Java API中的ArrayList和LinkedList实现

刺骨的言语ヽ痛彻心扉 2022-05-22 09:11 195阅读 0赞

要说数据结构里面最简单的应该就是表了，记得本科上课时候也是先讲表，顺序表和链表，平时写Java程序的时候用的最多的表结构也就是ArrayList和LinkedList了，ArrayList随机访问快（查找快），LinkedList随机插入和随机删除快，看自己的代码什么操作频繁选择相对快一点的表结构，这里就来看一下Java官方API 1.8 里面这两个表示怎样实现的。

import java.util.ArrayList;
    import java.util.LinkedList;

这两个类都在java.util包中，java.util是java集合类所在的包。

先看ArrayList，该类提供了List ADT的一种可增长数组的实现，使用ArrayList的优点在于set和get花费常数时间，缺点是新项的插入和现有项的删除代价昂贵，除非变动是在末端进行。

public class ArrayList<E> extends AbstractList<E>
 implements List<E>, RandomAccess, Cloneable, java.io.Serializable

可以看到ArrayList的基本结构是继承自AbstractList，并实现了List接口，再来看一下AbstractList

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E>

AbstractList继承自AbstracCollection并实现List接口，再看AbstractCollection

public abstract class AbstractCollection<E> implements Collection<E>

AbstractCollection实现了Collection接口

public interface Collection<E> extends Iterable<E>

Collection接口扩展了Iterable接口，Iterable是java.lang包中的接口，实现Iterable接口的类必须提供Iterator()方法，也就是迭代器，进而可以使用增强的for循环遍历。

ArrayList的继承路线就是AbstractCollection→AbstractList→ArrayList

再看LinkedList，该类提供了List ADT 的双链表实现。使用LinkedList的优点在于，新项的插入和现有项的删除均开销很小，缺点是它不容易作索引，对get调用时很昂贵。

public class LinkedList<E>
 extends AbstractSequentialList<E>
 implements List<E>, Deque<E>, Cloneable, java.io.Serializable

public abstract class AbstractSequentialList<E> extends AbstractList<E>

LinkedList的继承路线就是：AbstractCollection→AbstractList→AbstractSequentialList→ArrayList

这里多了个AbstractSequentialList类，从类名也能看出，Sequential是按次序的意思，AbstractSequentialList 只支持迭代器按顺序访问，不支持 RandomAccess，同时也实现了AbstractList 中没有实现或者没有支持的一些操作。

ArrayList 和LinkedList的基本实现原则都是实现Collection和List接口，中间提供了了Abstract类，每一层Abstract类都提供了接口中部分操作的实现，继承这样这样的抽象类相当于简化了接口的实现。

以下是ArrayList和LinkedList源码，这种代码看起来真的舒服，太懒注释木有翻译哈

package java.util;
 
 import java.util.function.Consumer;
 import java.util.function.Predicate;
 import java.util.function.UnaryOperator;
 import sun.misc.SharedSecrets;
 
 /**
 * Resizable-array implementation of the <tt>List</tt> interface. Implements
 * all optional list operations, and permits all elements, including
 * <tt>null</tt>. In addition to implementing the <tt>List</tt> interface,
 * this class provides methods to manipulate the size of the array that is
 * used internally to store the list. (This class is roughly equivalent to
 * <tt>Vector</tt>, except that it is unsynchronized.)
 *
 * The <tt>size</tt>, <tt>isEmpty</tt>, <tt>get</tt>, <tt>set</tt>,
 * <tt>iterator</tt>, and <tt>listIterator</tt> operations run in constant
 * time. The <tt>add</tt> operation runs in amortized constant time,
 * that is, adding n elements requires O(n) time. All of the other operations
 * run in linear time (roughly speaking). The constant factor is low compared
 * to that for the <tt>LinkedList</tt> implementation.
 *
 * Each <tt>ArrayList</tt> instance has a capacity. The capacity is
 * the size of the array used to store the elements in the list. It is always
 * at least as large as the list size. As elements are added to an ArrayList,
 * its capacity grows automatically. The details of the growth policy are not
 * specified beyond the fact that adding an element has constant amortized
 * time cost.
 *
 * An application can increase the capacity of an <tt>ArrayList</tt> instance
 * before adding a large number of elements using the <tt>ensureCapacity</tt>
 * operation. This may reduce the amount of incremental reallocation.
 *
 * Note that this implementation is not synchronized.
 * If multiple threads access an <tt>ArrayList</tt> instance concurrently,
 * and at least one of the threads modifies the list structurally, it
 * must be synchronized externally. (A structural modification is
 * any operation that adds or deletes one or more elements, or explicitly
 * resizes the backing array; merely setting the value of an element is not
 * a structural modification.) This is typically accomplished by
 * synchronizing on some object that naturally encapsulates the list.
 *
 * If no such object exists, the list should be "wrapped" using the
 * {@link Collections#synchronizedList Collections.synchronizedList}
 * method. This is best done at creation time, to prevent accidental
 * unsynchronized access to the list:<pre>
 * List list = Collections.synchronizedList(new ArrayList(...));</pre>
 *
 * <a name="fail-fast">
 * The iterators returned by this class's {@link #iterator() iterator} and
 * {@link #listIterator(int) listIterator} methods are fail-fast:</a>
 * if the list is structurally modified at any time after the iterator is
 * created, in any way except through the iterator's own
 * {@link ListIterator#remove() remove} or
 * {@link ListIterator#add(Object) add} methods, the iterator will throw a
 * {@link ConcurrentModificationException}. Thus, in the face of
 * concurrent modification, the iterator fails quickly and cleanly, rather
 * than risking arbitrary, non-deterministic behavior at an undetermined
 * time in the future.
 *
 * Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification. Fail-fast iterators
 * throw {@code ConcurrentModificationException} on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness: the fail-fast behavior of iterators
 * should be used only to detect bugs.
 *
 * This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author Josh Bloch
 * @author Neal Gafter
 * @see Collection
 * @see List
 * @see LinkedList
 * @see Vector
 * @since 1.2
 */
 
 public class ArrayList<E> extends AbstractList<E>
 implements List<E>, RandomAccess, Cloneable, java.io.Serializable
 {
 private static final long serialVersionUID = 8683452581122892189L;
 
 /**
 * Default initial capacity.
 */
 private static final int DEFAULT_CAPACITY = 10;
 
 /**
 * Shared empty array instance used for empty instances.
 */
 private static final Object[] EMPTY_ELEMENTDATA = {};
 
 /**
 * Shared empty array instance used for default sized empty instances. We
 * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
 * first element is added.
 */
 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
 
 /**
 * The array buffer into which the elements of the ArrayList are stored.
 * The capacity of the ArrayList is the length of this array buffer. Any
 * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
 * will be expanded to DEFAULT_CAPACITY when the first element is added.
 */
 transient Object[] elementData; // non-private to simplify nested class access
 
 /**
 * The size of the ArrayList (the number of elements it contains).
 *
 * @serial
 */
 private int size;
 
 /**
 * Constructs an empty list with the specified initial capacity.
 *
 * @param initialCapacity the initial capacity of the list
 * @throws IllegalArgumentException if the specified initial capacity
 * is negative
 */
 public ArrayList(int initialCapacity) {
 if (initialCapacity > 0) {
 this.elementData = new Object[initialCapacity];
 } else if (initialCapacity == 0) {
 this.elementData = EMPTY_ELEMENTDATA;
 } else {
 throw new IllegalArgumentException("Illegal Capacity: "+
 initialCapacity);
 }
 }
 
 /**
 * Constructs an empty list with an initial capacity of ten.
 */
 public ArrayList() {
 this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
 }
 
 /**
 * Constructs a list containing the elements of the specified
 * collection, in the order they are returned by the collection's
 * iterator.
 *
 * @param c the collection whose elements are to be placed into this list
 * @throws NullPointerException if the specified collection is null
 */
 public ArrayList(Collection<? extends E> c) {
 elementData = c.toArray();
 if ((size = elementData.length) != 0) {
 // c.toArray might (incorrectly) not return Object[] (see 6260652)
 if (elementData.getClass() != Object[].class)
 elementData = Arrays.copyOf(elementData, size, Object[].class);
 } else {
 // replace with empty array.
 this.elementData = EMPTY_ELEMENTDATA;
 }
 }
 
 /**
 * Trims the capacity of this <tt>ArrayList</tt> instance to be the
 * list's current size. An application can use this operation to minimize
 * the storage of an <tt>ArrayList</tt> instance.
 */
 public void trimToSize() {
 modCount++;
 if (size < elementData.length) {
 elementData = (size == 0)
 ? EMPTY_ELEMENTDATA
 : Arrays.copyOf(elementData, size);
 }
 }
 
 /**
 * Increases the capacity of this <tt>ArrayList</tt> instance, if
 * necessary, to ensure that it can hold at least the number of elements
 * specified by the minimum capacity argument.
 *
 * @param minCapacity the desired minimum capacity
 */
 public void ensureCapacity(int minCapacity) {
 int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
 // any size if not default element table
 ? 0
 // larger than default for default empty table. It's already
 // supposed to be at default size.
 : DEFAULT_CAPACITY;
 
 if (minCapacity > minExpand) {
 ensureExplicitCapacity(minCapacity);
 }
 }
 
 private static int calculateCapacity(Object[] elementData, int minCapacity) {
 if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
 return Math.max(DEFAULT_CAPACITY, minCapacity);
 }
 return minCapacity;
 }
 
 private void ensureCapacityInternal(int minCapacity) {
 ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
 }
 
 private void ensureExplicitCapacity(int minCapacity) {
 modCount++;
 
