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.common.annotations.J2ktIncompatible;
023import com.google.errorprone.annotations.CanIgnoreReturnValue;
024import com.google.errorprone.annotations.DoNotCall;
025import com.google.errorprone.annotations.DoNotMock;
026import java.io.InvalidObjectException;
027import java.io.ObjectInputStream;
028import java.io.Serializable;
029import java.util.AbstractCollection;
030import java.util.Collection;
031import java.util.Collections;
032import java.util.HashSet;
033import java.util.Iterator;
034import java.util.List;
035import java.util.Spliterator;
036import java.util.Spliterators;
037import java.util.function.Predicate;
038import javax.annotation.CheckForNull;
039import org.checkerframework.checker.nullness.qual.Nullable;
040
041/**
042 * A {@link Collection} whose contents will never change, and which offers a few additional
043 * guarantees detailed below.
044 *
045 * <p><b>Warning:</b> avoid <i>direct</i> usage of {@link ImmutableCollection} as a type (just as
046 * with {@link Collection} itself). Prefer subtypes such as {@link ImmutableSet} or {@link
047 * ImmutableList}, which have well-defined {@link #equals} semantics, thus avoiding a common source
048 * of bugs and confusion.
049 *
050 * <h3>About <i>all</i> {@code Immutable-} collections</h3>
051 *
052 * <p>The remainder of this documentation applies to every public {@code Immutable-} type in this
053 * package, whether it is a subtype of {@code ImmutableCollection} or not.
054 *
055 * <h4>Guarantees</h4>
056 *
057 * <p>Each makes the following guarantees:
058 *
059 * <ul>
060 *   <li><b>Shallow immutability.</b> Elements can never be added, removed or replaced in this
061 *       collection. This is a stronger guarantee than that of {@link
062 *       Collections#unmodifiableCollection}, whose contents change whenever the wrapped collection
063 *       is modified.
064 *   <li><b>Null-hostility.</b> This collection will never contain a null element.
065 *   <li><b>Deterministic iteration.</b> The iteration order is always well-defined, depending on
066 *       how the collection was created. Typically this is insertion order unless an explicit
067 *       ordering is otherwise specified (e.g. {@link ImmutableSortedSet#naturalOrder}). See the
068 *       appropriate factory method for details. View collections such as {@link
069 *       ImmutableMultiset#elementSet} iterate in the same order as the parent, except as noted.
070 *   <li><b>Thread safety.</b> It is safe to access this collection concurrently from multiple
071 *       threads.
072 *   <li><b>Integrity.</b> This type cannot be subclassed outside this package (which would allow
073 *       these guarantees to be violated).
074 * </ul>
075 *
076 * <h4>"Interfaces", not implementations</h4>
077 *
078 * <p>These are classes instead of interfaces to prevent external subtyping, but should be thought
079 * of as interfaces in every important sense. Each public class such as {@link ImmutableSet} is a
080 * <i>type</i> offering meaningful behavioral guarantees. This is substantially different from the
081 * case of (say) {@link HashSet}, which is an <i>implementation</i>, with semantics that were
082 * largely defined by its supertype.
083 *
084 * <p>For field types and method return types, you should generally use the immutable type (such as
085 * {@link ImmutableList}) instead of the general collection interface type (such as {@link List}).
086 * This communicates to your callers all of the semantic guarantees listed above, which is almost
087 * always very useful information.
088 *
089 * <p>On the other hand, a <i>parameter</i> type of {@link ImmutableList} is generally a nuisance to
090 * callers. Instead, accept {@link Iterable} and have your method or constructor body pass it to the
091 * appropriate {@code copyOf} method itself.
092 *
093 * <p>Expressing the immutability guarantee directly in the type that user code references is a
094 * powerful advantage. Although Java offers certain immutable collection factory methods, such as
095 * {@link Collections#singleton(Object)} and <a
096 * href="https://docs.oracle.com/javase/9/docs/api/java/util/Set.html#immutable">{@code Set.of}</a>,
097 * we recommend using <i>these</i> classes instead for this reason (as well as for consistency).
098 *
099 * <h4>Creation</h4>
100 *
101 * <p>Except for logically "abstract" types like {@code ImmutableCollection} itself, each {@code
102 * Immutable} type provides the static operations you need to obtain instances of that type. These
103 * usually include:
104 *
105 * <ul>
106 *   <li>Static methods named {@code of}, accepting an explicit list of elements or entries.
107 *   <li>Static methods named {@code copyOf} (or {@code copyOfSorted}), accepting an existing
108 *       collection whose contents should be copied.
109 *   <li>A static nested {@code Builder} class which can be used to populate a new immutable
110 *       instance.
