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 com.google.errorprone.annotations.DoNotCall;
026import com.google.errorprone.annotations.DoNotMock;
027import java.io.Serializable;
028import java.util.AbstractCollection;
029import java.util.Arrays;
030import java.util.Collection;
031import java.util.Collections;
032import java.util.HashSet;
033import java.util.Iterator;
034import java.util.List;
035import javax.annotation.CheckForNull;
036import org.checkerframework.checker.nullness.qual.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 {@link
059 *       Collections#unmodifiableCollection}, whose contents change whenever the wrapped collection
060 *       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
063 *       how the collection was created. Typically this is insertion order unless an explicit
064 *       ordering is otherwise specified (e.g. {@link ImmutableSortedSet#naturalOrder}). See the
065 *       appropriate factory method for details. View collections such as {@link
066 *       ImmutableMultiset#elementSet} iterate in the same order as the parent, except as noted.
067 *   <li><b>Thread safety.</b> It is safe to access this collection concurrently from multiple
068 *       threads.
069 *   <li><b>Integrity.</b> This type cannot be subclassed outside this package (which would allow
070 *       these guarantees to be violated).
071 * </ul>
072 *
073 * <h4>"Interfaces", not implementations</h4>
074 *
075 * <p>These are classes instead of interfaces to prevent external subtyping, but should be thought
076 * of as interfaces in every important sense. Each public class such as {@link ImmutableSet} is a
077 * <i>type</i> offering meaningful behavioral guarantees. This is substantially different from the
078 * case of (say) {@link HashSet}, which is an <i>implementation</i>, with semantics that were
079 * largely defined by its supertype.
080 *
081 * <p>For field types and method return types, you should generally use the immutable type (such as
082 * {@link ImmutableList}) instead of the general collection interface type (such as {@link List}).
083 * This communicates to your callers all of the semantic guarantees listed above, which is almost
084 * always very useful information.
085 *
086 * <p>On the other hand, a <i>parameter</i> type of {@link ImmutableList} is generally a nuisance to
087 * callers. Instead, accept {@link Iterable} and have your method or constructor body pass it to the
088 * appropriate {@code copyOf} method itself.
089 *
090 * <p>Expressing the immutability guarantee directly in the type that user code references is a
091 * powerful advantage. Although Java offers certain immutable collection factory methods, such as
092 * {@link Collections#singleton(Object)} and <a
093 * href="https://docs.oracle.com/javase/9/docs/api/java/util/Set.html#immutable">{@code Set.of}</a>,
094 * we recommend using <i>these</i> classes instead for this reason (as well as for consistency).
095 *
096 * <h4>Creation</h4>
097 *
098 * <p>Except for logically "abstract" types like {@code ImmutableCollection} itself, each {@code
099 * Immutable} type provides the static operations you need to obtain instances of that type. These
100 * usually include:
101 *
102 * <ul>
103 *   <li>Static methods named {@code of}, accepting an explicit list of elements or entries.
104 *   <li>Static methods named {@code copyOf} (or {@code copyOfSorted}), accepting an existing
105 *       collection whose contents should be copied.
106 *   <li>A static nested {@code Builder} class which can be used to populate a new immutable
107 *       instance.
108 * </ul>
109 *
110 * <h4>Warnings</h4>
111 *
112 * <ul>
113 *   <li><b>Warning:</b> as with any collection, it is almost always a bad idea to modify an element
114 *       (in a way that affects its {@link Object#equals} behavior) while it is contained in a
115 *       collection. Undefined behavior and bugs will result. It's generally best to avoid using
116 *       mutable objects as elements at all, as many users may expect your "immutable" object to be
117 *       <i>deeply</i> immutable.
118 * </ul>
119 *
120 * <h4>Performance notes</h4>
121 *
122 * <ul>
123 *   <li>Implementations can be generally assumed to prioritize memory efficiency, then speed of
124 *       access, and lastly speed of creation.
125 *   <li>The {@code copyOf} methods will sometimes recognize that the actual copy operation is
126 *       unnecessary; for example, {@code copyOf(copyOf(anArrayList))} should copy the data only
127 *       once. This reduces the expense of habitually making defensive copies at API boundaries.
128 *       However, the precise conditions for skipping the copy operation are undefined.
129 *   <li><b>Warning:</b> a view collection such as {@link ImmutableMap#keySet} or {@link
130 *       ImmutableList#subList} may retain a reference to the entire data set, preventing it from
131 *       being garbage collected. If some of the data is no longer reachable through other means,
132 *       this constitutes a memory leak. Pass the view collection to the appropriate {@code copyOf}
133 *       method to obtain a correctly-sized copy.