 // overflow-conscious code
 if (minCapacity - elementData.length > 0)
 grow(minCapacity);
 }
 
 /**
 * The maximum size of array to allocate.
 * Some VMs reserve some header words in an array.
 * Attempts to allocate larger arrays may result in
 * OutOfMemoryError: Requested array size exceeds VM limit
 */
 private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
 
 /**
 * Increases the capacity to ensure that it can hold at least the
 * number of elements specified by the minimum capacity argument.
 *
 * @param minCapacity the desired minimum capacity
 */
 private void grow(int minCapacity) {
 // overflow-conscious code
 int oldCapacity = elementData.length;
 int newCapacity = oldCapacity + (oldCapacity >> 1);
 if (newCapacity - minCapacity < 0)
 newCapacity = minCapacity;
 if (newCapacity - MAX_ARRAY_SIZE > 0)
 newCapacity = hugeCapacity(minCapacity);
 // minCapacity is usually close to size, so this is a win:
 elementData = Arrays.copyOf(elementData, newCapacity);
 }
 
 private static int hugeCapacity(int minCapacity) {
 if (minCapacity < 0) // overflow
 throw new OutOfMemoryError();
 return (minCapacity > MAX_ARRAY_SIZE) ?
 Integer.MAX_VALUE :
 MAX_ARRAY_SIZE;
 }
 
 /**
 * Returns the number of elements in this list.
 *
 * @return the number of elements in this list
 */
 public int size() {
 return size;
 }
 
 /**
 * Returns <tt>true</tt> if this list contains no elements.
 *
 * @return <tt>true</tt> if this list contains no elements
 */
 public boolean isEmpty() {
 return size == 0;
 }
 
 /**
 * Returns <tt>true</tt> if this list contains the specified element.
 * More formally, returns <tt>true</tt> if and only if this list contains
 * at least one element <tt>e</tt> such that
 * <tt>(o==null ? e==null : o.equals(e))</tt>.
 *
 * @param o element whose presence in this list is to be tested
 * @return <tt>true</tt> if this list contains the specified element
 */
 public boolean contains(Object o) {
 return indexOf(o) >= 0;
 }
 
 /**
 * Returns the index of the first occurrence of the specified element
 * in this list, or -1 if this list does not contain the element.
 * More formally, returns the lowest index <tt>i</tt> such that
 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
 * or -1 if there is no such index.
 */
 public int indexOf(Object o) {
 if (o == null) {
 for (int i = 0; i < size; i++)
 if (elementData[i]==null)
 return i;
 } else {
 for (int i = 0; i < size; i++)
 if (o.equals(elementData[i]))
 return i;
 }
 return -1;
 }
 
 /**
 * Returns the index of the last occurrence of the specified element
 * in this list, or -1 if this list does not contain the element.
 * More formally, returns the highest index <tt>i</tt> such that
 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
 * or -1 if there is no such index.
 */
 public int lastIndexOf(Object o) {
 if (o == null) {
 for (int i = size-1; i >= 0; i--)
 if (elementData[i]==null)
 return i;
 } else {
 for (int i = size-1; i >= 0; i--)
 if (o.equals(elementData[i]))
 return i;
 }
 return -1;
 }
 
 /**
 * Returns a shallow copy of this <tt>ArrayList</tt> instance. (The
 * elements themselves are not copied.)
 *
 * @return a clone of this <tt>ArrayList</tt> instance
 */
 public Object clone() {
 try {
 ArrayList<?> v = (ArrayList<?>) super.clone();
 v.elementData = Arrays.copyOf(elementData, size);
 v.modCount = 0;
 return v;
 } catch (CloneNotSupportedException e) {
 // this shouldn't happen, since we are Cloneable
 throw new InternalError(e);
 }
 }
 
 /**
 * Returns an array containing all of the elements in this list
 * in proper sequence (from first to last element).
 *
 * The returned array will be "safe" in that no references to it are
 * maintained by this list. (In other words, this method must allocate
 * a new array). The caller is thus free to modify the returned array.
 *
 * This method acts as bridge between array-based and collection-based
 * APIs.
 *
 * @return an array containing all of the elements in this list in
 * proper sequence
 */
 public Object[] toArray() {
 return Arrays.copyOf(elementData, size);
 }
 
 /**
 * Returns an array containing all of the elements in this list in proper
 * sequence (from first to last element); the runtime type of the returned
 * array is that of the specified array. If the list fits in the
 * specified array, it is returned therein. Otherwise, a new array is
 * allocated with the runtime type of the specified array and the size of
 * this list.
 *
 * If the list fits in the specified array with room to spare
 * (i.e., the array has more elements than the list), the element in
 * the array immediately following the end of the collection is set to
 * <tt>null</tt>. (This is useful in determining the length of the
 * list only if the caller knows that the list does not contain
 * any null elements.)
 *
 * @param a the array into which the elements of the list are to
 * be stored, if it is big enough; otherwise, a new array of the
 * same runtime type is allocated for this purpose.
 * @return an array containing the elements of the list
 * @throws ArrayStoreException if the runtime type of the specified array
 * is not a supertype of the runtime type of every element in
 * this list
 * @throws NullPointerException if the specified array is null
 */
 @SuppressWarnings("unchecked")
 public <T> T[] toArray(T[] a) {
 if (a.length < size)
 // Make a new array of a's runtime type, but my contents:
 return (T[]) Arrays.copyOf(elementData, size, a.getClass());
 System.arraycopy(elementData, 0, a, 0, size);
 if (a.length > size)
 a[size] = null;
 return a;
 }
 
 // Positional Access Operations
 
 @SuppressWarnings("unchecked")
 E elementData(int index) {
 return (E) elementData[index];
 }
 
 /**
 * Returns the element at the specified position in this list.
 *
 * @param index index of the element to return
 * @return the element at the specified position in this list
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
 public E get(int index) {
 rangeCheck(index);
 
 return elementData(index);
 }
 
 /**
 * Replaces the element at the specified position in this list with
 * the specified element.
 *
 * @param index index of the element to replace
 * @param element element to be stored at the specified position
 * @return the element previously at the specified position
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
 public E set(int index, E element) {
 rangeCheck(index);
 
 E oldValue = elementData(index);
 elementData[index] = element;
 return oldValue;
 }
 
 /**
 * Appends the specified element to the end of this list.
 *
 * @param e element to be appended to this list
 * @return <tt>true</tt> (as specified by {@link Collection#add})
 */
 public boolean add(E e) {
 ensureCapacityInternal(size + 1); // Increments modCount!!
 elementData[size++] = e;
 return true;
 }
 
 /**
 * Inserts the specified element at the specified position in this
 * list. Shifts the element currently at that position (if any) and
 * any subsequent elements to the right (adds one to their indices).
 *
 * @param index index at which the specified element is to be inserted
 * @param element element to be inserted
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
 public void add(int index, E element) {
 rangeCheckForAdd(index);
 
 ensureCapacityInternal(size + 1); // Increments modCount!!
 System.arraycopy(elementData, index, elementData, index + 1,
 size - index);
 elementData[index] = element;
 size++;
 }
 
 /**
 * Removes the element at the specified position in this list.
 * Shifts any subsequent elements to the left (subtracts one from their
 * indices).
 *
 * @param index the index of the element to be removed
 * @return the element that was removed from the list
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
 public E remove(int index) {
 rangeCheck(index);
 
 modCount++;
 E oldValue = elementData(index);
 
 int numMoved = size - index - 1;
 if (numMoved > 0)
 System.arraycopy(elementData, index+1, elementData, index,
 numMoved);
 elementData[--size] = null; // clear to let GC do its work
 
 return oldValue;
 }
 
 /**
 * Removes the first occurrence of the specified element from this list,
 * if it is present. If the list does not contain the element, it is
 * unchanged. More formally, removes the element with the lowest index
 * <tt>i</tt> such that
 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
 * (if such an element exists). Returns <tt>true</tt> if this list
 * contained the specified element (or equivalently, if this list
 * changed as a result of the call).
 *
 * @param o element to be removed from this list, if present
 * @return <tt>true</tt> if this list contained the specified element
 */
 public boolean remove(Object o) {
 if (o == null) {
 for (int index = 0; index < size; index++)
 if (elementData[index] == null) {
 fastRemove(index);
 return true;
 }
 } else {
 for (int index = 0; index < size; index++)
 if (o.equals(elementData[index])) {
 fastRemove(index);
 return true;
 }
 }
 return false;
 }
 
 /*
 * Private remove method that skips bounds checking and does not
 * return the value removed.
 */
 private void fastRemove(int index) {
 modCount++;
 int numMoved = size - index - 1;
 if (numMoved > 0)
 System.arraycopy(elementData, index+1, elementData, index,
 numMoved);
 elementData[--size] = null; // clear to let GC do its work
 }
 
 /**
 * Removes all of the elements from this list. The list will
 * be empty after this call returns.
 */
 public void clear() {
 modCount++;
 
 // clear to let GC do its work
 for (int i = 0; i < size; i++)
 elementData[i] = null;
 
 size = 0;
 }
 
 /**
 * Appends all of the elements in the specified collection to the end of
 * this list, in the order that they are returned by the
 * specified collection's Iterator. The behavior of this operation is
 * undefined if the specified collection is modified while the operation
 * is in progress. (This implies that the behavior of this call is
 * undefined if the specified collection is this list, and this
 * list is nonempty.)
 *
 * @param c collection containing elements to be added to this list
 * @return <tt>true</tt> if this list changed as a result of the call
 * @throws NullPointerException if the specified collection is null
 */
 public boolean addAll(Collection<? extends E> c) {
 Object[] a = c.toArray();
 int numNew = a.length;
 ensureCapacityInternal(size + numNew); // Increments modCount
 System.arraycopy(a, 0, elementData, size, numNew);
 size += numNew;
 return numNew != 0;
 }
 
