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 java.io.Serializable;
026import java.util.AbstractCollection;
027import java.util.ArrayList;
028import java.util.Arrays;
029import java.util.Collection;
030import java.util.Collections;
031import java.util.Iterator;
032import java.util.List;
033import java.util.Spliterator;
034import java.util.Spliterators;
035import java.util.function.Predicate;
036import javax.annotation.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
059 *     {@link Collections#unmodifiableCollection}, whose contents change whenever the wrapped
060 *     collection 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 how
063 *     the collection was created (see the appropriate factory method for details). View collections
064 *     such as {@link ImmutableMultiset#elementSet} iterate in the same order as the parent, except
065 *     as noted.
066 * <li><b>Thread safety.</b> It is safe to access this collection concurrently from multiple
067 *     threads.
068 * <li><b>Integrity.</b> This type cannot be subclassed outside this package (which would allow
069 *     these guarantees to be violated).
070 * </ul>
071 *
072 * <h4>"Interfaces", not implementations</h4>
073 *
074 * <p>Each public class, such as {@link ImmutableSet}, is a <i>type</i> offering meaningful
075 * behavioral guarantees -- not merely a specific <i>implementation</i> as in the case of, say,
076 * {@link ArrayList}. You should treat them as interfaces in every important sense of the word.
077 *
078 * <p>For field types and method return types, you should generally use the immutable type (such as
079 * {@link ImmutableList}) instead of the general collection interface type (such as {@link List}).
080 * This communicates to your callers all of the semantic guarantees listed above, which is almost
081 * always very useful information.
082 *
083 * <p>On the other hand, a <i>parameter</i> type of {@link ImmutableList} is generally a nuisance to
084 * callers. Instead, accept {@link Iterable} and have your method or constructor body pass it to the
085 * appropriate {@code copyOf} method itself.
086 *
087 * <h4>Creation</h4>
088 *
089 * <p>Except for logically "abstract" types like {@code ImmutableCollection} itself, each {@code
090 * Immutable} type provides the static operations you need to obtain instances of that type. These
091 * usually include:
092 *
093 * <ul>
094 * <li>Static methods named {@code of}, accepting an explicit list of elements or entries.
095 * <li>Static methods named {@code copyOf} (or {@code copyOfSorted}), accepting an existing
096 *     collection whose contents should be copied.
097 * <li>A static nested {@code Builder} class which can be used to populate a new immutable instance.
098 * </ul>
099 *
100 * <h4>Warnings</h4>
101 *
102 * <ul>
103 * <li><b>Warning:</b> as with any collection, it is almost always a bad idea to modify an element
104 *     (in a way that affects its {@link Object#equals} behavior) while it is contained in a
105 *     collection. Undefined behavior and bugs will result. It's generally best to avoid using
106 *     mutable objects as elements at all, as many users may expect your "immutable" object to be
107 *     <i>deeply</i> immutable.
108 * </ul>
109 *
110 * <h4>Performance notes</h4>
111 *
112 * <ul>
113 * <li>Implementations can be generally assumed to prioritize memory efficiency, then speed of
114 *     access, and lastly speed of creation.
115 * <li>The {@code copyOf} methods will sometimes recognize that the actual copy operation is
116 *     unnecessary; for example, {@code copyOf(copyOf(anArrayList))} should copy the data only once.
117 *     This reduces the expense of habitually making defensive copies at API boundaries. However,
118 *     the precise conditions for skipping the copy operation are undefined.
119 * <li><b>Warning:</b> a view collection such as {@link ImmutableMap#keySet} or {@link
120 *     ImmutableList#subList} may retain a reference to the entire data set, preventing it from
121 *     being garbage collected. If some of the data is no longer reachable through other means, this
122 *     constitutes a memory leak. Pass the view collection to the appropriate {@code copyOf} method
123 *     to obtain a correctly-sized copy.
124 * <li>The performance of using the associated {@code Builder} class can be assumed to be
125 *     no worse, and possibly better, than creating a mutable collection and copying it.
126 * <li>Implementations generally do not cache hash codes. If your element or key type has a slow
127 *     {@code hashCode} implementation, it should cache it itself.
128 * </ul>
129 *
130 * <h4>Example usage</h4>
131 *
132 * <pre>   {@code
133 *
134 *   class Foo {
135 *     private static final ImmutableSet<String> RESERVED_CODES =
136 *         ImmutableSet.of("AZ", "CQ", "ZX");
137 *
138 *     private final ImmutableSet<String> codes;
139 *
140 *     public Foo(Iterable<String> codes) {
141 *       this.codes = ImmutableSet.copyOf(codes);
142 *       checkArgument(Collections.disjoint(this.codes, RESERVED_CODES));
143 *     }
144 *   }}</pre>
145 *
146 * <h3>See also</h3>
147 *
148 * <p>See the Guava User Guide article on <a href=
149 * "https://github.com/google/guava/wiki/ImmutableCollectionsExplained">
150 * immutable collections</a>.
