Java tutorial
/* * 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 static com.google.common.collect.CollectPreconditions.checkNonnegative; import static com.google.common.collect.CollectPreconditions.checkRemove; 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.math.IntMath; import com.google.common.primitives.Ints; import java.io.Serializable; import java.math.RoundingMode; 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.CheckReturnValue; 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= * "https://github.com/google/guava/wiki/CollectionUtilitiesExplained#lists"> * {@code Lists}</a>. * * @author Kevin Bourrillion * @author Mike Bostock * @author Louis Wasserman * @since 2.0 */ @GwtCompatible(emulated = true) public final class Lists { private Lists() { } // ArrayList /** * Creates a <i>mutable</i>, empty {@code ArrayList} instance (for Java 6 and * earlier). * * <p><b>Note:</b> if mutability is not required, use {@link * ImmutableList#of()} instead. * * <p><b>Note for Java 7 and later:</b> this method is now unnecessary and * should be treated as deprecated. Instead, use the {@code ArrayList} * {@linkplain ArrayList#ArrayList() constructor} directly, taking advantage * of the new <a href="http://goo.gl/iz2Wi">"diamond" syntax</a>. */ @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> essentially the only reason to use this method is when you * will need to add or remove elements later. Otherwise, for non-null elements * use {@link ImmutableList#of()} (for varargs) or {@link * ImmutableList#copyOf(Object[])} (for an array) instead. If any elements * might be null, or you need support for {@link List#set(int, Object)}, use * {@link Arrays#asList}. * * <p>Note that even when you do need the ability to add or remove, this method * provides only a tiny bit of syntactic sugar for {@code newArrayList(}{@link * Arrays#asList asList}{@code (...))}, or for creating an empty list then * calling {@link Collections#addAll}. This method is not actually very useful * and will likely be deprecated in the future. */ @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) { checkNonnegative(arraySize, "arraySize"); // 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; a very thin shortcut for creating an empty list then calling * {@link Iterables#addAll}. * * <p><b>Note:</b> if mutability is not required and the elements are * non-null, use {@link ImmutableList#copyOf(Iterable)} instead. (Or, change * {@code elements} to be a {@link FluentIterable} and call * {@code elements.toList()}.) * * <p><b>Note for Java 7 and later:</b> if {@code elements} is a {@link * Collection}, you don't need this method. Use the {@code ArrayList} * {@linkplain ArrayList#ArrayList(Collection) constructor} directly, taking * advantage of the new <a href="http://goo.gl/iz2Wi">"diamond" syntax</a>. */ @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; a very thin shortcut for creating an empty list and then calling * {@link Iterators#addAll}. * * <p><b>Note:</b> if mutability is not required and the elements are * non-null, use {@link ImmutableList#copyOf(Iterator)} instead. */ @GwtCompatible(serializable = true) public static <E> ArrayList<E> newArrayList(Iterator<? extends E> elements) { ArrayList<E> list = newArrayList(); Iterators.addAll(list, elements); return list; } /** * Creates an {@code ArrayList} instance backed by an array with the specified * initial size; simply delegates to {@link ArrayList#ArrayList(int)}. * * <p><b>Note for Java 7 and later:</b> this method is now unnecessary and * should be treated as deprecated. Instead, use {@code new }{@link * ArrayList#ArrayList(int) ArrayList}{@code <>(int)} directly, taking * advantage of the new <a href="http://goo.gl/iz2Wi">"diamond" syntax</a>. * (Unlike here, there is no risk of overload ambiguity, since the {@code * ArrayList} constructors very wisely did not accept varargs.) * * @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) { checkNonnegative(initialArraySize, "initialArraySize"); // for GWT. return new ArrayList<E>(initialArraySize); } /** * Creates an {@code ArrayList} instance to hold {@code estimatedSize} * elements, <i>plus</i> an unspecified amount of padding; you almost * certainly mean to call {@link #newArrayListWithCapacity} (see that method * for further advice on usage). * * <p><b>Note:</b> This method will soon be deprecated. Even in the rare case * that you do want some amount of padding, it's best if you choose your * desired amount explicitly. * * @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 a <i>mutable</i>, empty {@code LinkedList} instance (for Java 6 and * earlier). * * <p><b>Note:</b> if you won't be adding any elements to the list, use {@link * ImmutableList#of()} instead. * * <p><b>Performance note:</b> {@link ArrayList} and {@link * java.util.ArrayDeque} consistently outperform {@code LinkedList} except in * certain rare and specific situations. Unless you have spent a lot of time * benchmarking your specific needs, use one of those instead. * * <p><b>Note for Java 7 and later:</b> this method is now unnecessary and * should be treated as deprecated. Instead, use the {@code LinkedList} * {@linkplain LinkedList#LinkedList() constructor} directly, taking advantage * of the new <a href="http://goo.gl/iz2Wi">"diamond" syntax</a>. */ @GwtCompatible(serializable = true) public static <E> LinkedList<E> newLinkedList() { return new LinkedList<E>(); } /** * Creates a <i>mutable</i> {@code LinkedList} instance containing the given * elements; a very thin shortcut for creating an empty list then calling * {@link Iterables#addAll}. * * <p><b>Note:</b> if mutability is not required and the elements are * non-null, use {@link ImmutableList#copyOf(Iterable)} instead. (Or, change * {@code elements} to be a {@link FluentIterable} and