 /**
 * Inserts all of the elements in the specified collection into this
 * list, starting at the specified position. Shifts the element
 * currently at that position (if any) and any subsequent elements to
 * the right (increases their indices). The new elements will appear
 * in the list in the order that they are returned by the
 * specified collection's iterator.
 *
 * @param index index at which to insert the first element from the
 * specified collection
 * @param c collection containing elements to be added to this list
 * @return <tt>true</tt> if this list changed as a result of the call
 * @throws IndexOutOfBoundsException {@inheritDoc}
 * @throws NullPointerException if the specified collection is null
 */
 public boolean addAll(int index, Collection<? extends E> c) {
 rangeCheckForAdd(index);
 
 Object[] a = c.toArray();
 int numNew = a.length;
 ensureCapacityInternal(size + numNew); // Increments modCount
 
 int numMoved = size - index;
 if (numMoved > 0)
 System.arraycopy(elementData, index, elementData, index + numNew,
 numMoved);
 
 System.arraycopy(a, 0, elementData, index, numNew);
 size += numNew;
 return numNew != 0;
 }
 
 /**
 * Removes from this list all of the elements whose index is between
 * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.
 * Shifts any succeeding elements to the left (reduces their index).
 * This call shortens the list by {@code (toIndex - fromIndex)} elements.
 * (If {@code toIndex==fromIndex}, this operation has no effect.)
 *
 * @throws IndexOutOfBoundsException if {@code fromIndex} or
 * {@code toIndex} is out of range
 * ({@code fromIndex < 0 ||
 * fromIndex >= size() ||
 * toIndex > size() ||
 * toIndex < fromIndex})
 */
 protected void removeRange(int fromIndex, int toIndex) {
 modCount++;
 int numMoved = size - toIndex;
 System.arraycopy(elementData, toIndex, elementData, fromIndex,
 numMoved);
 
 // clear to let GC do its work
 int newSize = size - (toIndex-fromIndex);
 for (int i = newSize; i < size; i++) {
 elementData[i] = null;
 }
 size = newSize;
 }
 
 /**
 * Checks if the given index is in range. If not, throws an appropriate
 * runtime exception. This method does *not* check if the index is
 * negative: It is always used immediately prior to an array access,
 * which throws an ArrayIndexOutOfBoundsException if index is negative.
 */
 private void rangeCheck(int index) {
 if (index >= size)
 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
 }
 
 /**
 * A version of rangeCheck used by add and addAll.
 */
 private void rangeCheckForAdd(int index) {
 if (index > size || index < 0)
 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
 }
 
 /**
 * Constructs an IndexOutOfBoundsException detail message.
 * Of the many possible refactorings of the error handling code,
 * this "outlining" performs best with both server and client VMs.
 */
 private String outOfBoundsMsg(int index) {
 return "Index: "+index+", Size: "+size;
 }
 
 /**
 * Removes from this list all of its elements that are contained in the
 * specified collection.
 *
 * @param c collection containing elements to be removed from this list
 * @return {@code true} if this list changed as a result of the call
 * @throws ClassCastException if the class of an element of this list
 * is incompatible with the specified collection
 * (<a href="Collection.html#optional-restrictions">optional</a>)
 * @throws NullPointerException if this list contains a null element and the
 * specified collection does not permit null elements
 * (<a href="Collection.html#optional-restrictions">optional</a>),
 * or if the specified collection is null
 * @see Collection#contains(Object)
 */
 public boolean removeAll(Collection<?> c) {
 Objects.requireNonNull(c);
 return batchRemove(c, false);
 }
 
 /**
 * Retains only the elements in this list that are contained in the
 * specified collection. In other words, removes from this list all
 * of its elements that are not contained in the specified collection.
 *
 * @param c collection containing elements to be retained in this list
 * @return {@code true} if this list changed as a result of the call
 * @throws ClassCastException if the class of an element of this list
 * is incompatible with the specified collection
 * (<a href="Collection.html#optional-restrictions">optional</a>)
 * @throws NullPointerException if this list contains a null element and the
 * specified collection does not permit null elements
 * (<a href="Collection.html#optional-restrictions">optional</a>),
 * or if the specified collection is null
 * @see Collection#contains(Object)
 */
 public boolean retainAll(Collection<?> c) {
 Objects.requireNonNull(c);
 return batchRemove(c, true);
 }
 
 private boolean batchRemove(Collection<?> c, boolean complement) {
 final Object[] elementData = this.elementData;
 int r = 0, w = 0;
 boolean modified = false;
 try {
 for (; r < size; r++)
 if (c.contains(elementData[r]) == complement)
 elementData[w++] = elementData[r];
 } finally {
 // Preserve behavioral compatibility with AbstractCollection,
 // even if c.contains() throws.
 if (r != size) {
 System.arraycopy(elementData, r,
 elementData, w,
 size - r);
 w += size - r;
 }
 if (w != size) {
 // clear to let GC do its work
 for (int i = w; i < size; i++)
 elementData[i] = null;
 modCount += size - w;
 size = w;
 modified = true;
 }
 }
 return modified;
 }
 
 /**
 * Save the state of the <tt>ArrayList</tt> instance to a stream (that
 * is, serialize it).
 *
 * @serialData The length of the array backing the <tt>ArrayList</tt>
 * instance is emitted (int), followed by all of its elements
 * (each an <tt>Object</tt>) in the proper order.
 */
 private void writeObject(java.io.ObjectOutputStream s)
 throws java.io.IOException{
 // Write out element count, and any hidden stuff
 int expectedModCount = modCount;
 s.defaultWriteObject();
 
 // Write out size as capacity for behavioural compatibility with clone()
 s.writeInt(size);
 
 // Write out all elements in the proper order.
 for (int i=0; i<size; i++) {
 s.writeObject(elementData[i]);
 }
 
 if (modCount != expectedModCount) {
 throw new ConcurrentModificationException();
 }
 }
 
 /**
 * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
 * deserialize it).
 */
 private void readObject(java.io.ObjectInputStream s)
 throws java.io.IOException, ClassNotFoundException {
 elementData = EMPTY_ELEMENTDATA;
 
 // Read in size, and any hidden stuff
 s.defaultReadObject();
 
 // Read in capacity
 s.readInt(); // ignored
 
 if (size > 0) {
 // be like clone(), allocate array based upon size not capacity
 int capacity = calculateCapacity(elementData, size);
 SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, capacity);
 ensureCapacityInternal(size);
 
 Object[] a = elementData;
 // Read in all elements in the proper order.
 for (int i=0; i<size; i++) {
 a[i] = s.readObject();
 }
 }
 }
 
 /**
 * Returns a list iterator over the elements in this list (in proper
 * sequence), starting at the specified position in the list.
 * The specified index indicates the first element that would be
 * returned by an initial call to {@link ListIterator#next next}.
 * An initial call to {@link ListIterator#previous previous} would
 * return the element with the specified index minus one.
 *
 * The returned list iterator is <a href="#fail-fast">fail-fast</a>.
 *
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
 public ListIterator<E> listIterator(int index) {
 if (index < 0 || index > size)
 throw new IndexOutOfBoundsException("Index: "+index);
 return new ListItr(index);
 }
 
 /**
 * Returns a list iterator over the elements in this list (in proper
 * sequence).
 *
 * The returned list iterator is <a href="#fail-fast">fail-fast</a>.
 *
 * @see #listIterator(int)
 */
 public ListIterator<E> listIterator() {
 return new ListItr(0);
 }
 
 /**
 * Returns an iterator over the elements in this list in proper sequence.
 *
 * The returned iterator is <a href="#fail-fast">fail-fast</a>.
 *
 * @return an iterator over the elements in this list in proper sequence
 */
 public Iterator<E> iterator() {
 return new Itr();
 }
 
 /**
 * An optimized version of AbstractList.Itr
 */
 private class Itr implements Iterator<E> {
 int cursor; // index of next element to return
 int lastRet = -1; // index of last element returned; -1 if no such
 int expectedModCount = modCount;
 
 Itr() {}
 
 public boolean hasNext() {
 return cursor != size;
 }
 
 @SuppressWarnings("unchecked")
 public E next() {
 checkForComodification();
 int i = cursor;
 if (i >= size)
 throw new NoSuchElementException();
 Object[] elementData = ArrayList.this.elementData;
 if (i >= elementData.length)
 throw new ConcurrentModificationException();
 cursor = i + 1;
 return (E) elementData[lastRet = i];
 }
 
 public void remove() {
 if (lastRet < 0)
 throw new IllegalStateException();
 checkForComodification();
 
 try {
 ArrayList.this.remove(lastRet);
 cursor = lastRet;
 lastRet = -1;
 expectedModCount = modCount;
 } catch (IndexOutOfBoundsException ex) {
 throw new ConcurrentModificationException();
 }
 }
 
 @Override
 @SuppressWarnings("unchecked")
 public void forEachRemaining(Consumer<? super E> consumer) {
 Objects.requireNonNull(consumer);
 final int size = ArrayList.this.size;
 int i = cursor;
 if (i >= size) {
 return;
 }
 final Object[] elementData = ArrayList.this.elementData;
 if (i >= elementData.length) {
 throw new ConcurrentModificationException();
 }
 while (i != size && modCount == expectedModCount) {
 consumer.accept((E) elementData[i++]);
 }
 // update once at end of iteration to reduce heap write traffic
 cursor = i;
 lastRet = i - 1;
 checkForComodification();
 }
 
 final void checkForComodification() {
 if (modCount != expectedModCount)
 throw new ConcurrentModificationException();
 }
 }
 