151 *
152 * @since 2.0
153 */
154@GwtCompatible(emulated = true)
155@SuppressWarnings("serial") // we're overriding default serialization
156// TODO(kevinb): I think we should push everything down to "BaseImmutableCollection" or something,
157// just to do everything we can to emphasize the "practically an interface" nature of this class.
158public abstract class ImmutableCollection<E> extends AbstractCollection<E> implements Serializable {
159  /*
160   * We expect SIZED (and SUBSIZED, if applicable) to be added by the spliterator factory methods.
161   * These are properties of the collection as a whole; SIZED and SUBSIZED are more properties of
162   * the spliterator implementation.
163   */
164  static final int SPLITERATOR_CHARACTERISTICS =
165      Spliterator.IMMUTABLE | Spliterator.NONNULL | Spliterator.ORDERED;
166
167  ImmutableCollection() {}
168
169  /**
170   * Returns an unmodifiable iterator across the elements in this collection.
171   */
172  @Override
173  public abstract UnmodifiableIterator<E> iterator();
174
175  @Override
176  public Spliterator<E> spliterator() {
177    return Spliterators.spliterator(this, SPLITERATOR_CHARACTERISTICS);
178  }
179
180  @Override
181  public final Object[] toArray() {
182    int size = size();
183    if (size == 0) {
184      return ObjectArrays.EMPTY_ARRAY;
185    }
186    Object[] result = new Object[size];
187    copyIntoArray(result, 0);
188    return result;
189  }
190
191  @CanIgnoreReturnValue
192  @Override
193  public final <T> T[] toArray(T[] other) {
194    checkNotNull(other);
195    int size = size();
196    if (other.length < size) {
197      other = ObjectArrays.newArray(other, size);
198    } else if (other.length > size) {
199      other[size] = null;
200    }
201    copyIntoArray(other, 0);
202    return other;
203  }
204
205  @Override
206  public abstract boolean contains(@Nullable Object object);
207
208  /**
209   * Guaranteed to throw an exception and leave the collection unmodified.
210   *
211   * @throws UnsupportedOperationException always
212   * @deprecated Unsupported operation.
213   */
214  @CanIgnoreReturnValue
215  @Deprecated
216  @Override
217  public final boolean add(E e) {
218    throw new UnsupportedOperationException();
219  }
220
221  /**
222   * Guaranteed to throw an exception and leave the collection unmodified.
223   *
224   * @throws UnsupportedOperationException always
225   * @deprecated Unsupported operation.
226   */
227  @CanIgnoreReturnValue
228  @Deprecated
229  @Override
230  public final boolean remove(Object object) {
231    throw new UnsupportedOperationException();
232  }
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 addAll(Collection<? extends E> newElements) {
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 removeAll(Collection<?> oldElements) {
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 removeIf(Predicate<? super E> filter) {
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  @Deprecated
280  @Override
281  public final boolean retainAll(Collection<?> elementsToKeep) {
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  @Deprecated
292  @Override
293  public final void clear() {
294    throw new UnsupportedOperationException();
295  }
296
297  /**
298   * Returns an {@code ImmutableList} containing the same elements, in the same order, as this
299   * collection.
300   *
301   * <p><b>Performance note:</b> in most cases this method can return quickly without actually
302   * copying anything. The exact circumstances under which the copy is performed are undefined and
303   * subject to change.
304   *
305   * @since 2.0
306   */
307  public ImmutableList<E> asList() {
308    switch (size()) {
309      case 0:
310        return ImmutableList.of();
311      case 1:
312        return ImmutableList.of(iterator().next());
313      default:
314        return new RegularImmutableAsList<E>(this, toArray());
315    }
316  }
317
318  /**
319   * Returns {@code true} if this immutable collection's implementation contains references to
320   * user-created objects that aren't accessible via this collection's methods. This is generally
321   * used to determine whether {@code copyOf} implementations should make an explicit copy to avoid
322   * memory leaks.
323   */
324  abstract boolean isPartialView();
325
326  /**
327   * Copies the contents of this immutable collection into the specified array at the specified
328   * offset.  Returns {@code offset + size()}.
329   */
330  @CanIgnoreReturnValue
331  int copyIntoArray(Object[] dst, int offset) {
332    for (E e : this) {
333      dst[offset++] = e;
334    }
335    return offset;
336  }
337
338  Object writeReplace() {
339    // We serialize by default to ImmutableList, the simplest thing that works.
340    return new ImmutableList.SerializedForm(toArray());
341  }
342
343  /**
344   * Abstract base class for builders of {@link ImmutableCollection} types.
345   *
346   * @since 10.0
347   */
348  public abstract static class Builder<E> {
349    static final int DEFAULT_INITIAL_CAPACITY = 4;
350
351    static int expandedCapacity(int oldCapacity, int minCapacity) {
352      if (minCapacity < 0) {
353        throw new AssertionError("cannot store more than MAX_VALUE elements");
354      }
355      // careful of overflow!
