path: root/guava/src/com/google/common/collect/Lists.java

/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkElementIndex;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkPositionIndex;
import static com.google.common.base.Preconditions.checkPositionIndexes;
import static com.google.common.base.Preconditions.checkState;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Function;
import com.google.common.base.Objects;
import com.google.common.primitives.Ints;

import java.io.Serializable;
import java.util.AbstractList;
import java.util.AbstractSequentialList;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.RandomAccess;
import java.util.concurrent.CopyOnWriteArrayList;

import javax.annotation.Nullable;

/**
 * Static utility methods pertaining to {@link List} instances. Also see this
 * class's counterparts {@link Sets}, {@link Maps} and {@link Queues}.
 *
 * <p>See the Guava User Guide article on <a href=
 * "http://code.google.com/p/guava-libraries/wiki/CollectionUtilitiesExplained#Lists">
 * {@code Lists}</a>.
 *
 * @author Kevin Bourrillion
 * @author Mike Bostock
 * @author Louis Wasserman
 * @since 2.0 (imported from Google Collections Library)
 */
@GwtCompatible(emulated = true)
public final class Lists {
  private Lists() {}

  // ArrayList

  /**
   * Creates a <i>mutable</i>, empty {@code ArrayList} instance.
   *
   * <p><b>Note:</b> if mutability is not required, use {@link
   * ImmutableList#of()} instead.
   *
   * @return a new, empty {@code ArrayList}
   */
  @GwtCompatible(serializable = true)
  public static <E> ArrayList<E> newArrayList() {
    return new ArrayList<E>();
  }

  /**
   * Creates a <i>mutable</i> {@code ArrayList} instance containing the given
   * elements.
   *
   * <p><b>Note:</b> if mutability is not required and the elements are
   * non-null, use an overload of {@link ImmutableList#of()} (for varargs) or
   * {@link ImmutableList#copyOf(Object[])} (for an array) instead.
   *
   * @param elements the elements that the list should contain, in order
   * @return a new {@code ArrayList} containing those elements
   */
  @GwtCompatible(serializable = true)
  public static <E> ArrayList<E> newArrayList(E... elements) {
    checkNotNull(elements); // for GWT
    // Avoid integer overflow when a large array is passed in
    int capacity = computeArrayListCapacity(elements.length);
    ArrayList<E> list = new ArrayList<E>(capacity);
    Collections.addAll(list, elements);
    return list;
  }

  @VisibleForTesting static int computeArrayListCapacity(int arraySize) {
    checkArgument(arraySize >= 0);

    // TODO(kevinb): Figure out the right behavior, and document it
    return Ints.saturatedCast(5L + arraySize + (arraySize / 10));
  }

  /**
   * Creates a <i>mutable</i> {@code ArrayList} instance containing the given
   * elements.
   *
   * <p><b>Note:</b> if mutability is not required and the elements are
   * non-null, use {@link ImmutableList#copyOf(Iterator)} instead.
   *
   * @param elements the elements that the list should contain, in order
   * @return a new {@code ArrayList} containing those elements
   */
  @GwtCompatible(serializable = true)
  public static <E> ArrayList<E> newArrayList(Iterable<? extends E> elements) {
    checkNotNull(elements); // for GWT
    // Let ArrayList's sizing logic work, if possible
    return (elements instanceof Collection)
        ? new ArrayList<E>(Collections2.cast(elements))
        : newArrayList(elements.iterator());
  }

  /**
   * Creates a <i>mutable</i> {@code ArrayList} instance containing the given
   * elements.
   *
   * <p><b>Note:</b> if mutability is not required and the elements are
   * non-null, use {@link ImmutableList#copyOf(Iterator)} instead.
   *
   * @param elements the elements that the list should contain, in order
   * @return a new {@code ArrayList} containing those elements
   */
  @GwtCompatible(serializable = true)
  public static <E> ArrayList<E> newArrayList(Iterator<? extends E> elements) {
    checkNotNull(elements); // for GWT
    ArrayList<E> list = newArrayList();
    while (elements.hasNext()) {
      list.add(elements.next());
    }
    return list;
  }

