001/*
002 * Copyright (C) 2007 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.checkArgument;
020import static com.google.common.base.Preconditions.checkElementIndex;
021import static com.google.common.base.Preconditions.checkNotNull;
022import static com.google.common.base.Preconditions.checkPositionIndexes;
023import static com.google.common.collect.CollectPreconditions.checkNonnegative;
024import static com.google.common.collect.ObjectArrays.checkElementsNotNull;
025import static com.google.common.collect.RegularImmutableList.EMPTY;
026
027import com.google.common.annotations.Beta;
028import com.google.common.annotations.GwtCompatible;
029import com.google.common.annotations.VisibleForTesting;
030import com.google.errorprone.annotations.CanIgnoreReturnValue;
031import java.io.InvalidObjectException;
032import java.io.ObjectInputStream;
033import java.io.Serializable;
034import java.util.Arrays;
035import java.util.Collection;
036import java.util.Collections;
037import java.util.Comparator;
038import java.util.Iterator;
039import java.util.List;
040import java.util.RandomAccess;
041import java.util.Spliterator;
042import java.util.function.Consumer;
043import java.util.function.UnaryOperator;
044import java.util.stream.Collector;
045import org.checkerframework.checker.nullness.compatqual.NullableDecl;
046
047/**
048 * A {@link List} whose contents will never change, with many other important properties detailed at
049 * {@link ImmutableCollection}.
050 *
051 * <p>See the Guava User Guide article on <a href=
052 * "https://github.com/google/guava/wiki/ImmutableCollectionsExplained"> immutable collections</a>.
053 *
054 * @see ImmutableMap
055 * @see ImmutableSet
056 * @author Kevin Bourrillion
057 * @since 2.0
058 */
059@GwtCompatible(serializable = true, emulated = true)
060@SuppressWarnings("serial") // we're overriding default serialization
061public abstract class ImmutableList<E> extends ImmutableCollection<E>
062    implements List<E>, RandomAccess {
063
064  /**
065   * Returns a {@code Collector} that accumulates the input elements into a new {@code
066   * ImmutableList}, in encounter order.
067   *
068   * @since 21.0
069   */
070  @Beta
071  public static <E> Collector<E, ?, ImmutableList<E>> toImmutableList() {
072    return CollectCollectors.toImmutableList();
073  }
074
075  /**
076   * Returns the empty immutable list. This list behaves and performs comparably to {@link
077   * Collections#emptyList}, and is preferable mainly for consistency and maintainability of your
078   * code.
079   */
080  // Casting to any type is safe because the list will never hold any elements.
081  @SuppressWarnings("unchecked")
082  public static <E> ImmutableList<E> of() {
083    return (ImmutableList<E>) EMPTY;
084  }
085
086  /**
087   * Returns an immutable list containing a single element. This list behaves and performs
088   * comparably to {@link Collections#singleton}, but will not accept a null element. It is
089   * preferable mainly for consistency and maintainability of your code.
090   *
091   * @throws NullPointerException if {@code element} is null
092   */
093  public static <E> ImmutableList<E> of(E element) {
094    return new SingletonImmutableList<E>(element);
095  }
096
097  /**
098   * Returns an immutable list containing the given elements, in order.
099   *
100   * @throws NullPointerException if any element is null
101   */
102  public static <E> ImmutableList<E> of(E e1, E e2) {
103    return construct(e1, e2);
104  }
105
106  /**
107   * Returns an immutable list containing the given elements, in order.
108   *
109   * @throws NullPointerException if any element is null
110   */
111  public static <E> ImmutableList<E> of(E e1, E e2, E e3) {
112    return construct(e1, e2, e3);
113  }
114
115  /**
116   * Returns an immutable list containing the given elements, in order.
117   *
118   * @throws NullPointerException if any element is null
119   */
120  public static <E> ImmutableList<E> of(E e1, E e2, E e3, E e4) {
121    return construct(e1, e2, e3, e4);
122  }
123
124  /**
125   * Returns an immutable list containing the given elements, in order.