356      int newCapacity = oldCapacity + (oldCapacity >> 1) + 1;
357      if (newCapacity < minCapacity) {
358        newCapacity = Integer.highestOneBit(minCapacity - 1) << 1;
359      }
360      if (newCapacity < 0) {
361        newCapacity = Integer.MAX_VALUE;
362        // guaranteed to be >= newCapacity
363      }
364      return newCapacity;
365    }
366
367    Builder() {}
368
369    /**
370     * Adds {@code element} to the {@code ImmutableCollection} being built.
371     *
372     * <p>Note that each builder class covariantly returns its own type from
373     * this method.
374     *
375     * @param element the element to add
376     * @return this {@code Builder} instance
377     * @throws NullPointerException if {@code element} is null
378     */
379    @CanIgnoreReturnValue
380    public abstract Builder<E> add(E element);
381
382    /**
383     * Adds each element of {@code elements} to the {@code ImmutableCollection}
384     * being built.
385     *
386     * <p>Note that each builder class overrides this method in order to
387     * covariantly return its own type.
388     *
389     * @param elements the elements to add
390     * @return this {@code Builder} instance
391     * @throws NullPointerException if {@code elements} is null or contains a
392     *     null element
393     */
394    @CanIgnoreReturnValue
395    public Builder<E> add(E... elements) {
396      for (E element : elements) {
397        add(element);
398      }
399      return this;
400    }
401
402    /**
403     * Adds each element of {@code elements} to the {@code ImmutableCollection}
404     * being built.
405     *
406     * <p>Note that each builder class overrides this method in order to
407     * covariantly return its own 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
412     *     null element
413     */
414    @CanIgnoreReturnValue
415    public Builder<E> addAll(Iterable<? extends E> elements) {
416      for (E element : elements) {
417        add(element);
418      }
419      return this;
420    }
421
422    /**
423     * Adds each element of {@code elements} to the {@code ImmutableCollection}
424     * being built.
425     *
426     * <p>Note that each builder class overrides this method in order to
427     * covariantly return its own type.
428     *
429     * @param elements the elements to add
430     * @return this {@code Builder} instance
431     * @throws NullPointerException if {@code elements} is null or contains a
432     *     null element
433     */
434    @CanIgnoreReturnValue
435    public Builder<E> addAll(Iterator<? extends E> elements) {
436      while (elements.hasNext()) {
437        add(elements.next());
438      }
439      return this;
440    }
441
442    /**
443     * Returns a newly-created {@code ImmutableCollection} of the appropriate
444     * type, containing the elements provided to this builder.
445     *
446     * <p>Note that each builder class covariantly returns the appropriate type
447     * of {@code ImmutableCollection} from this method.
448     */
449    public abstract ImmutableCollection<E> build();
450  }
451
452  abstract static class ArrayBasedBuilder<E> extends ImmutableCollection.Builder<E> {
453    Object[] contents;
454    int size;
455
456    ArrayBasedBuilder(int initialCapacity) {
457      checkNonnegative(initialCapacity, "initialCapacity");
458      this.contents = new Object[initialCapacity];
459      this.size = 0;
460    }
461
462    /**
463     * Expand the absolute capacity of the builder so it can accept at least
464     * the specified number of elements without being resized.
465     */
466    private void ensureCapacity(int minCapacity) {
467      if (contents.length < minCapacity) {
468        this.contents =
469            Arrays.copyOf(
470                this.contents, expandedCapacity(contents.length, minCapacity));
471      }
472    }
473
474    @CanIgnoreReturnValue
475    @Override
476    public ArrayBasedBuilder<E> add(E element) {
477      checkNotNull(element);
478      ensureCapacity(size + 1);
479      contents[size++] = element;
480      return this;
481    }
482
483    @CanIgnoreReturnValue
484    @Override
485    public Builder<E> add(E... elements) {
486      checkElementsNotNull(elements);
487      ensureCapacity(size + elements.length);
488      System.arraycopy(elements, 0, contents, size, elements.length);
489      size += elements.length;
490      return this;
491    }
492
493    @CanIgnoreReturnValue
494    @Override
495    public Builder<E> addAll(Iterable<? extends E> elements) {
496      if (elements instanceof Collection) {
497        Collection<?> collection = (Collection<?>) elements;
498        ensureCapacity(size + collection.size());
499      }
500      super.addAll(elements);
501      return this;
502    }
503
504    @CanIgnoreReturnValue
505    ArrayBasedBuilder<E> combine(ArrayBasedBuilder<E> builder) {
506      checkNotNull(builder);
507      ensureCapacity(size + builder.size);
508      System.arraycopy(builder.contents, 0, this.contents, size, builder.size);
509      size += builder.size;
510      return this;
511    }
512  }
513}