  /**
   * Creates an {@code ArrayList} instance backed by an array of the
   * <i>exact</i> size specified; equivalent to
   * {@link ArrayList#ArrayList(int)}.
   *
   * <p><b>Note:</b> if you know the exact size your list will be, consider
   * using a fixed-size list ({@link Arrays#asList(Object[])}) or an {@link
   * ImmutableList} instead of a growable {@link ArrayList}.
   *
   * <p><b>Note:</b> If you have only an <i>estimate</i> of the eventual size of
   * the list, consider padding this estimate by a suitable amount, or simply
   * use {@link #newArrayListWithExpectedSize(int)} instead.
   *
   * @param initialArraySize the exact size of the initial backing array for
   *     the returned array list ({@code ArrayList} documentation calls this
   *     value the "capacity")
   * @return a new, empty {@code ArrayList} which is guaranteed not to resize
   *     itself unless its size reaches {@code initialArraySize + 1}
   * @throws IllegalArgumentException if {@code initialArraySize} is negative
   */
  @GwtCompatible(serializable = true)
  public static <E> ArrayList<E> newArrayListWithCapacity(
      int initialArraySize) {
    checkArgument(initialArraySize >= 0);  // for GWT.
    return new ArrayList<E>(initialArraySize);
  }

  /**
   * Creates an {@code ArrayList} instance sized appropriately to hold an
   * <i>estimated</i> number of elements without resizing. A small amount of
   * padding is added in case the estimate is low.
   *
   * <p><b>Note:</b> If you know the <i>exact</i> number of elements the list
   * will hold, or prefer to calculate your own amount of padding, refer to
   * {@link #newArrayListWithCapacity(int)}.
   *
   * @param estimatedSize an estimate of the eventual {@link List#size()} of
   *     the new list
   * @return a new, empty {@code ArrayList}, sized appropriately to hold the
   *     estimated number of elements
   * @throws IllegalArgumentException if {@code estimatedSize} is negative
   */
  @GwtCompatible(serializable = true)
  public static <E> ArrayList<E> newArrayListWithExpectedSize(
      int estimatedSize) {
    return new ArrayList<E>(computeArrayListCapacity(estimatedSize));
  }

  // LinkedList

  /**
   * Creates an empty {@code LinkedList} instance.
   *
   * <p><b>Note:</b> if you need an immutable empty {@link List}, use
   * {@link ImmutableList#of()} instead.
   *
   * @return a new, empty {@code LinkedList}
   */
  @GwtCompatible(serializable = true)
  public static <E> LinkedList<E> newLinkedList() {
    return new LinkedList<E>();
  }

  /**
   * Creates a {@code LinkedList} instance containing the given elements.
   *
   * @param elements the elements that the list should contain, in order
   * @return a new {@code LinkedList} containing those elements
   */
  @GwtCompatible(serializable = true)
  public static <E> LinkedList<E> newLinkedList(
      Iterable<? extends E> elements) {
    LinkedList<E> list = newLinkedList();
    for (E element : elements) {
      list.add(element);
    }
    return list;
  }

  /**
   * Creates an empty {@code CopyOnWriteArrayList} instance.
   *
   * <p><b>Note:</b> if you need an immutable empty {@link List}, use
   * {@link Collections#emptyList} instead.
   *
   * @return a new, empty {@code CopyOnWriteArrayList}
   * @since 12.0
   */
  @GwtIncompatible("CopyOnWriteArrayList")
  public static <E> CopyOnWriteArrayList<E> newCopyOnWriteArrayList() {
    return new CopyOnWriteArrayList<E>();
  }

  /**
   * Creates a {@code CopyOnWriteArrayList} instance containing the given elements.
   *
   * @param elements the elements that the list should contain, in order
   * @return a new {@code CopyOnWriteArrayList} containing those elements
   * @since 12.0
   */
  @GwtIncompatible("CopyOnWriteArrayList")
  public static <E> CopyOnWriteArrayList<E> newCopyOnWriteArrayList(
      Iterable<? extends E> elements) {
    // We copy elements to an ArrayList first, rather than incurring the
    // quadratic cost of adding them to the COWAL directly.
    Collection<? extends E> elementsCollection = (elements instanceof Collection)
        ? Collections2.cast(elements)
        : newArrayList(elements);
    return new CopyOnWriteArrayList<E>(elementsCollection);
  }

  /**
   * Returns an unmodifiable list containing the specified first element and
   * backed by the specified array of additional elements. Changes to the {@code
   * rest} array will be reflected in the returned list. Unlike {@link
   * Arrays#asList}, the returned list is unmodifiable.
   *
   * <p>This is useful when a varargs method needs to use a signature such as
   * {@code (Foo firstFoo, Foo... moreFoos)}, in order to avoid overload
   * ambiguity or to enforce a minimum argument count.
   *
   * <p>The returned list is serializable and implements {@link RandomAccess}.
   *
   * @param first the first element
   * @param rest an array of additional elements, possibly empty
   * @return an unmodifiable list containing the specified elements
   */
  public static <E> List<E> asList(@Nullable E first, E[] rest) {
    return new OnePlusArrayList<E>(first, rest);
  }