 /**
 * An optimized version of AbstractList.ListItr
 */
 private class ListItr extends Itr implements ListIterator<E> {
 ListItr(int index) {
 super();
 cursor = index;
 }
 
 public boolean hasPrevious() {
 return cursor != 0;
 }
 
 public int nextIndex() {
 return cursor;
 }
 
 public int previousIndex() {
 return cursor - 1;
 }
 
 @SuppressWarnings("unchecked")
 public E previous() {
 checkForComodification();
 int i = cursor - 1;
 if (i < 0)
 throw new NoSuchElementException();
 Object[] elementData = ArrayList.this.elementData;
 if (i >= elementData.length)
 throw new ConcurrentModificationException();
 cursor = i;
 return (E) elementData[lastRet = i];
 }
 
 public void set(E e) {
 if (lastRet < 0)
 throw new IllegalStateException();
 checkForComodification();
 
 try {
 ArrayList.this.set(lastRet, e);
 } catch (IndexOutOfBoundsException ex) {
 throw new ConcurrentModificationException();
 }
 }
 
 public void add(E e) {
 checkForComodification();
 
 try {
 int i = cursor;
 ArrayList.this.add(i, e);
 cursor = i + 1;
 lastRet = -1;
 expectedModCount = modCount;
 } catch (IndexOutOfBoundsException ex) {
 throw new ConcurrentModificationException();
 }
 }
 }
 
 /**
 * Returns a view of the portion of this list between the specified
 * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive. (If
 * {@code fromIndex} and {@code toIndex} are equal, the returned list is
 * empty.) The returned list is backed by this list, so non-structural
 * changes in the returned list are reflected in this list, and vice-versa.
 * The returned list supports all of the optional list operations.
 *
 * This method eliminates the need for explicit range operations (of
 * the sort that commonly exist for arrays). Any operation that expects
 * a list can be used as a range operation by passing a subList view
 * instead of a whole list. For example, the following idiom
 * removes a range of elements from a list:
 * <pre>
 * list.subList(from, to).clear();
 * </pre>
 * Similar idioms may be constructed for {@link #indexOf(Object)} and
 * {@link #lastIndexOf(Object)}, and all of the algorithms in the
 * {@link Collections} class can be applied to a subList.
 *
 * The semantics of the list returned by this method become undefined if
 * the backing list (i.e., this list) is structurally modified in
 * any way other than via the returned list. (Structural modifications are
 * those that change the size of this list, or otherwise perturb it in such
 * a fashion that iterations in progress may yield incorrect results.)
 *
 * @throws IndexOutOfBoundsException {@inheritDoc}
 * @throws IllegalArgumentException {@inheritDoc}
 */
 public List<E> subList(int fromIndex, int toIndex) {
 subListRangeCheck(fromIndex, toIndex, size);
 return new SubList(this, 0, fromIndex, toIndex);
 }
 
 static void subListRangeCheck(int fromIndex, int toIndex, int size) {
 if (fromIndex < 0)
 throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
 if (toIndex > size)
 throw new IndexOutOfBoundsException("toIndex = " + toIndex);
 if (fromIndex > toIndex)
 throw new IllegalArgumentException("fromIndex(" + fromIndex +
 ") > toIndex(" + toIndex + ")");
 }
 
 private class SubList extends AbstractList<E> implements RandomAccess {
 private final AbstractList<E> parent;
 private final int parentOffset;
 private final int offset;
 int size;
 
 SubList(AbstractList<E> parent,
 int offset, int fromIndex, int toIndex) {
 this.parent = parent;
 this.parentOffset = fromIndex;
 this.offset = offset + fromIndex;
 this.size = toIndex - fromIndex;
 this.modCount = ArrayList.this.modCount;
 }
 
 public E set(int index, E e) {
 rangeCheck(index);
 checkForComodification();
 E oldValue = ArrayList.this.elementData(offset + index);
 ArrayList.this.elementData[offset + index] = e;
 return oldValue;
 }
 
 public E get(int index) {
 rangeCheck(index);
 checkForComodification();
 return ArrayList.this.elementData(offset + index);
 }
 
 public int size() {
 checkForComodification();
 return this.size;
 }
 
 public void add(int index, E e) {
 rangeCheckForAdd(index);
 checkForComodification();
 parent.add(parentOffset + index, e);
 this.modCount = parent.modCount;
 this.size++;
 }
 
 public E remove(int index) {
 rangeCheck(index);
 checkForComodification();
 E result = parent.remove(parentOffset + index);
 this.modCount = parent.modCount;
 this.size--;
 return result;
 }
 
 protected void removeRange(int fromIndex, int toIndex) {
 checkForComodification();
 parent.removeRange(parentOffset + fromIndex,
 parentOffset + toIndex);
 this.modCount = parent.modCount;
 this.size -= toIndex - fromIndex;
 }
 
 public boolean addAll(Collection<? extends E> c) {
 return addAll(this.size, c);
 }
 
 public boolean addAll(int index, Collection<? extends E> c) {
 rangeCheckForAdd(index);
 int cSize = c.size();
 if (cSize==0)
 return false;
 
 checkForComodification();
 parent.addAll(parentOffset + index, c);
 this.modCount = parent.modCount;
 this.size += cSize;
 return true;
 }
 
 public Iterator<E> iterator() {
 return listIterator();
 }
 
 public ListIterator<E> listIterator(final int index) {
 checkForComodification();
 rangeCheckForAdd(index);
 final int offset = this.offset;
 
 return new ListIterator<E>() {
 int cursor = index;
 int lastRet = -1;
 int expectedModCount = ArrayList.this.modCount;
 
 public boolean hasNext() {
 return cursor != SubList.this.size;
 }
 
 @SuppressWarnings("unchecked")
 public E next() {
 checkForComodification();
 int i = cursor;
 if (i >= SubList.this.size)
 throw new NoSuchElementException();
 Object[] elementData = ArrayList.this.elementData;
 if (offset + i >= elementData.length)
 throw new ConcurrentModificationException();
 cursor = i + 1;
 return (E) elementData[offset + (lastRet = i)];
 }
 
 public boolean hasPrevious() {
 return cursor != 0;
 }
 
 @SuppressWarnings("unchecked")
 public E previous() {
 checkForComodification();
 int i = cursor - 1;
 if (i < 0)
 throw new NoSuchElementException();
 Object[] elementData = ArrayList.this.elementData;
 if (offset + i >= elementData.length)
 throw new ConcurrentModificationException();
 cursor = i;
 return (E) elementData[offset + (lastRet = i)];
 }
 
 @SuppressWarnings("unchecked")
 public void forEachRemaining(Consumer<? super E> consumer) {
 Objects.requireNonNull(consumer);
 final int size = SubList.this.size;
 int i = cursor;
 if (i >= size) {
 return;
 }
 final Object[] elementData = ArrayList.this.elementData;
 if (offset + i >= elementData.length) {
 throw new ConcurrentModificationException();
 }
 while (i != size && modCount == expectedModCount) {
 consumer.accept((E) elementData[offset + (i++)]);
 }
 // update once at end of iteration to reduce heap write traffic
 lastRet = cursor = i;
 checkForComodification();
 }
 
 public int nextIndex() {
 return cursor;
 }
 
 public int previousIndex() {
 return cursor - 1;
 }
 
 public void remove() {
 if (lastRet < 0)
 throw new IllegalStateException();
 checkForComodification();
 
 try {
 SubList.this.remove(lastRet);
 cursor = lastRet;
 lastRet = -1;
 expectedModCount = ArrayList.this.modCount;
 } catch (IndexOutOfBoundsException ex) {
 throw new ConcurrentModificationException();
 }
 }
 
 public void set(E e) {
 if (lastRet < 0)
 throw new IllegalStateException();
 checkForComodification();
 
 try {
 ArrayList.this.set(offset + lastRet, e);
 } catch (IndexOutOfBoundsException ex) {
 throw new ConcurrentModificationException();
 }
 }
 
 public void add(E e) {
 checkForComodification();
 
 try {
 int i = cursor;
 SubList.this.add(i, e);
 cursor = i + 1;
 lastRet = -1;
 expectedModCount = ArrayList.this.modCount;
 } catch (IndexOutOfBoundsException ex) {
 throw new ConcurrentModificationException();
 }
 }
 
 final void checkForComodification() {
 if (expectedModCount != ArrayList.this.modCount)
 throw new ConcurrentModificationException();
 }
 };
 }
 
 public List<E> subList(int fromIndex, int toIndex) {
 subListRangeCheck(fromIndex, toIndex, size);
 return new SubList(this, offset, fromIndex, toIndex);
 }
 
 private void rangeCheck(int index) {
 if (index < 0 || index >= this.size)
 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
 }
 
 private void rangeCheckForAdd(int index) {
 if (index < 0 || index > this.size)
 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
 }
 
 private String outOfBoundsMsg(int index) {
 return "Index: "+index+", Size: "+this.size;
 }
 
 private void checkForComodification() {
 if (ArrayList.this.modCount != this.modCount)
 throw new ConcurrentModificationException();
 }
 
 public Spliterator<E> spliterator() {
 checkForComodification();
 return new ArrayListSpliterator<E>(ArrayList.this, offset,
 offset + this.size, this.modCount);
 }
 }
 