call * {@code elements.toList()}.) * * <p><b>Performance note:</b> {@link ArrayList} and {@link * java.util.ArrayDeque} consistently outperform {@code LinkedList} except in * certain rare and specific situations. Unless you have spent a lot of time * benchmarking your specific needs, use one of those instead. * * <p><b>Note for Java 7 and later:</b> if {@code elements} is a {@link * Collection}, you don't need this method. Use the {@code LinkedList} * {@linkplain LinkedList#LinkedList(Collection) constructor} directly, taking * advantage of the new <a href="http://goo.gl/iz2Wi">"diamond" syntax</a>. */ @GwtCompatible(serializable = true) public static <E> LinkedList<E> newLinkedList(Iterable<? extends E> elements) { LinkedList<E> list = newLinkedList(); Iterables.addAll(list, elements); 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> * * <p>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> * * <p>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> * * <p>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 * @since 19.0 */ public 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> * * <p>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> * * <p>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> * * <p>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 * @since 19.0 */ public 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. */ @CheckReturnValue 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 Iterator<T> iterator() { return listIterator(); } @Override public ListIterator<T> listIterator(int index) { return new TransformedListIterator<F, T>(fromList.listIterator(index)) { @Override T transform(F from) { return function.apply(from); } }; } @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) { checkElementIndex(index, size()); int start = index * size; int end = Math.min(start + size, list.size()); return list.subList(start, end); } @Override public int size() { return IntMath.divide(list.size(), size, RoundingMode.CEILING); } @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(); } } /** * 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 int size() { return sequence.length(); } } /** * 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 */ @CheckReturnValue public static <T> List<T> reverse(List<T> list) { if (list instanceof ImmutableList) { return ((ImmutableList<T>) list).reverse(); } else 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 int size() { return forwardList.size(); } @Override public List<T> subList(int fromIndex, int toIndex) { checkPositionIndexes(fromIndex, toIndex, size()); return reverse(forwardList.subList(reversePosition(toIndex), reversePosition(fromIndex))); } @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 canRemoveOrSet; @Override public void add(T e) { forwardIterator.add(e); forwardIterator.previous(); canRemoveOrSet = false; } @Override public boolean hasNext() { return forwardIterator.hasPrevious(); } @Override public boolean hasPrevious() { return forwardIterator.hasNext(); } @Override public T next() { if (!hasNext()) { throw new NoSuchElementException(); } canRemoveOrSet = true; return forwardIterator.previous(); } @Override public int nextIndex() { return reversePosition(forwardIterator.nextIndex()); } @Override public T previous() { if (!hasPrevious()) { throw new NoSuchElementException(); } canRemoveOrSet = true; return forwardIterator.next(); } @Override public int previousIndex() { return nextIndex() - 1; } @Override public void remove() { checkRemove(canRemoveOrSet); forwardIterator.remove(); canRemoveOrSet = false; } @Override public void set(T e) { checkState(canRemoveOrSet); 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) { // TODO(lowasser): worth optimizing for RandomAccess? 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<?> thisList, @Nullable Object other) { if (other == checkNotNull(thisList)) { return true; } if (!(other instanceof List)) { return false; } List<?> otherList = (List<?>) other; int size = thisList.size(); if (size != otherList.size()) { return false; } if (thisList instanceof RandomAccess && otherList instanceof RandomAccess) { // avoid allocation and use the faster loop for (int i = 0; i < size; i++) { if (!Objects.equal(thisList.get(i), otherList.get(i))) { return false; } } return true; } else { return Iterators.elementsEqual(thisList.iterator(), otherList.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) { if (list instanceof RandomAccess) { return indexOfRandomAccess(list, element); } else { ListIterator<?> listIterator = list.listIterator(); while (listIterator.hasNext()) { if (Objects.equal(element, listIterator.next())) { return listIterator.previousIndex(); } } return -1; } } private static int indexOfRandomAccess(List<?> list, @Nullable Object element) { int size = list.size(); if (element == null) { for (int i = 0; i < size; i++) { if (list.get(i) == null) { return i; } } } else { for (int i = 0; i < size; i++) { if (element.equals(list.get(i))) { return i; } } } return -1; } /** * An implementation of {@link List#lastIndexOf(Object)}. */ static int lastIndexOfImpl(List<?> list, @Nullable Object element) { if (list instanceof RandomAccess) { return lastIndexOfRandomAccess(list, element); } else { ListIterator<?> listIterator = list.listIterator(list.size()); while (listIterator.hasPrevious()) { if (Objects.equal(element, listIterator.previous())) { return listIterator.nextIndex(); } } return -1; } } private static int lastIndexOfRandomAccess(List<?> list, @Nullable Object element) { if (element == null) { for (int i = list.size() - 1; i >= 0; i--) { if (list.get(i) == null) { return i; } } } else { for (int i = list.size() - 1; i >= 0; i--) { if (element.equals(list.get(i))) { return i; } } } 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; } }