  /** @see Lists#asList(Object, Object[]) */
  private static class OnePlusArrayList<E> extends AbstractList<E>
      implements Serializable, RandomAccess {
    final E first;
    final E[] rest;

    OnePlusArrayList(@Nullable E first, E[] rest) {
      this.first = first;
      this.rest = checkNotNull(rest);
    }
    @Override public int size() {
      return rest.length + 1;
    }
    @Override public E get(int index) {
      // check explicitly so the IOOBE will have the right message
      checkElementIndex(index, size());
      return (index == 0) ? first : rest[index - 1];
    }
    private static final long serialVersionUID = 0;
  }

  /**
   * Returns an unmodifiable list containing the specified first and second
   * element, and backed by the specified array of additional elements. Changes
   * to the {@code rest} array will be reflected in the returned list. Unlike
   * {@link Arrays#asList}, the returned list is unmodifiable.
   *
   * <p>This is useful when a varargs method needs to use a signature such as
   * {@code (Foo firstFoo, Foo secondFoo, Foo... moreFoos)}, in order to avoid
   * overload ambiguity or to enforce a minimum argument count.
   *
   * <p>The returned list is serializable and implements {@link RandomAccess}.
   *
   * @param first the first element
   * @param second the second element
   * @param rest an array of additional elements, possibly empty
   * @return an unmodifiable list containing the specified elements
   */
  public static <E> List<E> asList(
      @Nullable E first, @Nullable E second, E[] rest) {
    return new TwoPlusArrayList<E>(first, second, rest);
  }

  /** @see Lists#asList(Object, Object, Object[]) */
  private static class TwoPlusArrayList<E> extends AbstractList<E>
      implements Serializable, RandomAccess {
    final E first;
    final E second;
    final E[] rest;

    TwoPlusArrayList(@Nullable E first, @Nullable E second, E[] rest) {
      this.first = first;
      this.second = second;
      this.rest = checkNotNull(rest);
    }
    @Override public int size() {
      return rest.length + 2;
    }
    @Override public E get(int index) {
      switch (index) {
        case 0:
          return first;
        case 1:
          return second;
        default:
          // check explicitly so the IOOBE will have the right message
          checkElementIndex(index, size());
          return rest[index - 2];
      }
    }
    private static final long serialVersionUID = 0;
  }

  /**
   * Returns every possible list that can be formed by choosing one element
   * from each of the given lists in order; the "n-ary
   * <a href="http://en.wikipedia.org/wiki/Cartesian_product">Cartesian
   * product</a>" of the lists. For example: <pre>   {@code
   *
   *   Lists.cartesianProduct(ImmutableList.of(
   *       ImmutableList.of(1, 2),
   *       ImmutableList.of("A", "B", "C")))}</pre>
   *
   * returns a list containing six lists in the following order:
   *
   * <ul>
   * <li>{@code ImmutableList.of(1, "A")}
   * <li>{@code ImmutableList.of(1, "B")}
   * <li>{@code ImmutableList.of(1, "C")}
   * <li>{@code ImmutableList.of(2, "A")}
   * <li>{@code ImmutableList.of(2, "B")}
   * <li>{@code ImmutableList.of(2, "C")}
   * </ul>
   *
   * The result is guaranteed to be in the "traditional", lexicographical
   * order for Cartesian products that you would get from nesting for loops:
   * <pre>   {@code
   *
   *   for (B b0 : lists.get(0)) {
   *     for (B b1 : lists.get(1)) {
   *       ...
   *       ImmutableList<B> tuple = ImmutableList.of(b0, b1, ...);
   *       // operate on tuple
   *     }
   *   }}</pre>
   *
   * Note that if any input list is empty, the Cartesian product will also be
   * empty. If no lists at all are provided (an empty list), the resulting
   * Cartesian product has one element, an empty list (counter-intuitive, but
   * mathematically consistent).
   *
   * <p><i>Performance notes:</i> while the cartesian product of lists of size
   * {@code m, n, p} is a list of size {@code m x n x p}, its actual memory
   * consumption is much smaller. When the cartesian product is constructed, the
   * input lists are merely copied. Only as the resulting list is iterated are
   * the individual lists created, and these are not retained after iteration.
   *
   * @param lists the lists to choose elements from, in the order that
   *     the elements chosen from those lists should appear in the resulting
   *     lists
   * @param <B> any common base class shared by all axes (often just {@link
   *     Object})
   * @return the Cartesian product, as an immutable list containing immutable
   *     lists
   * @throws IllegalArgumentException if the size of the cartesian product would
   *     be greater than {@link Integer#MAX_VALUE}
   * @throws NullPointerException if {@code lists}, any one of the {@code lists},
   *     or any element of a provided list is null
   */
  static <B> List<List<B>> cartesianProduct(
      List<? extends List<? extends B>> lists) {
    return CartesianList.create(lists);
  }