 @Override
 public void forEach(Consumer<? super E> action) {
 Objects.requireNonNull(action);
 final int expectedModCount = modCount;
 @SuppressWarnings("unchecked")
 final E[] elementData = (E[]) this.elementData;
 final int size = this.size;
 for (int i=0; modCount == expectedModCount && i < size; i++) {
 action.accept(elementData[i]);
 }
 if (modCount != expectedModCount) {
 throw new ConcurrentModificationException();
 }
 }
 
 /**
 * Creates a <a href="Spliterator.html#binding">late-binding</a>
 * and fail-fast {@link Spliterator} over the elements in this
 * list.
 *
 * The {@code Spliterator} reports {@link Spliterator#SIZED},
 * {@link Spliterator#SUBSIZED}, and {@link Spliterator#ORDERED}.
 * Overriding implementations should document the reporting of additional
 * characteristic values.
 *
 * @return a {@code Spliterator} over the elements in this list
 * @since 1.8
 */
 @Override
 public Spliterator<E> spliterator() {
 return new ArrayListSpliterator<>(this, 0, -1, 0);
 }
 
 /** Index-based split-by-two, lazily initialized Spliterator */
 static final class ArrayListSpliterator<E> implements Spliterator<E> {
 
 /*
 * If ArrayLists were immutable, or structurally immutable (no
 * adds, removes, etc), we could implement their spliterators
 * with Arrays.spliterator. Instead we detect as much
 * interference during traversal as practical without
 * sacrificing much performance. We rely primarily on
 * modCounts. These are not guaranteed to detect concurrency
 * violations, and are sometimes overly conservative about
 * within-thread interference, but detect enough problems to
 * be worthwhile in practice. To carry this out, we (1) lazily
 * initialize fence and expectedModCount until the latest
 * point that we need to commit to the state we are checking
 * against; thus improving precision. (This doesn't apply to
 * SubLists, that create spliterators with current non-lazy
 * values). (2) We perform only a single
 * ConcurrentModificationException check at the end of forEach
 * (the most performance-sensitive method). When using forEach
 * (as opposed to iterators), we can normally only detect
 * interference after actions, not before. Further
 * CME-triggering checks apply to all other possible
 * violations of assumptions for example null or too-small
 * elementData array given its size(), that could only have
 * occurred due to interference. This allows the inner loop
 * of forEach to run without any further checks, and
 * simplifies lambda-resolution. While this does entail a
 * number of checks, note that in the common case of
 * list.stream().forEach(a), no checks or other computation
 * occur anywhere other than inside forEach itself. The other
 * less-often-used methods cannot take advantage of most of
 * these streamlinings.
 */
 
 private final ArrayList<E> list;
 private int index; // current index, modified on advance/split
 private int fence; // -1 until used; then one past last index
 private int expectedModCount; // initialized when fence set
 
 /** Create new spliterator covering the given range */
 ArrayListSpliterator(ArrayList<E> list, int origin, int fence,
 int expectedModCount) {
 this.list = list; // OK if null unless traversed
 this.index = origin;
 this.fence = fence;
 this.expectedModCount = expectedModCount;
 }
 
 private int getFence() { // initialize fence to size on first use
 int hi; // (a specialized variant appears in method forEach)
 ArrayList<E> lst;
 if ((hi = fence) < 0) {
 if ((lst = list) == null)
 hi = fence = 0;
 else {
 expectedModCount = lst.modCount;
 hi = fence = lst.size;
 }
 }
 return hi;
 }
 
 public ArrayListSpliterator<E> trySplit() {
 int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
 return (lo >= mid) ? null : // divide range in half unless too small
 new ArrayListSpliterator<E>(list, lo, index = mid,
 expectedModCount);
 }
 
 public boolean tryAdvance(Consumer<? super E> action) {
 if (action == null)
 throw new NullPointerException();
 int hi = getFence(), i = index;
 if (i < hi) {
 index = i + 1;
 @SuppressWarnings("unchecked") E e = (E)list.elementData[i];
 action.accept(e);
 if (list.modCount != expectedModCount)
 throw new ConcurrentModificationException();
 return true;
 }
 return false;
 }
 
 public void forEachRemaining(Consumer<? super E> action) {
 int i, hi, mc; // hoist accesses and checks from loop
 ArrayList<E> lst; Object[] a;
 if (action == null)
 throw new NullPointerException();
 if ((lst = list) != null && (a = lst.elementData) != null) {
 if ((hi = fence) < 0) {
 mc = lst.modCount;
 hi = lst.size;
 }
 else
 mc = expectedModCount;
 if ((i = index) >= 0 && (index = hi) <= a.length) {
 for (; i < hi; ++i) {
 @SuppressWarnings("unchecked") E e = (E) a[i];
 action.accept(e);
 }
 if (lst.modCount == mc)
 return;
 }
 }
 throw new ConcurrentModificationException();
 }
 
 public long estimateSize() {
 return (long) (getFence() - index);
 }
 
 public int characteristics() {
 return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
 }
 }
 
 @Override
 public boolean removeIf(Predicate<? super E> filter) {
 Objects.requireNonNull(filter);
 // figure out which elements are to be removed
 // any exception thrown from the filter predicate at this stage
 // will leave the collection unmodified
 int removeCount = 0;
 final BitSet removeSet = new BitSet(size);
 final int expectedModCount = modCount;
 final int size = this.size;
 for (int i=0; modCount == expectedModCount && i < size; i++) {
 @SuppressWarnings("unchecked")
 final E element = (E) elementData[i];
 if (filter.test(element)) {
 removeSet.set(i);
 removeCount++;
 }
 }
 if (modCount != expectedModCount) {
 throw new ConcurrentModificationException();
 }
 
 // shift surviving elements left over the spaces left by removed elements
 final boolean anyToRemove = removeCount > 0;
 if (anyToRemove) {
 final int newSize = size - removeCount;
 for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
 i = removeSet.nextClearBit(i);
 elementData[j] = elementData[i];
 }
 for (int k=newSize; k < size; k++) {
 elementData[k] = null; // Let gc do its work
 }
 this.size = newSize;
 if (modCount != expectedModCount) {
 throw new ConcurrentModificationException();
 }
 modCount++;
 }
 
 return anyToRemove;
 }
 
 @Override
 @SuppressWarnings("unchecked")
 public void replaceAll(UnaryOperator<E> operator) {
 Objects.requireNonNull(operator);
 final int expectedModCount = modCount;
 final int size = this.size;
 for (int i=0; modCount == expectedModCount && i < size; i++) {
 elementData[i] = operator.apply((E) elementData[i]);
 }
 if (modCount != expectedModCount) {
 throw new ConcurrentModificationException();
 }
 modCount++;
 }
 
 @Override
 @SuppressWarnings("unchecked")
 public void sort(Comparator<? super E> c) {
 final int expectedModCount = modCount;
 Arrays.sort((E[]) elementData, 0, size, c);
 if (modCount != expectedModCount) {
 throw new ConcurrentModificationException();
 }
 modCount++;
 }
 }

LinkedList源码

package java.util;
 
 import java.util.function.Consumer;
 
 /**
 * Doubly-linked list implementation of the {@code List} and {@code Deque}
 * interfaces. Implements all optional list operations, and permits all
 * elements (including {@code null}).
 *
 * All of the operations perform as could be expected for a doubly-linked
 * list. Operations that index into the list will traverse the list from
 * the beginning or the end, whichever is closer to the specified index.
 *
 * Note that this implementation is not synchronized.
 * If multiple threads access a linked list concurrently, and at least
 * one of the threads modifies the list structurally, it must be
 * synchronized externally. (A structural modification is any operation
 * that adds or deletes one or more elements; merely setting the value of
 * an element is not a structural modification.) This is typically
 * accomplished by synchronizing on some object that naturally
 * encapsulates the list.
 *
 * If no such object exists, the list should be "wrapped" using the
 * {@link Collections#synchronizedList Collections.synchronizedList}
 * method. This is best done at creation time, to prevent accidental
 * unsynchronized access to the list:<pre>
 * List list = Collections.synchronizedList(new LinkedList(...));</pre>
 *
 * The iterators returned by this class's {@code iterator} and
 * {@code listIterator} methods are fail-fast: if the list is
 * structurally modified at any time after the iterator is created, in
 * any way except through the Iterator's own {@code remove} or
 * {@code add} methods, the iterator will throw a {@link
 * ConcurrentModificationException}. Thus, in the face of concurrent
 * modification, the iterator fails quickly and cleanly, rather than
 * risking arbitrary, non-deterministic behavior at an undetermined
 * time in the future.
 *
 * Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification. Fail-fast iterators
 * throw {@code ConcurrentModificationException} on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness: the fail-fast behavior of iterators
 * should be used only to detect bugs.
 *
 * This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author Josh Bloch
 * @see List
 * @see ArrayList
 * @since 1.2
 * @param <E> the type of elements held in this collection
 */
 
 public class LinkedList<E>
 extends AbstractSequentialList<E>
 implements List<E>, Deque<E>, Cloneable, java.io.Serializable
 {
 transient int size = 0;
 
 /**
 * Pointer to first node.
 * Invariant: (first == null && last == null) ||
 * (first.prev == null && first.item != null)
 */
 transient Node<E> first;
 
 /**
 * Pointer to last node.
 * Invariant: (first == null && last == null) ||
 * (last.next == null && last.item != null)
 */
 transient Node<E> last;
 