111 * </ul>
112 *
113 * <h4>Warnings</h4>
114 *
115 * <ul>
116 *   <li><b>Warning:</b> as with any collection, it is almost always a bad idea to modify an element
117 *       (in a way that affects its {@link Object#equals} behavior) while it is contained in a
118 *       collection. Undefined behavior and bugs will result. It's generally best to avoid using
119 *       mutable objects as elements at all, as many users may expect your "immutable" object to be
120 *       <i>deeply</i> immutable.
121 * </ul>
122 *
123 * <h4>Performance notes</h4>
124 *
125 * <ul>
126 *   <li>Implementations can be generally assumed to prioritize memory efficiency, then speed of
127 *       access, and lastly speed of creation.
128 *   <li>The {@code copyOf} methods will sometimes recognize that the actual copy operation is
129 *       unnecessary; for example, {@code copyOf(copyOf(anArrayList))} should copy the data only
130 *       once. This reduces the expense of habitually making defensive copies at API boundaries.
131 *       However, the precise conditions for skipping the copy operation are undefined.
132 *   <li><b>Warning:</b> a view collection such as {@link ImmutableMap#keySet} or {@link
133 *       ImmutableList#subList} may retain a reference to the entire data set, preventing it from
134 *       being garbage collected. If some of the data is no longer reachable through other means,
135 *       this constitutes a memory leak. Pass the view collection to the appropriate {@code copyOf}
136 *       method to obtain a correctly-sized copy.
137 *   <li>The performance of using the associated {@code Builder} class can be assumed to be no
138 *       worse, and possibly better, than creating a mutable collection and copying it.
139 *   <li>Implementations generally do not cache hash codes. If your element or key type has a slow
140 *       {@code hashCode} implementation, it should cache it itself.
141 * </ul>
142 *
143 * <h4>Example usage</h4>
144 *
145 * <pre>{@code
146 * class Foo {
147 *   private static final ImmutableSet<String> RESERVED_CODES =
148 *       ImmutableSet.of("AZ", "CQ", "ZX");
149 *
150 *   private final ImmutableSet<String> codes;
151 *
152 *   public Foo(Iterable<String> codes) {
153 *     this.codes = ImmutableSet.copyOf(codes);
154 *     checkArgument(Collections.disjoint(this.codes, RESERVED_CODES));
155 *   }
156 * }
157 * }</pre>
158 *
159 * <h3>See also</h3>
160 *
161 * <p>See the Guava User Guide article on <a href=
162 * "https://github.com/google/guava/wiki/ImmutableCollectionsExplained">immutable collections</a>.
163 *
164 * @since 2.0
165 */
166@DoNotMock("Use ImmutableList.of or another implementation")
167@GwtCompatible(emulated = true)
168@SuppressWarnings("serial") // we're overriding default serialization
169@ElementTypesAreNonnullByDefault
170// TODO(kevinb): I think we should push everything down to "BaseImmutableCollection" or something,
171// just to do everything we can to emphasize the "practically an interface" nature of this class.
172public abstract class ImmutableCollection<E> extends AbstractCollection<E> implements Serializable {
173  /*
174   * We expect SIZED (and SUBSIZED, if applicable) to be added by the spliterator factory methods.
175   * These are properties of the collection as a whole; SIZED and SUBSIZED are more properties of
176   * the spliterator implementation.
177   */
178  static final int SPLITERATOR_CHARACTERISTICS =
179      Spliterator.IMMUTABLE | Spliterator.NONNULL | Spliterator.ORDERED;
180
181  ImmutableCollection() {}
182
183  /** Returns an unmodifiable iterator across the elements in this collection. */
184  @Override
185  public abstract UnmodifiableIterator<E> iterator();
186
187  @Override
188  public Spliterator<E> spliterator() {
189    return Spliterators.spliterator(this, SPLITERATOR_CHARACTERISTICS);
190  }
191
192  private static final Object[] EMPTY_ARRAY = {};
193
194  @Override
195  @J2ktIncompatible // Incompatible return type change. Use inherited (unoptimized) implementation
196  public final Object[] toArray() {
197    return toArray(EMPTY_ARRAY);
198  }
199
200  @CanIgnoreReturnValue
201  @Override
202  /*
203   * This suppression is here for two reasons:
204   *
205   * 1. b/192354773 in our checker affects toArray declarations.
206   *
207   * 2. `other[size] = null` is unsound. We could "fix" this by requiring callers to pass in an
208   * array with a nullable element type. But probably they usually want an array with a non-nullable
209   * type. That said, we could *accept* a `@Nullable T[]` (which, given that we treat arrays as
210   * covariant, would still permit a plain `T[]`) and return a plain `T[]`. But of course that would
211   * require its own suppression, since it is also unsound. toArray(T[]) is just a mess from a
212   * nullness perspective. The signature below at least has the virtue of being relatively simple.