  /**
   * Returns every possible list that can be formed by choosing one element
   * from each of the given lists in order; the "n-ary
   * <a href="http://en.wikipedia.org/wiki/Cartesian_product">Cartesian
   * product</a>" of the lists. For example: <pre>   {@code
   *
   *   Lists.cartesianProduct(ImmutableList.of(
   *       ImmutableList.of(1, 2),
   *       ImmutableList.of("A", "B", "C")))}</pre>
   *
   * returns a list containing six lists in the following order:
   *
   * <ul>
   * <li>{@code ImmutableList.of(1, "A")}
   * <li>{@code ImmutableList.of(1, "B")}
   * <li>{@code ImmutableList.of(1, "C")}
   * <li>{@code ImmutableList.of(2, "A")}
   * <li>{@code ImmutableList.of(2, "B")}
   * <li>{@code ImmutableList.of(2, "C")}
   * </ul>
   *
   * The result is guaranteed to be in the "traditional", lexicographical
   * order for Cartesian products that you would get from nesting for loops:
   * <pre>   {@code
   *
   *   for (B b0 : lists.get(0)) {
   *     for (B b1 : lists.get(1)) {
   *       ...
   *       ImmutableList<B> tuple = ImmutableList.of(b0, b1, ...);
   *       // operate on tuple
   *     }
   *   }}</pre>
   *
   * Note that if any input list is empty, the Cartesian product will also be
   * empty. If no lists at all are provided (an empty list), the resulting
   * Cartesian product has one element, an empty list (counter-intuitive, but
   * mathematically consistent).
   *
   * <p><i>Performance notes:</i> while the cartesian product of lists of size
   * {@code m, n, p} is a list of size {@code m x n x p}, its actual memory
   * consumption is much smaller. When the cartesian product is constructed, the
   * input lists are merely copied. Only as the resulting list is iterated are
   * the individual lists created, and these are not retained after iteration.
   *
   * @param lists the lists to choose elements from, in the order that
   *     the elements chosen from those lists should appear in the resulting
   *     lists
   * @param <B> any common base class shared by all axes (often just {@link
   *     Object})
   * @return the Cartesian product, as an immutable list containing immutable
   *     lists
   * @throws IllegalArgumentException if the size of the cartesian product would
   *     be greater than {@link Integer#MAX_VALUE}
   * @throws NullPointerException if {@code lists}, any one of the
   *     {@code lists}, or any element of a provided list is null
   */
  static <B> List<List<B>> cartesianProduct(List<? extends B>... lists) {
    return cartesianProduct(Arrays.asList(lists));
  }

  /**
   * Returns a list that applies {@code function} to each element of {@code
   * fromList}. The returned list is a transformed view of {@code fromList};
   * changes to {@code fromList} will be reflected in the returned list and vice
   * versa.
   *
   * <p>Since functions are not reversible, the transform is one-way and new
   * items cannot be stored in the returned list. The {@code add},
   * {@code addAll} and {@code set} methods are unsupported in the returned
   * list.
   *
   * <p>The function is applied lazily, invoked when needed. This is necessary
   * for the returned list to be a view, but it means that the function will be
   * applied many times for bulk operations like {@link List#contains} and
   * {@link List#hashCode}. For this to perform well, {@code function} should be
   * fast. To avoid lazy evaluation when the returned list doesn't need to be a
   * view, copy the returned list into a new list of your choosing.
   *
   * <p>If {@code fromList} implements {@link RandomAccess}, so will the
   * returned list. The returned list is threadsafe if the supplied list and
   * function are.
   *
   * <p>If only a {@code Collection} or {@code Iterable} input is available, use
   * {@link Collections2#transform} or {@link Iterables#transform}.
   *
   * <p><b>Note:</b> serializing the returned list is implemented by serializing
   * {@code fromList}, its contents, and {@code function} -- <i>not</i> by
   * serializing the transformed values. This can lead to surprising behavior,
   * so serializing the returned list is <b>not recommended</b>. Instead,
   * copy the list using {@link ImmutableList#copyOf(Collection)} (for example),
   * then serialize the copy. Other methods similar to this do not implement
   * serialization at all for this reason.
   */
  public static <F, T> List<T> transform(
      List<F> fromList, Function<? super F, ? extends T> function) {
    return (fromList instanceof RandomAccess)
        ? new TransformingRandomAccessList<F, T>(fromList, function)
        : new TransformingSequentialList<F, T>(fromList, function);
  }