 /**
 * Constructs an empty list.
 */
 public LinkedList() {
 }
 
 /**
 * Constructs a list containing the elements of the specified
 * collection, in the order they are returned by the collection's
 * iterator.
 *
 * @param c the collection whose elements are to be placed into this list
 * @throws NullPointerException if the specified collection is null
 */
 public LinkedList(Collection<? extends E> c) {
 this();
 addAll(c);
 }
 
 /**
 * Links e as first element.
 */
 private void linkFirst(E e) {
 final Node<E> f = first;
 final Node<E> newNode = new Node<>(null, e, f);
 first = newNode;
 if (f == null)
 last = newNode;
 else
 f.prev = newNode;
 size++;
 modCount++;
 }
 
 /**
 * Links e as last element.
 */
 void linkLast(E e) {
 final Node<E> l = last;
 final Node<E> newNode = new Node<>(l, e, null);
 last = newNode;
 if (l == null)
 first = newNode;
 else
 l.next = newNode;
 size++;
 modCount++;
 }
 
 /**
 * Inserts element e before non-null Node succ.
 */
 void linkBefore(E e, Node<E> succ) {
 // assert succ != null;
 final Node<E> pred = succ.prev;
 final Node<E> newNode = new Node<>(pred, e, succ);
 succ.prev = newNode;
 if (pred == null)
 first = newNode;
 else
 pred.next = newNode;
 size++;
 modCount++;
 }
 
 /**
 * Unlinks non-null first node f.
 */
 private E unlinkFirst(Node<E> f) {
 // assert f == first && f != null;
 final E element = f.item;
 final Node<E> next = f.next;
 f.item = null;
 f.next = null; // help GC
 first = next;
 if (next == null)
 last = null;
 else
 next.prev = null;
 size--;
 modCount++;
 return element;
 }
 
 /**
 * Unlinks non-null last node l.
 */
 private E unlinkLast(Node<E> l) {
 // assert l == last && l != null;
 final E element = l.item;
 final Node<E> prev = l.prev;
 l.item = null;
 l.prev = null; // help GC
 last = prev;
 if (prev == null)
 first = null;
 else
 prev.next = null;
 size--;
 modCount++;
 return element;
 }
 
 /**
 * Unlinks non-null node x.
 */
 E unlink(Node<E> x) {
 // assert x != null;
 final E element = x.item;
 final Node<E> next = x.next;
 final Node<E> prev = x.prev;
 
 if (prev == null) {
 first = next;
 } else {
 prev.next = next;
 x.prev = null;
 }
 
 if (next == null) {
 last = prev;
 } else {
 next.prev = prev;
 x.next = null;
 }
 
 x.item = null;
 size--;
 modCount++;
 return element;
 }
 
 /**
 * Returns the first element in this list.
 *
 * @return the first element in this list
 * @throws NoSuchElementException if this list is empty
 */
 public E getFirst() {
 final Node<E> f = first;
 if (f == null)
 throw new NoSuchElementException();
 return f.item;
 }
 
 /**
 * Returns the last element in this list.
 *
 * @return the last element in this list
 * @throws NoSuchElementException if this list is empty
 */
 public E getLast() {
 final Node<E> l = last;
 if (l == null)
 throw new NoSuchElementException();
 return l.item;
 }
 
 /**
 * Removes and returns the first element from this list.
 *
 * @return the first element from this list
 * @throws NoSuchElementException if this list is empty
 */
 public E removeFirst() {
 final Node<E> f = first;
 if (f == null)
 throw new NoSuchElementException();
 return unlinkFirst(f);
 }
 
 /**
 * Removes and returns the last element from this list.
 *
 * @return the last element from this list
 * @throws NoSuchElementException if this list is empty
 */
 public E removeLast() {
 final Node<E> l = last;
 if (l == null)
 throw new NoSuchElementException();
 return unlinkLast(l);
 }
 
 /**
 * Inserts the specified element at the beginning of this list.
 *
 * @param e the element to add
 */
 public void addFirst(E e) {
 linkFirst(e);
 }
 
 /**
 * Appends the specified element to the end of this list.
 *
 * This method is equivalent to {@link #add}.
 *
 * @param e the element to add
 */
 public void addLast(E e) {
 linkLast(e);
 }
 
 /**
 * Returns {@code true} if this list contains the specified element.
 * More formally, returns {@code true} if and only if this list contains
 * at least one element {@code e} such that
 * <tt>(o==null ? e==null : o.equals(e))</tt>.
 *
 * @param o element whose presence in this list is to be tested
 * @return {@code true} if this list contains the specified element
 */
 public boolean contains(Object o) {
 return indexOf(o) != -1;
 }
 
 /**
 * Returns the number of elements in this list.
 *
 * @return the number of elements in this list
 */
 public int size() {
 return size;
 }
 
 /**
 * Appends the specified element to the end of this list.
 *
 * This method is equivalent to {@link #addLast}.
 *
 * @param e element to be appended to this list
 * @return {@code true} (as specified by {@link Collection#add})
 */
 public boolean add(E e) {
 linkLast(e);
 return true;
 }
 
 /**
 * Removes the first occurrence of the specified element from this list,
 * if it is present. If this list does not contain the element, it is
 * unchanged. More formally, removes the element with the lowest index
 * {@code i} such that
 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
 * (if such an element exists). Returns {@code true} if this list
 * contained the specified element (or equivalently, if this list
 * changed as a result of the call).
 *
 * @param o element to be removed from this list, if present
 * @return {@code true} if this list contained the specified element
 */
 public boolean remove(Object o) {
 if (o == null) {
 for (Node<E> x = first; x != null; x = x.next) {
 if (x.item == null) {
 unlink(x);
 return true;
 }
 }
 } else {
 for (Node<E> x = first; x != null; x = x.next) {
 if (o.equals(x.item)) {
 unlink(x);
 return true;
 }
 }
 }
 return false;
 }
 
 /**
 * Appends all of the elements in the specified collection to the end of
 * this list, in the order that they are returned by the specified
 * collection's iterator. The behavior of this operation is undefined if
 * the specified collection is modified while the operation is in
 * progress. (Note that this will occur if the specified collection is
 * this list, and it's nonempty.)
 *
 * @param c collection containing elements to be added to this list
 * @return {@code true} if this list changed as a result of the call
 * @throws NullPointerException if the specified collection is null
 */
 public boolean addAll(Collection<? extends E> c) {
 return addAll(size, c);
 }
 
 /**
 * Inserts all of the elements in the specified collection into this
 * list, starting at the specified position. Shifts the element
 * currently at that position (if any) and any subsequent elements to
 * the right (increases their indices). The new elements will appear
 * in the list in the order that they are returned by the
 * specified collection's iterator.
 *
 * @param index index at which to insert the first element
 * from the specified collection
 * @param c collection containing elements to be added to this list
 * @return {@code true} if this list changed as a result of the call
 * @throws IndexOutOfBoundsException {@inheritDoc}
 * @throws NullPointerException if the specified collection is null
 */
 public boolean addAll(int index, Collection<? extends E> c) {
 checkPositionIndex(index);
 
 Object[] a = c.toArray();
 int numNew = a.length;
 if (numNew == 0)
 return false;
 
 Node<E> pred, succ;
 if (index == size) {
 succ = null;
 pred = last;
 } else {
 succ = node(index);
 pred = succ.prev;
 }
 
 for (Object o : a) {
 @SuppressWarnings("unchecked") E e = (E) o;
 Node<E> newNode = new Node<>(pred, e, null);
 if (pred == null)
 first = newNode;
 else
 pred.next = newNode;
 pred = newNode;
 }
 
 if (succ == null) {
 last = pred;
 } else {
 pred.next = succ;
 succ.prev = pred;
 }
 
 size += numNew;
 modCount++;
 return true;
 }
 
 /**
 * Removes all of the elements from this list.
 * The list will be empty after this call returns.
 */
 public void clear() {
 // Clearing all of the links between nodes is "unnecessary", but:
 // - helps a generational GC if the discarded nodes inhabit
 // more than one generation
 // - is sure to free memory even if there is a reachable Iterator
 for (Node<E> x = first; x != null; ) {
 Node<E> next = x.next;
 x.item = null;
 x.next = null;
 x.prev = null;
 x = next;
 }
 first = last = null;
 size = 0;
 modCount++;
 }
 
 
 // Positional Access Operations
 
 /**
 * Returns the element at the specified position in this list.
 *
 * @param index index of the element to return
 * @return the element at the specified position in this list
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
 public E get(int index) {
 checkElementIndex(index);
 return node(index).item;
 }
 
 /**
 * Replaces the element at the specified position in this list with the
 * specified element.
 *
 * @param index index of the element to replace
 * @param element element to be stored at the specified position
 * @return the element previously at the specified position
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
 public E set(int index, E element) {
 checkElementIndex(index);
 Node<E> x = node(index);
 E oldVal = x.item;
 x.item = element;
 return oldVal;
 }
 
 /**
 * Inserts the specified element at the specified position in this list.
 * Shifts the element currently at that position (if any) and any
 * subsequent elements to the right (adds one to their indices).
 *
 * @param index index at which the specified element is to be inserted
 * @param element element to be inserted
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
 public void add(int index, E element) {
 checkPositionIndex(index);
 
 if (index == size)
 linkLast(element);
 else
 linkBefore(element, node(index));
 }
 
 /**
 * Removes the element at the specified position in this list. Shifts any
 * subsequent elements to the left (subtracts one from their indices).
 * Returns the element that was removed from the list.
 *
 * @param index the index of the element to be removed
 * @return the element previously at the specified position
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
 public E remove(int index) {
 checkElementIndex(index);
 return unlink(node(index));
 }
 
