【Java集合源码02】LinkedList源码分析

向右看齐 2022-12-26 04:49 214阅读 0赞

# 简介 #

LinkedList是一个实现了List接口和Deque接口的双端链表。 LinkedList底层的链表结构使它支持高效的插入和删除操作，另外它实现了Deque接口，使得LinkedList类也具有队列的特性; LinkedList不是线程安全的，如果想使LinkedList变成线程安全的，可以调用静态类Collections类中的synchronizedList方法：

List list=Collections.synchronizedList(new LinkedList(...));

![20201202170755.png][]

# 实现原理 #

双端链表

![20201202194657.png][]

双端链表节点：

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;
 }
 }

# 源码解析 #

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)
 * 头结点
 */
 transient Node<E> first;
 
 /**
 * Pointer to last node.
 * Invariant: (first == null && last == null) ||
 * (last.next == null && last.item != null)
 * 尾结点
 */
 transient Node<E> last;
 
 /**
 * Constructs an empty list.
 * 无参构造函数
 */
 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
 * 有参构造函数
 */
 public LinkedList(Collection<? extends E> c) {
 
 this();
 addAll(c);
 }
 
 /**
 * Links e as first element.
 * 把e设置为头结点
 */
 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.
 * 把e设置为尾结点
 */
 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.
 * 插入一个结点e在succ之前
 */
 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.
 删除第一个结点
 */
 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.
 * 删除最后一个结点
 */
 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.
 * 删除结点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
 * 返回头结点的值
 */
 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
 * 返回尾结点的值
 */
 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
 * 删除第一个结点并返回其值
 */
 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
 * 删除最后一个结点并返回其值
 */
 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
 * 添加e结点作为首结点
 */
 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
 * 添加e结点作为尾结点
 */
 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
 * 代码比较简单，不太想注释了，把上面几个看懂就可以了
 */
 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;
 // succ第index个结点，pred是succ的前驱结点
 Node<E> pred, succ;
 if (index == size) {
 
 succ = null;
 pred = last;
 } else {
 
 // 如果依次存入tom、jack、rose，第0个结点是tom，第2个结点是rose
 // node函数用于找到第index个结点
 succ = node(index);
 pred = succ.prev;
 }
 // 循环插入结点
 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.
 * 清空链表
 */
 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.
 * 判断插入位置index是否合法
 */
 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);
 // 这里用点小技巧，index < size / 2，说明index在前半段，否则index在后半段
 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
 * 返回最后出现o的位置，也就是出现o，index最大的位置
 */
 public int lastIndexOf(Object o) {
 
 int index = size;
 // 逆向遍历
 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;
 }

#### 推荐阅读 ####

*  [机器学习资料汇总][Link 1]
 *  [吴恩达《机器学习》视频、作业、源码][Link 2]
 *  [106页《Python进阶》中文版正式发布][106_Python]
 *  [李航《统计学习方法》第二版完整课件][Link 3]
 *  [机器学习数学全书，1900页PDF下载][1900_PDF]

--------------------

![gzh.jpg][]

[20201202170755.png]: /images/20221120/d238a7d8e35b486a8d16f5e3f2397afb.png
[20201202194657.png]: /images/20221120/5221bf267ba44ede897f7a4229efe05e.png
[Link 1]: https://mp.weixin.qq.com/s/3nOkk_Yt9D7Qp1WaWEjyZQ
[Link 2]: https://mp.weixin.qq.com/s/dErZNtBYbVA7ItPm7T_HIw
[106_Python]: https://mp.weixin.qq.com/s/_WEuuxj-QgihijjLz7NJ9g
[Link 3]: https://mp.weixin.qq.com/s/xah47OWuu8ahAUa1aFFo4Q
[1900_PDF]: https://mp.weixin.qq.com/s/9BuyhdwuHiHH3ksVUe44ZQ
[gzh.jpg]: /images/20221120/0ea485b2d1b7495eabe446a31fbfc726.png