  /**
   * Implementation of a sequential transforming list.
   *
   * @see Lists#transform
   */
  private static class TransformingSequentialList<F, T>
      extends AbstractSequentialList<T> implements Serializable {
    final List<F> fromList;
    final Function<? super F, ? extends T> function;

    TransformingSequentialList(
        List<F> fromList, Function<? super F, ? extends T> function) {
      this.fromList = checkNotNull(fromList);
      this.function = checkNotNull(function);
    }
    /**
     * The default implementation inherited is based on iteration and removal of
     * each element which can be overkill. That's why we forward this call
     * directly to the backing list.
     */
    @Override public void clear() {
      fromList.clear();
    }
    @Override public int size() {
      return fromList.size();
    }
    @Override public ListIterator<T> listIterator(final int index) {
      return new TransformedListIterator<F, T>(fromList.listIterator(index)) {
        @Override
        T transform(F from) {
          return function.apply(from);
        }
      };
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Implementation of a transforming random access list. We try to make as many
   * of these methods pass-through to the source list as possible so that the
   * performance characteristics of the source list and transformed list are
   * similar.
   *
   * @see Lists#transform
   */
  private static class TransformingRandomAccessList<F, T>
      extends AbstractList<T> implements RandomAccess, Serializable {
    final List<F> fromList;
    final Function<? super F, ? extends T> function;

    TransformingRandomAccessList(
        List<F> fromList, Function<? super F, ? extends T> function) {
      this.fromList = checkNotNull(fromList);
      this.function = checkNotNull(function);
    }
    @Override public void clear() {
      fromList.clear();
    }
    @Override public T get(int index) {
      return function.apply(fromList.get(index));
    }
    @Override public boolean isEmpty() {
      return fromList.isEmpty();
    }
    @Override public T remove(int index) {
      return function.apply(fromList.remove(index));
    }
    @Override public int size() {
      return fromList.size();
    }
    private static final long serialVersionUID = 0;
  }

  /**
   * Returns consecutive {@linkplain List#subList(int, int) sublists} of a list,
   * each of the same size (the final list may be smaller). For example,
   * partitioning a list containing {@code [a, b, c, d, e]} with a partition
   * size of 3 yields {@code [[a, b, c], [d, e]]} -- an outer list containing
   * two inner lists of three and two elements, all in the original order.
   *
   * <p>The outer list is unmodifiable, but reflects the latest state of the
   * source list. The inner lists are sublist views of the original list,
   * produced on demand using {@link List#subList(int, int)}, and are subject
   * to all the usual caveats about modification as explained in that API.
   *
   * @param list the list to return consecutive sublists of
   * @param size the desired size of each sublist (the last may be
   *     smaller)
   * @return a list of consecutive sublists
   * @throws IllegalArgumentException if {@code partitionSize} is nonpositive
   */
  public static <T> List<List<T>> partition(List<T> list, int size) {
    checkNotNull(list);
    checkArgument(size > 0);
    return (list instanceof RandomAccess)
        ? new RandomAccessPartition<T>(list, size)
        : new Partition<T>(list, size);
  }

  private static class Partition<T> extends AbstractList<List<T>> {
    final List<T> list;
    final int size;

    Partition(List<T> list, int size) {
      this.list = list;
      this.size = size;
    }

    @Override public List<T> get(int index) {
      int listSize = size();
      checkElementIndex(index, listSize);
      int start = index * size;
      int end = Math.min(start + size, list.size());
      return list.subList(start, end);
    }

    @Override public int size() {
      // TODO(user): refactor to common.math.IntMath.divide
      int result = list.size() / size;
      if (result * size != list.size()) {
        result++;
      }
      return result;
    }