126   *
127   * @throws NullPointerException if any element is null
128   */
129  public static <E> ImmutableList<E> of(E e1, E e2, E e3, E e4, E e5) {
130    return construct(e1, e2, e3, e4, e5);
131  }
132
133  /**
134   * Returns an immutable list containing the given elements, in order.
135   *
136   * @throws NullPointerException if any element is null
137   */
138  public static <E> ImmutableList<E> of(E e1, E e2, E e3, E e4, E e5, E e6) {
139    return construct(e1, e2, e3, e4, e5, e6);
140  }
141
142  /**
143   * Returns an immutable list containing the given elements, in order.
144   *
145   * @throws NullPointerException if any element is null
146   */
147  public static <E> ImmutableList<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7) {
148    return construct(e1, e2, e3, e4, e5, e6, e7);
149  }
150
151  /**
152   * Returns an immutable list containing the given elements, in order.
153   *
154   * @throws NullPointerException if any element is null
155   */
156  public static <E> ImmutableList<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8) {
157    return construct(e1, e2, e3, e4, e5, e6, e7, e8);
158  }
159
160  /**
161   * Returns an immutable list containing the given elements, in order.
162   *
163   * @throws NullPointerException if any element is null
164   */
165  public static <E> ImmutableList<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9) {
166    return construct(e1, e2, e3, e4, e5, e6, e7, e8, e9);
167  }
168
169  /**
170   * Returns an immutable list containing the given elements, in order.
171   *
172   * @throws NullPointerException if any element is null
173   */
174  public static <E> ImmutableList<E> of(
175      E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10) {
176    return construct(e1, e2, e3, e4, e5, e6, e7, e8, e9, e10);
177  }
178
179  /**
180   * Returns an immutable list containing the given elements, in order.
181   *
182   * @throws NullPointerException if any element is null
183   */
184  public static <E> ImmutableList<E> of(
185      E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10, E e11) {
186    return construct(e1, e2, e3, e4, e5, e6, e7, e8, e9, e10, e11);
187  }
188
189  // These go up to eleven. After that, you just get the varargs form, and
190  // whatever warnings might come along with it. :(
191
192  /**
193   * Returns an immutable list containing the given elements, in order.
194   *
195   * <p>The array {@code others} must not be longer than {@code Integer.MAX_VALUE - 12}.
196   *
197   * @throws NullPointerException if any element is null
198   * @since 3.0 (source-compatible since 2.0)
199   */
200  @SafeVarargs // For Eclipse. For internal javac we have disabled this pointless type of warning.
201  public static <E> ImmutableList<E> of(
202      E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10, E e11, E e12, E... others) {
203    checkArgument(
204        others.length <= Integer.MAX_VALUE - 12,
205        "the total number of elements must fit in an int");
206    Object[] array = new Object[12 + others.length];
207    array[0] = e1;
208    array[1] = e2;
209    array[2] = e3;
210    array[3] = e4;
211    array[4] = e5;
212    array[5] = e6;
213    array[6] = e7;
214    array[7] = e8;
215    array[8] = e9;
216    array[9] = e10;
217    array[10] = e11;
218    array[11] = e12;
219    System.arraycopy(others, 0, array, 12, others.length);
220    return construct(array);
221  }
222
223  /**
224   * Returns an immutable list containing the given elements, in order. If {@code elements} is a
225   * {@link Collection}, this method behaves exactly as {@link #copyOf(Collection)}; otherwise, it
226   * behaves exactly as {@code copyOf(elements.iterator()}.
227   *
228   * @throws NullPointerException if any of {@code elements} is null
229   */
230  public static <E> ImmutableList<E> copyOf(Iterable<? extends E> elements) {
231    checkNotNull(elements); // TODO(kevinb): is this here only for GWT?
232    return (elements instanceof Collection)
233        ? copyOf((Collection<? extends E>) elements)
234        : copyOf(elements.iterator());
235  }
236
237  /**
238   * Returns an immutable list containing the given elements, in order.
239   *
240   * <p>Despite the method name, this method attempts to avoid actually copying the data when it is
241   * safe to do so. The exact circumstances under which a copy will or will not be performed are
242   * undocumented and subject to change.