 /**
 * Tells if the argument is the index of an existing element.
 */
 private boolean isElementIndex(int index) {
 return index >= 0 && index < size;
 }
 
 /**
 * Tells if the argument is the index of a valid position for an
 * iterator or an add operation.
 */
 private boolean isPositionIndex(int index) {
 return index >= 0 && index <= size;
 }
 
 /**
 * Constructs an IndexOutOfBoundsException detail message.
 * Of the many possible refactorings of the error handling code,
 * this "outlining" performs best with both server and client VMs.
 */
 private String outOfBoundsMsg(int index) {
 return "Index: "+index+", Size: "+size;
 }
 
 private void checkElementIndex(int index) {
 if (!isElementIndex(index))
 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
 }
 
 private void checkPositionIndex(int index) {
 if (!isPositionIndex(index))
 throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
 }
 
 /**
 * Returns the (non-null) Node at the specified element index.
 */
 Node<E> node(int index) {
 // assert isElementIndex(index);
 
 if (index < (size >> 1)) {
 Node<E> x = first;
 for (int i = 0; i < index; i++)
 x = x.next;
 return x;
 } else {
 Node<E> x = last;
 for (int i = size - 1; i > index; i--)
 x = x.prev;
 return x;
 }
 }
 
 // Search Operations
 
 /**
 * Returns the index of the first occurrence of the specified element
 * in this list, or -1 if this list does not contain the element.
 * More formally, returns the lowest index {@code i} such that
 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
 * or -1 if there is no such index.
 *
 * @param o element to search for
 * @return the index of the first occurrence of the specified element in
 * this list, or -1 if this list does not contain the element
 */
 public int indexOf(Object o) {
 int index = 0;
 if (o == null) {
 for (Node<E> x = first; x != null; x = x.next) {
 if (x.item == null)
 return index;
 index++;
 }
 } else {
 for (Node<E> x = first; x != null; x = x.next) {
 if (o.equals(x.item))
 return index;
 index++;
 }
 }
 return -1;
 }
 
 /**
 * Returns the index of the last occurrence of the specified element
 * in this list, or -1 if this list does not contain the element.
 * More formally, returns the highest index {@code i} such that
 * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
 * or -1 if there is no such index.
 *
 * @param o element to search for
 * @return the index of the last occurrence of the specified element in
 * this list, or -1 if this list does not contain the element
 */
 public int lastIndexOf(Object o) {
 int index = size;
 if (o == null) {
 for (Node<E> x = last; x != null; x = x.prev) {
 index--;
 if (x.item == null)
 return index;
 }
 } else {
 for (Node<E> x = last; x != null; x = x.prev) {
 index--;
 if (o.equals(x.item))
 return index;
 }
 }
 return -1;
 }
 
 // Queue operations.
 
 /**
 * Retrieves, but does not remove, the head (first element) of this list.
 *
 * @return the head of this list, or {@code null} if this list is empty
 * @since 1.5
 */
 public E peek() {
 final Node<E> f = first;
 return (f == null) ? null : f.item;
 }
 
 /**
 * Retrieves, but does not remove, the head (first element) of this list.
 *
 * @return the head of this list
 * @throws NoSuchElementException if this list is empty
 * @since 1.5
 */
 public E element() {
 return getFirst();
 }
 
 /**
 * Retrieves and removes the head (first element) of this list.
 *
 * @return the head of this list, or {@code null} if this list is empty
 * @since 1.5
 */
 public E poll() {
 final Node<E> f = first;
 return (f == null) ? null : unlinkFirst(f);
 }
 
 /**
 * Retrieves and removes the head (first element) of this list.
 *
 * @return the head of this list
 * @throws NoSuchElementException if this list is empty
 * @since 1.5
 */
 public E remove() {
 return removeFirst();
 }
 
 /**
 * Adds the specified element as the tail (last element) of this list.
 *
 * @param e the element to add
 * @return {@code true} (as specified by {@link Queue#offer})
 * @since 1.5
 */
 public boolean offer(E e) {
 return add(e);
 }
 
 // Deque operations
 /**
 * Inserts the specified element at the front of this list.
 *
 * @param e the element to insert
 * @return {@code true} (as specified by {@link Deque#offerFirst})
 * @since 1.6
 */
 public boolean offerFirst(E e) {
 addFirst(e);
 return true;
 }
 
 /**
 * Inserts the specified element at the end of this list.
 *
 * @param e the element to insert
 * @return {@code true} (as specified by {@link Deque#offerLast})
 * @since 1.6
 */
 public boolean offerLast(E e) {
 addLast(e);
 return true;
 }
 
 /**
 * Retrieves, but does not remove, the first element of this list,
 * or returns {@code null} if this list is empty.
 *
 * @return the first element of this list, or {@code null}
 * if this list is empty
 * @since 1.6
 */
 public E peekFirst() {
 final Node<E> f = first;
 return (f == null) ? null : f.item;
 }
 
 /**
 * Retrieves, but does not remove, the last element of this list,
 * or returns {@code null} if this list is empty.
 *
 * @return the last element of this list, or {@code null}
 * if this list is empty
 * @since 1.6
 */
 public E peekLast() {
 final Node<E> l = last;
 return (l == null) ? null : l.item;
 }
 
 /**
 * Retrieves and removes the first element of this list,
 * or returns {@code null} if this list is empty.
 *
 * @return the first element of this list, or {@code null} if
 * this list is empty
 * @since 1.6
 */
 public E pollFirst() {
 final Node<E> f = first;
 return (f == null) ? null : unlinkFirst(f);
 }
 
 /**
 * Retrieves and removes the last element of this list,
 * or returns {@code null} if this list is empty.
 *
 * @return the last element of this list, or {@code null} if
 * this list is empty
 * @since 1.6
 */
 public E pollLast() {
 final Node<E> l = last;
 return (l == null) ? null : unlinkLast(l);
 }
 
 /**
 * Pushes an element onto the stack represented by this list. In other
 * words, inserts the element at the front of this list.
 *
 * This method is equivalent to {@link #addFirst}.
 *
 * @param e the element to push
 * @since 1.6
 */
 public void push(E e) {
 addFirst(e);
 }
 
 /**
 * Pops an element from the stack represented by this list. In other
 * words, removes and returns the first element of this list.
 *
 * This method is equivalent to {@link #removeFirst()}.
 *
 * @return the element at the front of this list (which is the top
 * of the stack represented by this list)
 * @throws NoSuchElementException if this list is empty
 * @since 1.6
 */
 public E pop() {
 return removeFirst();
 }
 
 /**
 * Removes the first occurrence of the specified element in this
 * list (when traversing the list from head to tail). If the list
 * does not contain the element, it is unchanged.
 *
 * @param o element to be removed from this list, if present
 * @return {@code true} if the list contained the specified element
 * @since 1.6
 */
 public boolean removeFirstOccurrence(Object o) {
 return remove(o);
 }
 
 /**
 * Removes the last occurrence of the specified element in this
 * list (when traversing the list from head to tail). If the list
 * does not contain the element, it is unchanged.
 *
 * @param o element to be removed from this list, if present
 * @return {@code true} if the list contained the specified element
 * @since 1.6
 */
 public boolean removeLastOccurrence(Object o) {
 if (o == null) {
 for (Node<E> x = last; x != null; x = x.prev) {
 if (x.item == null) {
 unlink(x);
 return true;
 }
 }
 } else {
 for (Node<E> x = last; x != null; x = x.prev) {
 if (o.equals(x.item)) {
 unlink(x);
 return true;
 }
 }
 }
 return false;
 }
 
 /**
 * Returns a list-iterator of the elements in this list (in proper
 * sequence), starting at the specified position in the list.
 * Obeys the general contract of {@code List.listIterator(int)}.
 *
 * The list-iterator is fail-fast: if the list is structurally
 * modified at any time after the Iterator is created, in any way except
 * through the list-iterator's own {@code remove} or {@code add}
 * methods, the list-iterator will throw a
 * {@code ConcurrentModificationException}. Thus, in the face of
 * concurrent modification, the iterator fails quickly and cleanly, rather
 * than risking arbitrary, non-deterministic behavior at an undetermined
 * time in the future.
 *
 * @param index index of the first element to be returned from the
 * list-iterator (by a call to {@code next})
 * @return a ListIterator of the elements in this list (in proper
 * sequence), starting at the specified position in the list
 * @throws IndexOutOfBoundsException {@inheritDoc}
 * @see List#listIterator(int)
 */
 public ListIterator<E> listIterator(int index) {
 checkPositionIndex(index);
 return new ListItr(index);
 }
 
 private class ListItr implements ListIterator<E> {
 private Node<E> lastReturned;
 private Node<E> next;
 private int nextIndex;
 private int expectedModCount = modCount;
 
 ListItr(int index) {
 // assert isPositionIndex(index);
 next = (index == size) ? null : node(index);
 nextIndex = index;
 }
 
 public boolean hasNext() {
 return nextIndex < size;
 }
 
 public E next() {
 checkForComodification();
 if (!hasNext())
 throw new NoSuchElementException();
 
 lastReturned = next;
 next = next.next;
 nextIndex++;
 return lastReturned.item;
 }
 
 public boolean hasPrevious() {
 return nextIndex > 0;
 }
 
 public E previous() {
 checkForComodification();
 if (!hasPrevious())
 throw new NoSuchElementException();
 
 lastReturned = next = (next == null) ? last : next.prev;
 nextIndex--;
 return lastReturned.item;
 }
 
 public int nextIndex() {
 return nextIndex;
 }
 
 public int previousIndex() {
 return nextIndex - 1;
 }
 
 public void remove() {
 checkForComodification();
 if (lastReturned == null)
 throw new IllegalStateException();
 