    @Override public boolean isEmpty() {
      return list.isEmpty();
    }
  }

  private static class RandomAccessPartition<T> extends Partition<T>
      implements RandomAccess {
    RandomAccessPartition(List<T> list, int size) {
      super(list, size);
    }
  }

  /**
   * Returns a view of the specified string as an immutable list of {@code
   * Character} values.
   *
   * @since 7.0
   */
  @Beta public static ImmutableList<Character> charactersOf(String string) {
    return new StringAsImmutableList(checkNotNull(string));
  }

  @SuppressWarnings("serial") // serialized using ImmutableList serialization
  private static final class StringAsImmutableList
      extends ImmutableList<Character> {

    private final String string;

    StringAsImmutableList(String string) {
      this.string = string;
    }

    @Override public int indexOf(@Nullable Object object) {
      return (object instanceof Character)
          ? string.indexOf((Character) object) : -1;
    }

    @Override public int lastIndexOf(@Nullable Object object) {
      return (object instanceof Character)
          ? string.lastIndexOf((Character) object) : -1;
    }

    @Override public ImmutableList<Character> subList(
        int fromIndex, int toIndex) {
      checkPositionIndexes(fromIndex, toIndex, size()); // for GWT
      return charactersOf(string.substring(fromIndex, toIndex));
    }

    @Override boolean isPartialView() {
      return false;
    }

    @Override public Character get(int index) {
      checkElementIndex(index, size()); // for GWT
      return string.charAt(index);
    }

    @Override public int size() {
      return string.length();
    }

    @Override public boolean equals(@Nullable Object obj) {
      if (!(obj instanceof List)) {
        return false;
      }
      List<?> list = (List<?>) obj;
      int n = string.length();
      if (n != list.size()) {
        return false;
      }
      Iterator<?> iterator = list.iterator();
      for (int i = 0; i < n; i++) {
        Object elem = iterator.next();
        if (!(elem instanceof Character)
            || ((Character) elem).charValue() != string.charAt(i)) {
          return false;
        }
      }
      return true;
    }

    int hash = 0;

    @Override public int hashCode() {
      int h = hash;
      if (h == 0) {
        h = 1;
        for (int i = 0; i < string.length(); i++) {
          h = h * 31 + string.charAt(i);
        }
        hash = h;
      }
      return h;
    }
  }

  /**
   * Returns a view of the specified {@code CharSequence} as a {@code
   * List<Character>}, viewing {@code sequence} as a sequence of Unicode code
   * units. The view does not support any modification operations, but reflects
   * any changes to the underlying character sequence.
   *
   * @param sequence the character sequence to view as a {@code List} of
   *        characters
   * @return an {@code List<Character>} view of the character sequence
   * @since 7.0
   */
  @Beta public static List<Character> charactersOf(CharSequence sequence) {
    return new CharSequenceAsList(checkNotNull(sequence));
  }

  private static final class CharSequenceAsList
      extends AbstractList<Character> {
    private final CharSequence sequence;

    CharSequenceAsList(CharSequence sequence) {
      this.sequence = sequence;
    }

    @Override public Character get(int index) {
      checkElementIndex(index, size()); // for GWT
      return sequence.charAt(index);
    }

    @Override public boolean contains(@Nullable Object o) {
      return indexOf(o) >= 0;
    }

    @Override public int indexOf(@Nullable Object o) {
      if (o instanceof Character) {
        char c = (Character) o;
        for (int i = 0; i < sequence.length(); i++) {
          if (sequence.charAt(i) == c) {
            return i;
          }
        }
      }
      return -1;
    }

    @Override public int lastIndexOf(@Nullable Object o) {
      if (o instanceof Character) {
        char c = ((Character) o).charValue();
        for (int i = sequence.length() - 1; i >= 0; i--) {
          if (sequence.charAt(i) == c) {
            return i;
          }
        }
      }
      return -1;
    }

    @Override public int size() {
      return sequence.length();
    }

    @Override public List<Character> subList(int fromIndex, int toIndex) {
      checkPositionIndexes(fromIndex, toIndex, size()); // for GWT
      return charactersOf(sequence.subSequence(fromIndex, toIndex));
    }

    @Override public int hashCode() {
      int hash = 1;
      for (int i = 0; i < sequence.length(); i++) {
        hash = hash * 31 + sequence.charAt(i);
      }
      return hash;
    }

