will not contain 552 * nulls in its first `n` elements. 553 */ 554 System.arraycopy(elements, 0, contents, size, n); 555 size += n; 556 } 557 558 @CanIgnoreReturnValue 559 @Override 560 public Builder<E> addAll(Iterable<? extends E> elements) { 561 if (elements instanceof Collection) { 562 Collection<?> collection = (Collection<?>) elements; 563 ensureRoomFor(collection.size()); 564 if (collection instanceof ImmutableCollection) { 565 ImmutableCollection<?> immutableCollection = (ImmutableCollection<?>) collection; 566 size = immutableCollection.copyIntoArray(contents, size); 567 return this; 568 } 569 } 570 super.addAll(elements); 571 return this; 572 } 573 } 574 575 private static final long serialVersionUID = 0xdecaf; 576}