 Node<E> lastNext = lastReturned.next;
 unlink(lastReturned);
 if (next == lastReturned)
 next = lastNext;
 else
 nextIndex--;
 lastReturned = null;
 expectedModCount++;
 }
 
 public void set(E e) {
 if (lastReturned == null)
 throw new IllegalStateException();
 checkForComodification();
 lastReturned.item = e;
 }
 
 public void add(E e) {
 checkForComodification();
 lastReturned = null;
 if (next == null)
 linkLast(e);
 else
 linkBefore(e, next);
 nextIndex++;
 expectedModCount++;
 }
 
 public void forEachRemaining(Consumer<? super E> action) {
 Objects.requireNonNull(action);
 while (modCount == expectedModCount && nextIndex < size) {
 action.accept(next.item);
 lastReturned = next;
 next = next.next;
 nextIndex++;
 }
 checkForComodification();
 }
 
 final void checkForComodification() {
 if (modCount != expectedModCount)
 throw new ConcurrentModificationException();
 }
 }
 
 private static class Node<E> {
 E item;
 Node<E> next;
 Node<E> prev;
 
 Node(Node<E> prev, E element, Node<E> next) {
 this.item = element;
 this.next = next;
 this.prev = prev;
 }
 }
 
 /**
 * @since 1.6
 */
 public Iterator<E> descendingIterator() {
 return new DescendingIterator();
 }
 
 /**
 * Adapter to provide descending iterators via ListItr.previous
 */
 private class DescendingIterator implements Iterator<E> {
 private final ListItr itr = new ListItr(size());
 public boolean hasNext() {
 return itr.hasPrevious();
 }
 public E next() {
 return itr.previous();
 }
 public void remove() {
 itr.remove();
 }
 }
 
 @SuppressWarnings("unchecked")
 private LinkedList<E> superClone() {
 try {
 return (LinkedList<E>) super.clone();
 } catch (CloneNotSupportedException e) {
 throw new InternalError(e);
 }
 }
 
 /**
 * Returns a shallow copy of this {@code LinkedList}. (The elements
 * themselves are not cloned.)
 *
 * @return a shallow copy of this {@code LinkedList} instance
 */
 public Object clone() {
 LinkedList<E> clone = superClone();
 
 // Put clone into "virgin" state
 clone.first = clone.last = null;
 clone.size = 0;
 clone.modCount = 0;
 
 // Initialize clone with our elements
 for (Node<E> x = first; x != null; x = x.next)
 clone.add(x.item);
 
 return clone;
 }
 
 /**
 * Returns an array containing all of the elements in this list
 * in proper sequence (from first to last element).
 *
 * The returned array will be "safe" in that no references to it are
 * maintained by this list. (In other words, this method must allocate
 * a new array). The caller is thus free to modify the returned array.
 *
 * This method acts as bridge between array-based and collection-based
 * APIs.
 *
 * @return an array containing all of the elements in this list
 * in proper sequence
 */
 public Object[] toArray() {
 Object[] result = new Object[size];
 int i = 0;
 for (Node<E> x = first; x != null; x = x.next)
 result[i++] = x.item;
 return result;
 }
 
 /**
 * Returns an array containing all of the elements in this list in
 * proper sequence (from first to last element); the runtime type of
 * the returned array is that of the specified array. If the list fits
 * in the specified array, it is returned therein. Otherwise, a new
 * array is allocated with the runtime type of the specified array and
 * the size of this list.
 *
 * If the list fits in the specified array with room to spare (i.e.,
 * the array has more elements than the list), the element in the array
 * immediately following the end of the list is set to {@code null}.
 * (This is useful in determining the length of the list only if
 * the caller knows that the list does not contain any null elements.)
 *
 * Like the {@link #toArray()} method, this method acts as bridge between
 * array-based and collection-based APIs. Further, this method allows
 * precise control over the runtime type of the output array, and may,
 * under certain circumstances, be used to save allocation costs.
 *
 * Suppose {@code x} is a list known to contain only strings.
 * The following code can be used to dump the list into a newly
 * allocated array of {@code String}:
 *
 * <pre>
 * String[] y = x.toArray(new String[0]);</pre>
 *
 * Note that {@code toArray(new Object[0])} is identical in function to
 * {@code toArray()}.
 *
 * @param a the array into which the elements of the list are to
 * be stored, if it is big enough; otherwise, a new array of the
 * same runtime type is allocated for this purpose.
 * @return an array containing the elements of the list
 * @throws ArrayStoreException if the runtime type of the specified array
 * is not a supertype of the runtime type of every element in
 * this list
 * @throws NullPointerException if the specified array is null
 */
 @SuppressWarnings("unchecked")
 public <T> T[] toArray(T[] a) {
 if (a.length < size)
 a = (T[])java.lang.reflect.Array.newInstance(
 a.getClass().getComponentType(), size);
 int i = 0;
 Object[] result = a;
 for (Node<E> x = first; x != null; x = x.next)
 result[i++] = x.item;
 
 if (a.length > size)
 a[size] = null;
 
 return a;
 }
 
 private static final long serialVersionUID = 876323262645176354L;
 
 /**
 * Saves the state of this {@code LinkedList} instance to a stream
 * (that is, serializes it).
 *
 * @serialData The size of the list (the number of elements it
 * contains) is emitted (int), followed by all of its
 * elements (each an Object) in the proper order.
 */
 private void writeObject(java.io.ObjectOutputStream s)
 throws java.io.IOException {
 // Write out any hidden serialization magic
 s.defaultWriteObject();
 
 // Write out size
 s.writeInt(size);
 
 // Write out all elements in the proper order.
 for (Node<E> x = first; x != null; x = x.next)
 s.writeObject(x.item);
 }
 
 /**
 * Reconstitutes this {@code LinkedList} instance from a stream
 * (that is, deserializes it).
 */
 @SuppressWarnings("unchecked")
 private void readObject(java.io.ObjectInputStream s)
 throws java.io.IOException, ClassNotFoundException {
 // Read in any hidden serialization magic
 s.defaultReadObject();
 
 // Read in size
 int size = s.readInt();
 
 // Read in all elements in the proper order.
 for (int i = 0; i < size; i++)
 linkLast((E)s.readObject());
 }
 
 /**
 * Creates a <a href="Spliterator.html#binding">late-binding</a>
 * and fail-fast {@link Spliterator} over the elements in this
 * list.
 *
 * The {@code Spliterator} reports {@link Spliterator#SIZED} and
 * {@link Spliterator#ORDERED}. Overriding implementations should document
 * the reporting of additional characteristic values.
 *
 * @implNote
 * The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED}
 * and implements {@code trySplit} to permit limited parallelism..
 *
 * @return a {@code Spliterator} over the elements in this list
 * @since 1.8
 */
 @Override
 public Spliterator<E> spliterator() {
 return new LLSpliterator<E>(this, -1, 0);
 }
 
 /** A customized variant of Spliterators.IteratorSpliterator */
 static final class LLSpliterator<E> implements Spliterator<E> {
 static final int BATCH_UNIT = 1 << 10; // batch array size increment
 static final int MAX_BATCH = 1 << 25; // max batch array size;
 final LinkedList<E> list; // null OK unless traversed
 Node<E> current; // current node; null until initialized
 int est; // size estimate; -1 until first needed
 int expectedModCount; // initialized when est set
 int batch; // batch size for splits
 
 LLSpliterator(LinkedList<E> list, int est, int expectedModCount) {
 this.list = list;
 this.est = est;
 this.expectedModCount = expectedModCount;
 }
 
 final int getEst() {
 int s; // force initialization
 final LinkedList<E> lst;
 if ((s = est) < 0) {
 if ((lst = list) == null)
 s = est = 0;
 else {
 expectedModCount = lst.modCount;
 current = lst.first;
 s = est = lst.size;
 }
 }
 return s;
 }
 
 public long estimateSize() { return (long) getEst(); }
 
 public Spliterator<E> trySplit() {
 Node<E> p;
 int s = getEst();
 if (s > 1 && (p = current) != null) {
 int n = batch + BATCH_UNIT;
 if (n > s)
 n = s;
 if (n > MAX_BATCH)
 n = MAX_BATCH;
 Object[] a = new Object[n];
 int j = 0;
 do { a[j++] = p.item; } while ((p = p.next) != null && j < n);
 current = p;
 batch = j;
 est = s - j;
 return Spliterators.spliterator(a, 0, j, Spliterator.ORDERED);
 }
 return null;
 }
 
 public void forEachRemaining(Consumer<? super E> action) {
 Node<E> p; int n;
 if (action == null) throw new NullPointerException();
 if ((n = getEst()) > 0 && (p = current) != null) {
 current = null;
 est = 0;
 do {
 E e = p.item;
 p = p.next;
 action.accept(e);
 } while (p != null && --n > 0);
 }
 if (list.modCount != expectedModCount)
 throw new ConcurrentModificationException();
 }
 
 public boolean tryAdvance(Consumer<? super E> action) {
 Node<E> p;
 if (action == null) throw new NullPointerException();
 if (getEst() > 0 && (p = current) != null) {
 --est;
 E e = p.item;
 current = p.next;
 action.accept(e);
 if (list.modCount != expectedModCount)
 throw new ConcurrentModificationException();
 return true;
 }
 return false;
 }
 
 public int characteristics() {
 return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
 }
 }
 
 }

注：源码来自官方JDK1.8