    @Override public boolean equals(@Nullable Object o) {
      if (!(o instanceof List)) {
        return false;
      }
      List<?> list = (List<?>) o;
      int n = sequence.length();
      if (n != list.size()) {
        return false;
      }
      Iterator<?> iterator = list.iterator();
      for (int i = 0; i < n; i++) {
        Object elem = iterator.next();
        if (!(elem instanceof Character)
            || ((Character) elem).charValue() != sequence.charAt(i)) {
          return false;
        }
      }
      return true;
    }
  }

  /**
   * Returns a reversed view of the specified list. For example, {@code
   * Lists.reverse(Arrays.asList(1, 2, 3))} returns a list containing {@code 3,
   * 2, 1}. The returned list is backed by this list, so changes in the returned
   * list are reflected in this list, and vice-versa. The returned list supports
   * all of the optional list operations supported by this list.
   *
   * <p>The returned list is random-access if the specified list is random
   * access.
   *
   * @since 7.0
   */
  public static <T> List<T> reverse(List<T> list) {
    if (list instanceof ReverseList) {
      return ((ReverseList<T>) list).getForwardList();
    } else if (list instanceof RandomAccess) {
      return new RandomAccessReverseList<T>(list);
    } else {
      return new ReverseList<T>(list);
    }
  }

  private static class ReverseList<T> extends AbstractList<T> {
    private final List<T> forwardList;

    ReverseList(List<T> forwardList) {
      this.forwardList = checkNotNull(forwardList);
    }

    List<T> getForwardList() {
      return forwardList;
    }

    private int reverseIndex(int index) {
      int size = size();
      checkElementIndex(index, size);
      return (size - 1) - index;
    }

    private int reversePosition(int index) {
      int size = size();
      checkPositionIndex(index, size);
      return size - index;
    }

    @Override public void add(int index, @Nullable T element) {
      forwardList.add(reversePosition(index), element);
    }

    @Override public void clear() {
      forwardList.clear();
    }

    @Override public T remove(int index) {
      return forwardList.remove(reverseIndex(index));
    }

    @Override protected void removeRange(int fromIndex, int toIndex) {
      subList(fromIndex, toIndex).clear();
    }

    @Override public T set(int index, @Nullable T element) {
      return forwardList.set(reverseIndex(index), element);
    }

    @Override public T get(int index) {
      return forwardList.get(reverseIndex(index));
    }

    @Override public boolean isEmpty() {
      return forwardList.isEmpty();
    }

    @Override public int size() {
      return forwardList.size();
    }

    @Override public boolean contains(@Nullable Object o) {
      return forwardList.contains(o);
    }

    @Override public boolean containsAll(Collection<?> c) {
      return forwardList.containsAll(c);
    }

    @Override public List<T> subList(int fromIndex, int toIndex) {
      checkPositionIndexes(fromIndex, toIndex, size());
      return reverse(forwardList.subList(
          reversePosition(toIndex), reversePosition(fromIndex)));
    }

    @Override public int indexOf(@Nullable Object o) {
      int index = forwardList.lastIndexOf(o);
      return (index >= 0) ? reverseIndex(index) : -1;
    }

    @Override public int lastIndexOf(@Nullable Object o) {
      int index = forwardList.indexOf(o);
      return (index >= 0) ? reverseIndex(index) : -1;
    }

    @Override public Iterator<T> iterator() {
      return listIterator();
    }

    @Override public ListIterator<T> listIterator(int index) {
      int start = reversePosition(index);
      final ListIterator<T> forwardIterator = forwardList.listIterator(start);
      return new ListIterator<T>() {

        boolean canRemove;
        boolean canSet;

        @Override public void add(T e) {
          forwardIterator.add(e);
          forwardIterator.previous();
          canSet = canRemove = false;
        }

        @Override public boolean hasNext() {
          return forwardIterator.hasPrevious();
        }

        @Override public boolean hasPrevious() {
          return forwardIterator.hasNext();
        }

        @Override public T next() {
          if (!hasNext()) {
            throw new NoSuchElementException();
          }
          canSet = canRemove = true;
          return forwardIterator.previous();
        }

        @Override public int nextIndex() {
          return reversePosition(forwardIterator.nextIndex());
        }

        @Override public T previous() {
          if (!hasPrevious()) {
            throw new NoSuchElementException();
          }
          canSet = canRemove = true;
          return forwardIterator.next();
        }

        @Override public int previousIndex() {
          return nextIndex() - 1;
        }

        @Override public void remove() {
          checkState(canRemove);
          forwardIterator.remove();
          canRemove = canSet = false;
        }