243   *
244   * <p>Note that if {@code list} is a {@code List<String>}, then {@code ImmutableList.copyOf(list)}
245   * returns an {@code ImmutableList<String>} containing each of the strings in {@code list}, while
246   * ImmutableList.of(list)} returns an {@code ImmutableList<List<String>>} containing one element
247   * (the given list itself).
248   *
249   * <p>This method is safe to use even when {@code elements} is a synchronized or concurrent
250   * collection that is currently being modified by another thread.
251   *
252   * @throws NullPointerException if any of {@code elements} is null
253   */
254  public static <E> ImmutableList<E> copyOf(Collection<? extends E> elements) {
255    if (elements instanceof ImmutableCollection) {
256      @SuppressWarnings("unchecked") // all supported methods are covariant
257      ImmutableList<E> list = ((ImmutableCollection<E>) elements).asList();
258      return list.isPartialView() ? ImmutableList.<E>asImmutableList(list.toArray()) : list;
259    }
260    return construct(elements.toArray());
261  }
262
263  /**
264   * Returns an immutable list containing the given elements, in order.
265   *
266   * @throws NullPointerException if any of {@code elements} is null
267   */
268  public static <E> ImmutableList<E> copyOf(Iterator<? extends E> elements) {
269    // We special-case for 0 or 1 elements, but going further is madness.
270    if (!elements.hasNext()) {
271      return of();
272    }
273    E first = elements.next();
274    if (!elements.hasNext()) {
275      return of(first);
276    } else {
277      return new ImmutableList.Builder<E>().add(first).addAll(elements).build();
278    }
279  }
280
281  /**
282   * Returns an immutable list containing the given elements, in order.
283   *
284   * @throws NullPointerException if any of {@code elements} is null
285   * @since 3.0
286   */
287  public static <E> ImmutableList<E> copyOf(E[] elements) {
288    switch (elements.length) {
289      case 0:
290        return of();
291      case 1:
292        return of(elements[0]);
293      default:
294        return construct(elements.clone());
295    }
296  }
297
298  /**
299   * Returns an immutable list containing the given elements, sorted according to their natural
300   * order. The sorting algorithm used is stable, so elements that compare as equal will stay in the
301   * order in which they appear in the input.
302   *
303   * <p>If your data has no duplicates, or you wish to deduplicate elements, use {@code
304   * ImmutableSortedSet.copyOf(elements)}; if you want a {@code List} you can use its {@code
305   * asList()} view.
306   *
307   * <p><b>Java 8 users:</b> If you want to convert a {@link java.util.stream.Stream} to a sorted
308   * {@code ImmutableList}, use {@code stream.sorted().collect(toImmutableList())}.
309   *
310   * @throws NullPointerException if any element in the input is null
311   * @since 21.0
312   */
313  public static <E extends Comparable<? super E>> ImmutableList<E> sortedCopyOf(
314      Iterable<? extends E> elements) {
315    Comparable<?>[] array = Iterables.toArray(elements, new Comparable<?>[0]);
316    checkElementsNotNull((Object[]) array);
317    Arrays.sort(array);
318    return asImmutableList(array);
319  }
320
321  /**
322   * Returns an immutable list containing the given elements, in sorted order relative to the
323   * specified comparator. The sorting algorithm used is stable, so elements that compare as equal
324   * will stay in the order in which they appear in the input.
325   *
326   * <p>If your data has no duplicates, or you wish to deduplicate elements, use {@code
327   * ImmutableSortedSet.copyOf(comparator, elements)}; if you want a {@code List} you can use its
328   * {@code asList()} view.
329   *
330   * <p><b>Java 8 users:</b> If you want to convert a {@link java.util.stream.Stream} to a sorted
331   * {@code ImmutableList}, use {@code stream.sorted(comparator).collect(toImmutableList())}.
