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