134 *   <li>The performance of using the associated {@code Builder} class can be assumed to be no
135 *       worse, and possibly better, than creating a mutable collection and copying it.
136 *   <li>Implementations generally do not cache hash codes. If your element or key type has a slow
137 *       {@code hashCode} implementation, it should cache it itself.
138 * </ul>
139 *
140 * <h4>Example usage</h4>
141 *
142 * <pre>{@code
143 * class Foo {
144 *   private static final ImmutableSet<String> RESERVED_CODES =
145 *       ImmutableSet.of("AZ", "CQ", "ZX");
146 *
147 *   private final ImmutableSet<String> codes;
148 *
149 *   public Foo(Iterable<String> codes) {
150 *     this.codes = ImmutableSet.copyOf(codes);
151 *     checkArgument(Collections.disjoint(this.codes, RESERVED_CODES));
152 *   }
153 * }
154 * }</pre>
155 *
156 * <h3>See also</h3>
157 *
158 * <p>See the Guava User Guide article on <a href=
159 * "https://github.com/google/guava/wiki/ImmutableCollectionsExplained">immutable collections</a>.
160 *
161 * @since 2.0
162 */
163@DoNotMock("Use ImmutableList.of or another implementation")
164@GwtCompatible(emulated = true)
165@SuppressWarnings("serial") // we're overriding default serialization
166@ElementTypesAreNonnullByDefault
167// TODO(kevinb): I think we should push everything down to "BaseImmutableCollection" or something,
168// just to do everything we can to emphasize the "practically an interface" nature of this class.
169public abstract class ImmutableCollection<E> extends AbstractCollection<E> implements Serializable {
170
171  ImmutableCollection() {}
172
173  /** Returns an unmodifiable iterator across the elements in this collection. */
174  @Override
175  public abstract UnmodifiableIterator<E> iterator();
176
177  private static final Object[] EMPTY_ARRAY = {};
178
179  @Override
180  public final Object[] toArray() {
181    return toArray(EMPTY_ARRAY);
182  }
183
184  @CanIgnoreReturnValue
185  @Override
186  /*
187   * This suppression is here for two reasons:
188   *
189   * 1. b/192354773 in our checker affects toArray declarations.
190   *
191   * 2. `other[size] = null` is unsound. We could "fix" this by requiring callers to pass in an
192   * array with a nullable element type. But probably they usually want an array with a non-nullable
193   * type. That said, we could *accept* a `@Nullable T[]` (which, given that we treat arrays as
194   * covariant, would still permit a plain `T[]`) and return a plain `T[]`. But of course that would
195   * require its own suppression, since it is also unsound. toArray(T[]) is just a mess from a
196   * nullness perspective. The signature below at least has the virtue of being relatively simple.
197   */
198  @SuppressWarnings("nullness")
199  public final <T extends @Nullable Object> T[] toArray(T[] other) {
200    checkNotNull(other);
201    int size = size();
202
203    if (other.length < size) {
204      Object[] internal = internalArray();
205      if (internal != null) {
206        return Platform.copy(internal, internalArrayStart(), internalArrayEnd(), other);
207      }
208      other = ObjectArrays.newArray(other, size);
209    } else if (other.length > size) {
210      other[size] = null;
211    }
212    copyIntoArray(other, 0);
213    return other;
214  }
215
216  /** If this collection is backed by an array of its elements in insertion order, returns it. */
217  @CheckForNull
218  @Nullable
219  Object[] internalArray() {
220    return null;
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 start.
226   */
227  int internalArrayStart() {
228    throw new UnsupportedOperationException();
229  }
230
231  /**
232   * If this collection is backed by an array of its elements in insertion order, returns the offset
233   * where this collection's elements end.
234   */
235  int internalArrayEnd() {
236    throw new UnsupportedOperationException();
237  }
238
239  @Override
240  public abstract boolean contains(@CheckForNull Object object);
241
242  /**
243   * Guaranteed to throw an exception and leave the collection unmodified.
244   *
245   * @throws UnsupportedOperationException always
246   * @deprecated Unsupported operation.
247   */
248  @CanIgnoreReturnValue
249  @Deprecated
250  @Override
251  @DoNotCall("Always throws UnsupportedOperationException")
252  public final boolean add(E e) {
253    throw new UnsupportedOperationException();
254  }
255
256  /**
257   * Guaranteed to throw an exception and leave the collection unmodified.