332   *
333   * @throws NullPointerException if any element in the input is null
334   * @since 21.0
335   */
336  public static <E> ImmutableList<E> sortedCopyOf(
337      Comparator<? super E> comparator, Iterable<? extends E> elements) {
338    checkNotNull(comparator);
339    @SuppressWarnings("unchecked") // all supported methods are covariant
340    E[] array = (E[]) Iterables.toArray(elements);
341    checkElementsNotNull(array);
342    Arrays.sort(array, comparator);
343    return asImmutableList(array);
344  }
345
346  /** Views the array as an immutable list. Checks for nulls; does not copy. */
347  private static <E> ImmutableList<E> construct(Object... elements) {
348    return asImmutableList(checkElementsNotNull(elements));
349  }
350
351  /**
352   * Views the array as an immutable list. Does not check for nulls; does not copy.
353   *
354   * <p>The array must be internally created.
355   */
356  static <E> ImmutableList<E> asImmutableList(Object[] elements) {
357    return asImmutableList(elements, elements.length);
358  }
359
360  /**
361   * Views the array as an immutable list. Copies if the specified range does not cover the complete
362   * array. Does not check for nulls.
363   */
364  static <E> ImmutableList<E> asImmutableList(Object[] elements, int length) {
365    switch (length) {
366      case 0:
367        return of();
368      case 1:
369        return of((E) elements[0]);
370      default:
371        if (length < elements.length) {
372          elements = Arrays.copyOf(elements, length);
373        }
374        return new RegularImmutableList<E>(elements);
375    }
376  }
377
378  ImmutableList() {}
379
380  // This declaration is needed to make List.iterator() and
381  // ImmutableCollection.iterator() consistent.
382  @Override
383  public UnmodifiableIterator<E> iterator() {
384    return listIterator();
385  }
386
387  @Override
388  public UnmodifiableListIterator<E> listIterator() {
389    return listIterator(0);
390  }
391
392  @Override
393  public UnmodifiableListIterator<E> listIterator(int index) {
394    return new AbstractIndexedListIterator<E>(size(), index) {
395      @Override
396      protected E get(int index) {
397        return ImmutableList.this.get(index);
398      }
399    };
400  }
401
402  @Override
403  public void forEach(Consumer<? super E> consumer) {
404    checkNotNull(consumer);
405    int n = size();
406    for (int i = 0; i < n; i++) {
407      consumer.accept(get(i));
408    }
409  }
410
411  @Override
412  public int indexOf(@NullableDecl Object object) {
413    return (object == null) ? -1 : Lists.indexOfImpl(this, object);
414  }
415
416  @Override
417  public int lastIndexOf(@NullableDecl Object object) {
418    return (object == null) ? -1 : Lists.lastIndexOfImpl(this, object);
419  }
420
421  @Override
422  public boolean contains(@NullableDecl Object object) {
423    return indexOf(object) >= 0;
424  }
425
426  // constrain the return type to ImmutableList<E>
427
428  /**
429   * Returns an immutable list of the elements between the specified {@code fromIndex}, inclusive,
430   * and {@code toIndex}, exclusive. (If {@code fromIndex} and {@code toIndex} are equal, the empty
431   * immutable list is returned.)
432   */
433  @Override
434  public ImmutableList<E> subList(int fromIndex, int toIndex) {
435    checkPositionIndexes(fromIndex, toIndex, size());
436    int length = toIndex - fromIndex;
437    if (length == size()) {
438      return this;
439    } else if (length == 0) {
440      return of();
441    } else if (length == 1) {
442      return of(get(fromIndex));
443    } else {
444      return subListUnchecked(fromIndex, toIndex);
445    }
446  }
447
448  /**
449   * Called by the default implementation of {@link #subList} when {@code toIndex - fromIndex > 1},
450   * after index validation has already been performed.
