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