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.errorprone.annotations.CanIgnoreReturnValue; 025import com.google.errorprone.annotations.DoNotCall; 026import com.google.errorprone.annotations.DoNotMock; 027import java.io.Serializable; 028import java.util.AbstractCollection; 029import java.util.Arrays; 030import java.util.Collection; 031import java.util.Collections; 032import java.util.HashSet; 033import java.util.Iterator; 034import java.util.List; 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 ImmutableCollection() {} 172 173 /** Returns an unmodifiable iterator across the elements in this collection. */ 174 @Override 175 public abstract UnmodifiableIterator<E> iterator(); 176 177 private static final Object[] EMPTY_ARRAY = {}; 178 179 @Override 180 public final Object[] toArray() { 181 return toArray(EMPTY_ARRAY); 182 } 183 184 @CanIgnoreReturnValue 185 @Override 186 /* 187 * This suppression is here for two reasons: 188 * 189 * 1. b/192354773 in our checker affects toArray declarations. 190 * 191 * 2. `other[size] = null` is unsound. We could "fix" this by requiring callers to pass in an 192 * array with a nullable element type. But probably they usually want an array with a non-nullable 193 * type. That said, we could *accept* a `@Nullable T[]` (which, given that we treat arrays as 194 * covariant, would still permit a plain `T[]`) and return a plain `T[]`. But of course that would 195 * require its own suppression, since it is also unsound. toArray(T[]) is just a mess from a 196 * nullness perspective. The signature below at least has the virtue of being relatively simple. 197 */ 198 @SuppressWarnings("nullness") 199 public final <T extends @Nullable Object> T[] toArray(T[] other) { 200 checkNotNull(other); 201 int size = size(); 202 203 if (other.length < size) { 204 Object[] internal = internalArray(); 205 if (internal != null) { 206 return Platform.copy(internal, internalArrayStart(), internalArrayEnd(), other); 207 } 208 other = ObjectArrays.newArray(other, size); 209 } else if (other.length > size) { 210 other[size] = null; 211 } 212 copyIntoArray(other, 0); 213 return other; 214 } 215 216 /** If this collection is backed by an array of its elements in insertion order, returns it. */ 217 @CheckForNull 218 @Nullable 219 Object[] internalArray() { 220 return null; 221 } 222 223 /** 224 * If this collection is backed by an array of its elements in insertion order, returns the offset 225 * where this collection's elements start. 226 */ 227 int internalArrayStart() { 228 throw new UnsupportedOperationException(); 229 } 230 231 /** 232 * If this collection is backed by an array of its elements in insertion order, returns the offset 233 * where this collection's elements end. 234 */ 235 int internalArrayEnd() { 236 throw new UnsupportedOperationException(); 237 } 238 239 @Override 240 public abstract boolean contains(@CheckForNull Object object); 241 242 /** 243 * Guaranteed to throw an exception and leave the collection unmodified. 244 * 245 * @throws UnsupportedOperationException always 246 * @deprecated Unsupported operation. 247 */ 248 @CanIgnoreReturnValue 249 @Deprecated 250 @Override 251 @DoNotCall("Always throws UnsupportedOperationException") 252 public final boolean add(E e) { 253 throw new UnsupportedOperationException(); 254 } 255 256 /** 257 * Guaranteed to throw an exception and leave the collection unmodified. 258 * 259 * @throws UnsupportedOperationException always 260 * @deprecated Unsupported operation. 261 */ 262 @CanIgnoreReturnValue 263 @Deprecated 264 @Override 265 @DoNotCall("Always throws UnsupportedOperationException") 266 public final boolean remove(@CheckForNull Object object) { 267 throw new UnsupportedOperationException(); 268 } 269 270 /** 271 * Guaranteed to throw an exception and leave the collection unmodified. 272 * 273 * @throws UnsupportedOperationException always 274 * @deprecated Unsupported operation. 275 */ 276 @CanIgnoreReturnValue 277 @Deprecated 278 @Override 279 @DoNotCall("Always throws UnsupportedOperationException") 280 public final boolean addAll(Collection<? extends E> newElements) { 281 throw new UnsupportedOperationException(); 282 } 283 284 /** 285 * Guaranteed to throw an exception and leave the collection unmodified. 