451   */
452  ImmutableList<E> subListUnchecked(int fromIndex, int toIndex) {
453    return new SubList(fromIndex, toIndex - fromIndex);
454  }
455
456  class SubList extends ImmutableList<E> {
457    final transient int offset;
458    final transient int length;
459
460    SubList(int offset, int length) {
461      this.offset = offset;
462      this.length = length;
463    }
464
465    @Override
466    public int size() {
467      return length;
468    }
469
470    @Override
471    public E get(int index) {
472      checkElementIndex(index, length);
473      return ImmutableList.this.get(index + offset);
474    }
475
476    @Override
477    public ImmutableList<E> subList(int fromIndex, int toIndex) {
478      checkPositionIndexes(fromIndex, toIndex, length);
479      return ImmutableList.this.subList(fromIndex + offset, toIndex + offset);
480    }
481
482    @Override
483    boolean isPartialView() {
484      return true;
485    }
486  }
487
488  /**
489   * Guaranteed to throw an exception and leave the list unmodified.
490   *
491   * @throws UnsupportedOperationException always
492   * @deprecated Unsupported operation.
493   */
494  @CanIgnoreReturnValue
495  @Deprecated
496  @Override
497  public final boolean addAll(int index, Collection<? extends E> newElements) {
498    throw new UnsupportedOperationException();
499  }
500
501  /**
502   * Guaranteed to throw an exception and leave the list unmodified.
503   *
504   * @throws UnsupportedOperationException always
505   * @deprecated Unsupported operation.
506   */
507  @CanIgnoreReturnValue
508  @Deprecated
509  @Override
510  public final E set(int index, E element) {
511    throw new UnsupportedOperationException();
512  }
513
514  /**
515   * Guaranteed to throw an exception and leave the list unmodified.
516   *
517   * @throws UnsupportedOperationException always
518   * @deprecated Unsupported operation.
519   */
520  @Deprecated
521  @Override
522  public final void add(int index, E element) {
523    throw new UnsupportedOperationException();
524  }
525
526  /**
527   * Guaranteed to throw an exception and leave the list unmodified.
528   *
529   * @throws UnsupportedOperationException always
530   * @deprecated Unsupported operation.
531   */
532  @CanIgnoreReturnValue
533  @Deprecated
534  @Override
535  public final E remove(int index) {
536    throw new UnsupportedOperationException();
537  }
538
539  /**
540   * Guaranteed to throw an exception and leave the list unmodified.
541   *
542   * @throws UnsupportedOperationException always
543   * @deprecated Unsupported operation.
544   */
545  @Deprecated
546  @Override
547  public final void replaceAll(UnaryOperator<E> operator) {
548    throw new UnsupportedOperationException();
549  }
550
551  /**
552   * Guaranteed to throw an exception and leave the list unmodified.
553   *
554   * @throws UnsupportedOperationException always
555   * @deprecated Unsupported operation.
556   */
557  @Deprecated
558  @Override
559  public final void sort(Comparator<? super E> c) {
560    throw new UnsupportedOperationException();
561  }
562
563  /**
564   * Returns this list instance.
565   *
566   * @since 2.0
567   */
568  @Override
569  public final ImmutableList<E> asList() {
570    return this;
571  }
572
573  @Override
574  public Spliterator<E> spliterator() {
575    return CollectSpliterators.indexed(size(), SPLITERATOR_CHARACTERISTICS, this::get);
576  }
577
578  @Override
579  int copyIntoArray(Object[] dst, int offset) {
580    // this loop is faster for RandomAccess instances, which ImmutableLists are
581    int size = size();
582    for (int i = 0; i < size; i++) {
583      dst[offset + i] = get(i);
584    }
585    return offset + size;
586  }
587
588  /**
589   * Returns a view of this immutable list in reverse order. For example, {@code ImmutableList.of(1,
590   * 2, 3).reverse()} is equivalent to {@code ImmutableList.of(3, 2, 1)}.