258   *
259   * @throws UnsupportedOperationException always
260   * @deprecated Unsupported operation.
261   */
262  @CanIgnoreReturnValue
263  @Deprecated
264  @Override
265  @DoNotCall("Always throws UnsupportedOperationException")
266  public final boolean remove(@CheckForNull Object object) {
267    throw new UnsupportedOperationException();
268  }
269
270  /**
271   * Guaranteed to throw an exception and leave the collection unmodified.
272   *
273   * @throws UnsupportedOperationException always
274   * @deprecated Unsupported operation.
275   */
276  @CanIgnoreReturnValue
277  @Deprecated
278  @Override
279  @DoNotCall("Always throws UnsupportedOperationException")
280  public final boolean addAll(Collection<? extends E> newElements) {
281    throw new UnsupportedOperationException();
282  }
283
284  /**
285   * Guaranteed to throw an exception and leave the collection unmodified.
286   *
287   * @throws UnsupportedOperationException always
288   * @deprecated Unsupported operation.
289   */
290  @CanIgnoreReturnValue
291  @Deprecated
292  @Override
293  @DoNotCall("Always throws UnsupportedOperationException")
294  public final boolean removeAll(Collection<?> oldElements) {
295    throw new UnsupportedOperationException();
296  }
297
298  /**
299   * Guaranteed to throw an exception and leave the collection unmodified.
300   *
301   * @throws UnsupportedOperationException always
302   * @deprecated Unsupported operation.
303   */
304  @CanIgnoreReturnValue
305  @Deprecated
306  @Override
307  @DoNotCall("Always throws UnsupportedOperationException")
308  public final boolean retainAll(Collection<?> elementsToKeep) {
309    throw new UnsupportedOperationException();
310  }
311
312  /**
313   * Guaranteed to throw an exception and leave the collection unmodified.
314   *
315   * @throws UnsupportedOperationException always
316   * @deprecated Unsupported operation.
317   */
318  @Deprecated
319  @Override
320  @DoNotCall("Always throws UnsupportedOperationException")
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    return isEmpty() ? ImmutableList.<E>of() : ImmutableList.<E>asImmutableList(toArray());
337  }
338
339  /**
340   * Returns {@code true} if this immutable collection's implementation contains references to
341   * user-created objects that aren't accessible via this collection's methods. This is generally
342   * used to determine whether {@code copyOf} implementations should make an explicit copy to avoid
343   * memory leaks.
344   */
345  abstract boolean isPartialView();
346
347  /**
348   * Copies the contents of this immutable collection into the specified array at the specified
349   * offset. Returns {@code offset + size()}.
350   */
351  @CanIgnoreReturnValue
352  int copyIntoArray(@Nullable Object[] dst, int offset) {
353    for (E e : this) {
354      dst[offset++] = e;
355    }
356    return offset;
357  }
358
359  Object writeReplace() {
360    // We serialize by default to ImmutableList, the simplest thing that works.
361    return new ImmutableList.SerializedForm(toArray());
362  }
363
364  /**
365   * Abstract base class for builders of {@link ImmutableCollection} types.
366   *
367   * @since 10.0
368   */
369  @DoNotMock
370  public abstract static class Builder<E> {
371    static final int DEFAULT_INITIAL_CAPACITY = 4;
372
373    static int expandedCapacity(int oldCapacity, int minCapacity) {
374      if (minCapacity < 0) {
375        throw new AssertionError("cannot store more than MAX_VALUE elements");
376      }
377      // careful of overflow!
378      int newCapacity = oldCapacity + (oldCapacity >> 1) + 1;
379      if (newCapacity < minCapacity) {
380        newCapacity = Integer.highestOneBit(minCapacity - 1) << 1;
381      }
382      if (newCapacity < 0) {
383        newCapacity = Integer.MAX_VALUE;
384        // guaranteed to be >= newCapacity
385      }
386      return newCapacity;
387    }
388
389    Builder() {}
390
391    /**
392     * Adds {@code element} to the {@code ImmutableCollection} being built.
393     *
394     * <p>Note that each builder class covariantly returns its own type from this method.
395     *
396     * @param element the element to add
397     * @return this {@code Builder} instance
398     * @throws NullPointerException if {@code element} is null
399     */
400    @CanIgnoreReturnValue
401    public abstract Builder<E> add(E element);
402
403    /**
404     * Adds each element of {@code elements} to the {@code ImmutableCollection} being built.