213   */
214  @SuppressWarnings("nullness")
215  public final <T extends @Nullable Object> T[] toArray(T[] other) {
216    checkNotNull(other);
217    int size = size();
218
219    if (other.length < size) {
220      Object[] internal = internalArray();
221      if (internal != null) {
222        return Platform.copy(internal, internalArrayStart(), internalArrayEnd(), other);
223      }
224      other = ObjectArrays.newArray(other, size);
225    } else if (other.length > size) {
226      other[size] = null;
227    }
228    copyIntoArray(other, 0);
229    return other;
230  }
231
232  /** If this collection is backed by an array of its elements in insertion order, returns it. */
233  @CheckForNull
234  Object[] internalArray() {
235    return null;
236  }
237
238  /**
239   * If this collection is backed by an array of its elements in insertion order, returns the offset
240   * where this collection's elements start.
241   */
242  int internalArrayStart() {
243    throw new UnsupportedOperationException();
244  }
245
246  /**
247   * If this collection is backed by an array of its elements in insertion order, returns the offset
248   * where this collection's elements end.
249   */
250  int internalArrayEnd() {
251    throw new UnsupportedOperationException();
252  }
253
254  @Override
255  public abstract boolean contains(@CheckForNull Object object);
256
257  /**
258   * Guaranteed to throw an exception and leave the collection unmodified.
259   *
260   * @throws UnsupportedOperationException always
261   * @deprecated Unsupported operation.
262   */
263  @CanIgnoreReturnValue
264  @Deprecated
265  @Override
266  @DoNotCall("Always throws UnsupportedOperationException")
267  public final boolean add(E e) {
268    throw new UnsupportedOperationException();
269  }
270
271  /**
272   * Guaranteed to throw an exception and leave the collection unmodified.
273   *
274   * @throws UnsupportedOperationException always
275   * @deprecated Unsupported operation.
276   */
277  @CanIgnoreReturnValue
278  @Deprecated
279  @Override
280  @DoNotCall("Always throws UnsupportedOperationException")
281  public final boolean remove(@CheckForNull Object object) {
282    throw new UnsupportedOperationException();
283  }
284
285  /**
286   * Guaranteed to throw an exception and leave the collection unmodified.
287   *
288   * @throws UnsupportedOperationException always
289   * @deprecated Unsupported operation.
290   */
291  @CanIgnoreReturnValue
292  @Deprecated
293  @Override
294  @DoNotCall("Always throws UnsupportedOperationException")
295  public final boolean addAll(Collection<? extends E> newElements) {
296    throw new UnsupportedOperationException();
297  }
298
299  /**
300   * Guaranteed to throw an exception and leave the collection unmodified.
301   *
302   * @throws UnsupportedOperationException always
303   * @deprecated Unsupported operation.
304   */
305  @CanIgnoreReturnValue
306  @Deprecated
307  @Override
308  @DoNotCall("Always throws UnsupportedOperationException")
309  public final boolean removeAll(Collection<?> oldElements) {
310    throw new UnsupportedOperationException();
311  }
312
313  /**
314   * Guaranteed to throw an exception and leave the collection unmodified.
315   *
316   * @throws UnsupportedOperationException always
317   * @deprecated Unsupported operation.
318   */
319  @CanIgnoreReturnValue
320  @Deprecated
321  @Override
322  @DoNotCall("Always throws UnsupportedOperationException")
323  public final boolean removeIf(Predicate<? super E> filter) {
324    throw new UnsupportedOperationException();
325  }
326
327  /**
328   * Guaranteed to throw an exception and leave the collection unmodified.
329   *
330   * @throws UnsupportedOperationException always
331   * @deprecated Unsupported operation.
332   */
333  @Deprecated
334  @Override
335  @DoNotCall("Always throws UnsupportedOperationException")
336  public final boolean retainAll(Collection<?> elementsToKeep) {
337    throw new UnsupportedOperationException();
338  }
339
340  /**
341   * Guaranteed to throw an exception and leave the collection unmodified.
342   *
343   * @throws UnsupportedOperationException always
344   * @deprecated Unsupported operation.
345   */
346  @Deprecated
347  @Override
348  @DoNotCall("Always throws UnsupportedOperationException")
349  public final void clear() {
350    throw new UnsupportedOperationException();
351  }
352
353  /**
354   * Returns an {@code ImmutableList} containing the same elements, in the same order, as this
355   * collection.
356   *
357   * <p><b>Performance note:</b> in most cases this method can return quickly without actually
358   * copying anything. The exact circumstances under which the copy is performed are undefined and
359   * subject to change.