591   *
592   * @return a view of this immutable list in reverse order
593   * @since 7.0
594   */
595  public ImmutableList<E> reverse() {
596    return (size() <= 1) ? this : new ReverseImmutableList<E>(this);
597  }
598
599  private static class ReverseImmutableList<E> extends ImmutableList<E> {
600    private final transient ImmutableList<E> forwardList;
601
602    ReverseImmutableList(ImmutableList<E> backingList) {
603      this.forwardList = backingList;
604    }
605
606    private int reverseIndex(int index) {
607      return (size() - 1) - index;
608    }
609
610    private int reversePosition(int index) {
611      return size() - index;
612    }
613
614    @Override
615    public ImmutableList<E> reverse() {
616      return forwardList;
617    }
618
619    @Override
620    public boolean contains(@NullableDecl Object object) {
621      return forwardList.contains(object);
622    }
623
624    @Override
625    public int indexOf(@NullableDecl Object object) {
626      int index = forwardList.lastIndexOf(object);
627      return (index >= 0) ? reverseIndex(index) : -1;
628    }
629
630    @Override
631    public int lastIndexOf(@NullableDecl Object object) {
632      int index = forwardList.indexOf(object);
633      return (index >= 0) ? reverseIndex(index) : -1;
634    }
635
636    @Override
637    public ImmutableList<E> subList(int fromIndex, int toIndex) {
638      checkPositionIndexes(fromIndex, toIndex, size());
639      return forwardList.subList(reversePosition(toIndex), reversePosition(fromIndex)).reverse();
640    }
641
642    @Override
643    public E get(int index) {
644      checkElementIndex(index, size());
645      return forwardList.get(reverseIndex(index));
646    }
647
648    @Override
649    public int size() {
650      return forwardList.size();
651    }
652
653    @Override
654    boolean isPartialView() {
655      return forwardList.isPartialView();
656    }
657  }
658
659  @Override
660  public boolean equals(@NullableDecl Object obj) {
661    return Lists.equalsImpl(this, obj);
662  }
663
664  @Override
665  public int hashCode() {
666    int hashCode = 1;
667    int n = size();
668    for (int i = 0; i < n; i++) {
669      hashCode = 31 * hashCode + get(i).hashCode();
670
671      hashCode = ~~hashCode;
672      // needed to deal with GWT integer overflow
673    }
674    return hashCode;
675  }
676
677  /*
678   * Serializes ImmutableLists as their logical contents. This ensures that
679   * implementation types do not leak into the serialized representation.
680   */
681  static class SerializedForm implements Serializable {
682    final Object[] elements;
683
684    SerializedForm(Object[] elements) {
685      this.elements = elements;
686    }
687
688    Object readResolve() {
689      return copyOf(elements);
690    }
691
692    private static final long serialVersionUID = 0;
693  }
694
695  private void readObject(ObjectInputStream stream) throws InvalidObjectException {
696    throw new InvalidObjectException("Use SerializedForm");
697  }
698
699  @Override
700  Object writeReplace() {
701    return new SerializedForm(toArray());
702  }
703
704  /**
705   * Returns a new builder. The generated builder is equivalent to the builder created by the {@link
706   * Builder} constructor.
707   */
708  public static <E> Builder<E> builder() {
709    return new Builder<E>();
710  }
711
712  /**
713   * Returns a new builder, expecting the specified number of elements to be added.
714   *
715   * <p>If {@code expectedSize} is exactly the number of elements added to the builder before {@link
716   * Builder#build} is called, the builder is likely to perform better than an unsized {@link
717   * #builder()} would have.
718   *
719   * <p>It is not specified if any performance benefits apply if {@code expectedSize} is close to,
720   * but not exactly, the number of elements added to the builder.
721   *
722   * @since 23.1
723   */
724  @Beta
725  public static <E> Builder<E> builderWithExpectedSize(int expectedSize) {
726    checkNonnegative(expectedSize, "expectedSize");
727    return new ImmutableList.Builder<E>(expectedSize);
728  }
729
730  /**
731   * A builder for creating immutable list instances, especially {@code public static final} lists
732   * ("constant lists"). Example:
733   *
734   * <pre>{@code
735   * public static final ImmutableList<Color> GOOGLE_COLORS
736   *     = new ImmutableList.Builder<Color>()
737   *         .addAll(WEBSAFE_COLORS)
738   *         .add(new Color(0, 191, 255))
739   *         .build();
740   * }</pre>
741   *
742   * <p>Elements appear in the resulting list in the same order they were added to the builder.