405     *
406     * <p>Note that each builder class overrides this method in order to covariantly return its own
407     * 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 null element
412     */
413    @CanIgnoreReturnValue
414    public Builder<E> add(E... elements) {
415      for (E element : elements) {
416        add(element);
417      }
418      return this;
419    }
420
421    /**
422     * Adds each element of {@code elements} to the {@code ImmutableCollection} being built.
423     *
424     * <p>Note that each builder class overrides this method in order to covariantly return its own
425     * type.
426     *
427     * @param elements the elements to add
428     * @return this {@code Builder} instance
429     * @throws NullPointerException if {@code elements} is null or contains a null element
430     */
431    @CanIgnoreReturnValue
432    public Builder<E> addAll(Iterable<? extends E> elements) {
433      for (E element : elements) {
434        add(element);
435      }
436      return this;
437    }
438
439    /**
440     * Adds each element of {@code elements} to the {@code ImmutableCollection} being built.
441     *
442     * <p>Note that each builder class overrides this method in order to covariantly return its own
443     * type.
444     *
445     * @param elements the elements to add
446     * @return this {@code Builder} instance
447     * @throws NullPointerException if {@code elements} is null or contains a null element
448     */
449    @CanIgnoreReturnValue
450    public Builder<E> addAll(Iterator<? extends E> elements) {
451      while (elements.hasNext()) {
452        add(elements.next());
453      }
454      return this;
455    }
456
457    /**
458     * Returns a newly-created {@code ImmutableCollection} of the appropriate type, containing the
459     * elements provided to this builder.
460     *
461     * <p>Note that each builder class covariantly returns the appropriate type of {@code
462     * ImmutableCollection} from this method.
463     */
464    public abstract ImmutableCollection<E> build();
465  }
466
467  abstract static class ArrayBasedBuilder<E> extends ImmutableCollection.Builder<E> {
468    // The first `size` elements are non-null.
469    @Nullable Object[] contents;
470    int size;
471    boolean forceCopy;
472
473    ArrayBasedBuilder(int initialCapacity) {
474      checkNonnegative(initialCapacity, "initialCapacity");
475      this.contents = new @Nullable Object[initialCapacity];
476      this.size = 0;
477    }
478
479    /*
480     * Expand the absolute capacity of the builder so it can accept at least the specified number of
481     * elements without being resized. Also, if we've already built a collection backed by the
482     * current array, create a new array.
483     */
484    private void getReadyToExpandTo(int minCapacity) {
485      if (contents.length < minCapacity) {
486        this.contents =
487            Arrays.copyOf(this.contents, expandedCapacity(contents.length, minCapacity));
488        forceCopy = false;
489      } else if (forceCopy) {
490        this.contents = contents.clone();
491        forceCopy = false;
492      }
493    }
494
495    @CanIgnoreReturnValue
496    @Override
497    public ArrayBasedBuilder<E> add(E element) {
498      checkNotNull(element);
499      getReadyToExpandTo(size + 1);
500      contents[size++] = element;
501      return this;
502    }
503
504    @CanIgnoreReturnValue
505    @Override
506    public Builder<E> add(E... elements) {
507      addAll(elements, elements.length);
508      return this;
509    }
510
511    final void addAll(@Nullable Object[] elements, int n) {
512      checkElementsNotNull(elements, n);
513      getReadyToExpandTo(size + n);
514      /*
515       * The following call is not statically checked, since arraycopy accepts plain Object for its
516       * parameters. If it were statically checked, the checker would still be OK with it, since
517       * we're copying into a `contents` array whose type allows it to contain nulls. Still, it's
518       * worth noting that we promise not to put nulls into the array in the first `size` elements.
519       * We uphold that promise here because our callers promise that `elements` will not contain
520       * nulls in its first `n` elements.
521       */
522      System.arraycopy(elements, 0, contents, size, n);
523      size += n;
524    }
525
526    @CanIgnoreReturnValue
527    @Override
528    public Builder<E> addAll(Iterable<? extends E> elements) {
529      if (elements instanceof Collection) {
530        Collection<?> collection = (Collection<?>) elements;
531        getReadyToExpandTo(size + collection.size());
532        if (collection instanceof ImmutableCollection) {
533          ImmutableCollection<?> immutableCollection = (ImmutableCollection<?>) collection;
534          size = immutableCollection.copyIntoArray(contents, size);
535          return this;
536        }
537      }
538      super.addAll(elements);
539      return this;
540    }
541  }
542}