286 * 287 * @throws UnsupportedOperationException always 288 * @deprecated Unsupported operation. 289 */ 290 @CanIgnoreReturnValue 291 @Deprecated 292 @Override 293 @DoNotCall("Always throws UnsupportedOperationException") 294 public final boolean removeAll(Collection<?> oldElements) { 295 throw new UnsupportedOperationException(); 296 } 297 298 /** 299 * Guaranteed to throw an exception and leave the collection unmodified. 300 * 301 * @throws UnsupportedOperationException always 302 * @deprecated Unsupported operation. 303 */ 304 @CanIgnoreReturnValue 305 @Deprecated 306 @Override 307 @DoNotCall("Always throws UnsupportedOperationException") 308 public final boolean retainAll(Collection<?> elementsToKeep) { 309 throw new UnsupportedOperationException(); 310 } 311 312 /** 313 * Guaranteed to throw an exception and leave the collection unmodified. 314 * 315 * @throws UnsupportedOperationException always 316 * @deprecated Unsupported operation. 317 */ 318 @Deprecated 319 @Override 320 @DoNotCall("Always throws UnsupportedOperationException") 321 public final void clear() { 322 throw new UnsupportedOperationException(); 323 } 324 325 /** 326 * Returns an {@code ImmutableList} containing the same elements, in the same order, as this 327 * collection. 328 * 329 * <p><b>Performance note:</b> in most cases this method can return quickly without actually 330 * copying anything. The exact circumstances under which the copy is performed are undefined and 331 * subject to change. 332 * 333 * @since 2.0 334 */ 335 public ImmutableList<E> asList() { 336 return isEmpty() ? ImmutableList.<E>of() : ImmutableList.<E>asImmutableList(toArray()); 337 } 338 339 /** 340 * Returns {@code true} if this immutable collection's implementation contains references to 341 * user-created objects that aren't accessible via this collection's methods. This is generally 342 * used to determine whether {@code copyOf} implementations should make an explicit copy to avoid 343 * memory leaks. 344 */ 345 abstract boolean isPartialView(); 346 347 /** 348 * Copies the contents of this immutable collection into the specified array at the specified 349 * offset. Returns {@code offset + size()}. 350 */ 351 @CanIgnoreReturnValue 352 int copyIntoArray(@Nullable Object[] dst, int offset) { 353 for (E e : this) { 354 dst[offset++] = e; 355 } 356 return offset; 357 } 358 359 Object writeReplace() { 360 // We serialize by default to ImmutableList, the simplest thing that works. 361 return new ImmutableList.SerializedForm(toArray()); 362 } 363 364 /** 365 * Abstract base class for builders of {@link ImmutableCollection} types. 366 * 367 * @since 10.0 368 */ 369 @DoNotMock 370 public abstract static class Builder<E> { 371 static final int DEFAULT_INITIAL_CAPACITY = 4; 372 373 static int expandedCapacity(int oldCapacity, int minCapacity) { 374 if (minCapacity < 0) { 375 throw new AssertionError("cannot store more than MAX_VALUE elements"); 376 } 377 // careful of overflow! 378 int newCapacity = oldCapacity + (oldCapacity >> 1) + 1; 379 if (newCapacity < minCapacity) { 380 newCapacity = Integer.highestOneBit(minCapacity - 1) << 1; 381 } 382 if (newCapacity < 0) { 383 newCapacity = Integer.MAX_VALUE; 384 // guaranteed to be >= newCapacity 385 } 386 return newCapacity; 387 } 388 389 Builder() {} 390 391 /** 392 * Adds {@code element} to the {@code ImmutableCollection} being built. 393 * 394 * <p>Note that each builder class covariantly returns its own type from this method. 395 * 396 * @param element the element to add 397 * @return this {@code Builder} instance 398 * @throws NullPointerException if {@code element} is null 399 */ 400 @CanIgnoreReturnValue 401 public abstract Builder<E> add(E element); 402 403 /** 404 * Adds each element of {@code elements} to the {@code ImmutableCollection} being built. 405 * 406 * <p>Note that each builder class overrides this method in order to covariantly return its own 407 * type. 408 * 409 * @param elements the elements to add 410 * @return this {@code Builder} instance 411 * @throws NullPointerException if {@code elements} is null or contains a null element 412 */ 413 @CanIgnoreReturnValue 414 public Builder<E> add(E... elements) { 415 for (E element : elements) { 416 add(element); 417 } 418 return this; 419 } 420 421 /** 422 * Adds each element of {@code elements} to the {@code ImmutableCollection} being built. 