743   *
744   * <p>Builder instances can be reused; it is safe to call {@link #build} multiple times to build
745   * multiple lists in series. Each new list contains all the elements of the ones created before
746   * it.
747   *
748   * @since 2.0
749   */
750  public static final class Builder<E> extends ImmutableCollection.Builder<E> {
751    @VisibleForTesting Object[] contents;
752    private int size;
753    private boolean forceCopy;
754
755    /**
756     * Creates a new builder. The returned builder is equivalent to the builder generated by {@link
757     * ImmutableList#builder}.
758     */
759    public Builder() {
760      this(DEFAULT_INITIAL_CAPACITY);
761    }
762
763    Builder(int capacity) {
764      this.contents = new Object[capacity];
765      this.size = 0;
766    }
767
768    private void getReadyToExpandTo(int minCapacity) {
769      if (contents.length < minCapacity) {
770        this.contents = Arrays.copyOf(contents, expandedCapacity(contents.length, minCapacity));
771        forceCopy = false;
772      } else if (forceCopy) {
773        contents = Arrays.copyOf(contents, contents.length);
774        forceCopy = false;
775      }
776    }
777
778    /**
779     * Adds {@code element} to the {@code ImmutableList}.
780     *
781     * @param element the element to add
782     * @return this {@code Builder} object
783     * @throws NullPointerException if {@code element} is null
784     */
785    @CanIgnoreReturnValue
786    @Override
787    public Builder<E> add(E element) {
788      checkNotNull(element);
789      getReadyToExpandTo(size + 1);
790      contents[size++] = element;
791      return this;
792    }
793
794    /**
795     * Adds each element of {@code elements} to the {@code ImmutableList}.
796     *
797     * @param elements the {@code Iterable} to add to the {@code ImmutableList}
798     * @return this {@code Builder} object
799     * @throws NullPointerException if {@code elements} is null or contains a null element
800     */
801    @CanIgnoreReturnValue
802    @Override
803    public Builder<E> add(E... elements) {
804      checkElementsNotNull(elements);
805      add(elements, elements.length);
806      return this;
807    }
808
809    private void add(Object[] elements, int n) {
810      getReadyToExpandTo(size + n);
811      System.arraycopy(elements, 0, contents, size, n);
812      size += n;
813    }
814
815    /**
816     * Adds each element of {@code elements} to the {@code ImmutableList}.
817     *
818     * @param elements the {@code Iterable} to add to the {@code ImmutableList}
819     * @return this {@code Builder} object
820     * @throws NullPointerException if {@code elements} is null or contains a null element
821     */
822    @CanIgnoreReturnValue
823    @Override
824    public Builder<E> addAll(Iterable<? extends E> elements) {
825      checkNotNull(elements);
826      if (elements instanceof Collection) {
827        Collection<?> collection = (Collection<?>) elements;
828        getReadyToExpandTo(size + collection.size());
829        if (collection instanceof ImmutableCollection) {
830          ImmutableCollection<?> immutableCollection = (ImmutableCollection<?>) collection;
831          size = immutableCollection.copyIntoArray(contents, size);
832          return this;
833        }
834      }
835      super.addAll(elements);
836      return this;
837    }
838
839    /**
840     * Adds each element of {@code elements} to the {@code ImmutableList}.
841     *
842     * @param elements the {@code Iterable} to add to the {@code ImmutableList}
843     * @return this {@code Builder} object
844     * @throws NullPointerException if {@code elements} is null or contains a null element
845     */
846    @CanIgnoreReturnValue
847    @Override
848    public Builder<E> addAll(Iterator<? extends E> elements) {
849      super.addAll(elements);
850      return this;
851    }
852
853    @CanIgnoreReturnValue
854    Builder<E> combine(Builder<E> builder) {
855      checkNotNull(builder);
856      add(builder.contents, builder.size);
857      return this;
858    }
859
860    /**
861     * Returns a newly-created {@code ImmutableList} based on the contents of the {@code Builder}.
862     */
863    @Override
864    public ImmutableList<E> build() {
865      forceCopy = true;
866      return asImmutableList(contents, size);
867    }
868  }
869}