        @Override public void set(T e) {
          checkState(canSet);
          forwardIterator.set(e);
        }
      };
    }
  }

  private static class RandomAccessReverseList<T> extends ReverseList<T>
      implements RandomAccess {
    RandomAccessReverseList(List<T> forwardList) {
      super(forwardList);
    }
  }

  /**
   * An implementation of {@link List#hashCode()}.
   */
  static int hashCodeImpl(List<?> list) {
    int hashCode = 1;
    for (Object o : list) {
      hashCode = 31 * hashCode + (o == null ? 0 : o.hashCode());

      hashCode = ~~hashCode;
      // needed to deal with GWT integer overflow
    }
    return hashCode;
  }

  /**
   * An implementation of {@link List#equals(Object)}.
   */
  static boolean equalsImpl(List<?> list, @Nullable Object object) {
    if (object == checkNotNull(list)) {
      return true;
    }
    if (!(object instanceof List)) {
      return false;
    }

    List<?> o = (List<?>) object;

    return list.size() == o.size()
        && Iterators.elementsEqual(list.iterator(), o.iterator());
  }

  /**
   * An implementation of {@link List#addAll(int, Collection)}.
   */
  static <E> boolean addAllImpl(
      List<E> list, int index, Iterable<? extends E> elements) {
    boolean changed = false;
    ListIterator<E> listIterator = list.listIterator(index);
    for (E e : elements) {
      listIterator.add(e);
      changed = true;
    }
    return changed;
  }

  /**
   * An implementation of {@link List#indexOf(Object)}.
   */
  static int indexOfImpl(List<?> list, @Nullable Object element){
    ListIterator<?> listIterator = list.listIterator();
    while (listIterator.hasNext()) {
      if (Objects.equal(element, listIterator.next())) {
        return listIterator.previousIndex();
      }
    }
    return -1;
  }

  /**
   * An implementation of {@link List#lastIndexOf(Object)}.
   */
  static int lastIndexOfImpl(List<?> list, @Nullable Object element){
    ListIterator<?> listIterator = list.listIterator(list.size());
    while (listIterator.hasPrevious()) {
      if (Objects.equal(element, listIterator.previous())) {
        return listIterator.nextIndex();
      }
    }
    return -1;
  }

  /**
   * Returns an implementation of {@link List#listIterator(int)}.
   */
  static <E> ListIterator<E> listIteratorImpl(List<E> list, int index) {
    return new AbstractListWrapper<E>(list).listIterator(index);
  }

  /**
   * An implementation of {@link List#subList(int, int)}.
   */
  static <E> List<E> subListImpl(
      final List<E> list, int fromIndex, int toIndex) {
    List<E> wrapper;
    if (list instanceof RandomAccess) {
      wrapper = new RandomAccessListWrapper<E>(list) {
        @Override public ListIterator<E> listIterator(int index) {
          return backingList.listIterator(index);
        }

        private static final long serialVersionUID = 0;
      };
    } else {
      wrapper = new AbstractListWrapper<E>(list) {
        @Override public ListIterator<E> listIterator(int index) {
          return backingList.listIterator(index);
        }

        private static final long serialVersionUID = 0;
      };
    }
    return wrapper.subList(fromIndex, toIndex);
  }

  private static class AbstractListWrapper<E> extends AbstractList<E> {
    final List<E> backingList;

    AbstractListWrapper(List<E> backingList) {
      this.backingList = checkNotNull(backingList);
    }

    @Override public void add(int index, E element) {
      backingList.add(index, element);
    }

    @Override public boolean addAll(int index, Collection<? extends E> c) {
      return backingList.addAll(index, c);
    }

    @Override public E get(int index) {
      return backingList.get(index);
    }

    @Override public E remove(int index) {
      return backingList.remove(index);
    }

    @Override public E set(int index, E element) {
      return backingList.set(index, element);
    }

    @Override public boolean contains(Object o) {
      return backingList.contains(o);
    }

    @Override public int size() {
      return backingList.size();
    }
  }

  private static class RandomAccessListWrapper<E>
      extends AbstractListWrapper<E> implements RandomAccess {
    RandomAccessListWrapper(List<E> backingList) {
      super(backingList);
    }
  }

  /**
   * Used to avoid http://bugs.sun.com/view_bug.do?bug_id=6558557
   */
  static <T> List<T> cast(Iterable<T> iterable) {
    return (List<T>) iterable;
  }
}