423 * 424 * <p>Note that each builder class overrides this method in order to covariantly return its own 425 * type. 426 * 427 * @param elements the elements to add 428 * @return this {@code Builder} instance 429 * @throws NullPointerException if {@code elements} is null or contains a null element 430 */ 431 @CanIgnoreReturnValue 432 public Builder<E> addAll(Iterable<? extends E> elements) { 433 for (E element : elements) { 434 add(element); 435 } 436 return this; 437 } 438 439 /** 440 * Adds each element of {@code elements} to the {@code ImmutableCollection} being built. 441 * 442 * <p>Note that each builder class overrides this method in order to covariantly return its own 443 * type. 444 * 445 * @param elements the elements to add 446 * @return this {@code Builder} instance 447 * @throws NullPointerException if {@code elements} is null or contains a null element 448 */ 449 @CanIgnoreReturnValue 450 public Builder<E> addAll(Iterator<? extends E> elements) { 451 while (elements.hasNext()) { 452 add(elements.next()); 453 } 454 return this; 455 } 456 457 /** 458 * Returns a newly-created {@code ImmutableCollection} of the appropriate type, containing the 459 * elements provided to this builder. 460 * 461 * <p>Note that each builder class covariantly returns the appropriate type of {@code 462 * ImmutableCollection} from this method. 463 */ 464 public abstract ImmutableCollection<E> build(); 465 } 466 467 abstract static class ArrayBasedBuilder<E> extends ImmutableCollection.Builder<E> { 468 // The first `size` elements are non-null. 469 @Nullable Object[] contents; 470 int size; 471 boolean forceCopy; 472 473 ArrayBasedBuilder(int initialCapacity) { 474 checkNonnegative(initialCapacity, "initialCapacity"); 475 this.contents = new @Nullable Object[initialCapacity]; 476 this.size = 0; 477 } 478 479 /* 480 * Expand the absolute capacity of the builder so it can accept at least the specified number of 481 * elements without being resized. Also, if we've already built a collection backed by the 482 * current array, create a new array. 483 */ 484 private void getReadyToExpandTo(int minCapacity) { 485 if (contents.length < minCapacity) { 486 this.contents = 487 Arrays.copyOf(this.contents, expandedCapacity(contents.length, minCapacity)); 488 forceCopy = false; 489 } else if (forceCopy) { 490 this.contents = contents.clone(); 491 forceCopy = false; 492 } 493 } 494 495 @CanIgnoreReturnValue 496 @Override 497 public ArrayBasedBuilder<E> add(E element) { 498 checkNotNull(element); 499 getReadyToExpandTo(size + 1); 500 contents[size++] = element; 501 return this; 502 } 503 504 @CanIgnoreReturnValue 505 @Override 506 public Builder<E> add(E... elements) { 507 addAll(elements, elements.length); 508 return this; 509 } 510 511 final void addAll(@Nullable Object[] elements, int n) { 512 checkElementsNotNull(elements, n); 513 getReadyToExpandTo(size + n); 514 /* 515 * The following call is not statically checked, since arraycopy accepts plain Object for its 516 * parameters. If it were statically checked, the checker would still be OK with it, since 517 * we're copying into a `contents` array whose type allows it to contain nulls. Still, it's 518 * worth noting that we promise not to put nulls into the array in the first `size` elements. 519 * We uphold that promise here because our callers promise that `elements` will not contain 520 * nulls in its first `n` elements. 521 */ 522 System.arraycopy(elements, 0, contents, size, n); 523 size += n; 524 } 525 526 @CanIgnoreReturnValue 527 @Override 528 public Builder<E> addAll(Iterable<? extends E> elements) { 529 if (elements instanceof Collection) { 530 Collection<?> collection = (Collection<?>) elements; 531 getReadyToExpandTo(size + collection.size()); 532 if (collection instanceof ImmutableCollection) { 533 ImmutableCollection<?> immutableCollection = (ImmutableCollection<?>) collection; 534 size = immutableCollection.copyIntoArray(contents, size); 535 return this; 536 } 537 } 538 super.addAll(elements); 539 return this; 540 } 541 } 542}