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