360   *
361   * @since 2.0
362   */
363  public ImmutableList<E> asList() {
364    switch (size()) {
365      case 0:
366        return ImmutableList.of();
367      case 1:
368        return ImmutableList.of(iterator().next());
369      default:
370        return new RegularImmutableAsList<E>(this, toArray());
371    }
372  }
373
374  /**
375   * Returns {@code true} if this immutable collection's implementation contains references to
376   * user-created objects that aren't accessible via this collection's methods. This is generally
377   * used to determine whether {@code copyOf} implementations should make an explicit copy to avoid
378   * memory leaks.
379   */
380  abstract boolean isPartialView();
381
382  /**
383   * Copies the contents of this immutable collection into the specified array at the specified
384   * offset. Returns {@code offset + size()}.
385   */
386  @CanIgnoreReturnValue
387  int copyIntoArray(@Nullable Object[] dst, int offset) {
388    for (E e : this) {
389      dst[offset++] = e;
390    }
391    return offset;
392  }
393
394  @J2ktIncompatible // serialization
395  Object writeReplace() {
396    // We serialize by default to ImmutableList, the simplest thing that works.
397    return new ImmutableList.SerializedForm(toArray());
398  }
399
400  @J2ktIncompatible // serialization
401  private void readObject(ObjectInputStream stream) throws InvalidObjectException {
402    throw new InvalidObjectException("Use SerializedForm");
403  }
404
405  /**
406   * Abstract base class for builders of {@link ImmutableCollection} types.
407   *
408   * @since 10.0
409   */
410  @DoNotMock
411  public abstract static class Builder<E> {
412    static final int DEFAULT_INITIAL_CAPACITY = 4;
413
414    static int expandedCapacity(int oldCapacity, int minCapacity) {
415      if (minCapacity < 0) {
416        throw new AssertionError("cannot store more than MAX_VALUE elements");
417      }
418      // careful of overflow!
419      int newCapacity = oldCapacity + (oldCapacity >> 1) + 1;
420      if (newCapacity < minCapacity) {
421        newCapacity = Integer.highestOneBit(minCapacity - 1) << 1;
422      }
423      if (newCapacity < 0) {
424        newCapacity = Integer.MAX_VALUE;
425        // guaranteed to be >= newCapacity
426      }
427      return newCapacity;
428    }
429
430    Builder() {}
431
432    /**
433     * Adds {@code element} to the {@code ImmutableCollection} being built.
434     *
435     * <p>Note that each builder class covariantly returns its own type from this method.
436     *
437     * @param element the element to add
438     * @return this {@code Builder} instance
439     * @throws NullPointerException if {@code element} is null
440     */
441    @CanIgnoreReturnValue
442    public abstract Builder<E> add(E element);
443
444    /**
445     * Adds each element of {@code elements} to the {@code ImmutableCollection} being built.
446     *
447     * <p>Note that each builder class overrides this method in order to covariantly return its own
448     * type.
449     *
450     * @param elements the elements to add
451     * @return this {@code Builder} instance
452     * @throws NullPointerException if {@code elements} is null or contains a null element
453     */
454    @CanIgnoreReturnValue
455    public Builder<E> add(E... elements) {
456      for (E element : elements) {
457        add(element);
458      }
459      return this;
460    }
461
462    /**
463     * Adds each element of {@code elements} to the {@code ImmutableCollection} being built.
464     *
465     * <p>Note that each builder class overrides this method in order to covariantly return its own
466     * type.
467     *
468     * @param elements the elements to add
469     * @return this {@code Builder} instance
470     * @throws NullPointerException if {@code elements} is null or contains a null element
471     */
472    @CanIgnoreReturnValue
473    public Builder<E> addAll(Iterable<? extends E> elements) {
474      for (E element : elements) {
475        add(element);
476      }
477      return this;
478    }
479
480    /**
481     * Adds each element of {@code elements} to the {@code ImmutableCollection} being built.
482     *
483     * <p>Note that each builder class overrides this method in order to covariantly return its own
484     * type.
485     *
486     * @param elements the elements to add
487     * @return this {@code Builder} instance
488     * @throws NullPointerException if {@code elements} is null or contains a null element
489     */
490    @CanIgnoreReturnValue
491    public Builder<E> addAll(Iterator<? extends E> elements) {
492      while (elements.hasNext()) {
493        add(elements.next());
494      }
495      return this;
496    }
497
498    /**
499     * Returns a newly-created {@code ImmutableCollection} of the appropriate type, containing the
500     * elements provided to this builder.
501     *
502     * <p>Note that each builder class covariantly returns the appropriate type of {@code
503     * ImmutableCollection} from this method.
504     */
505    public abstract ImmutableCollection<E> build();
506  }
507}