LinkedList源码对其定义为

Doubly-linked list implementation of the List and Deque interfaces

即LinkedList是一个双向链表结构，它是以节点的形式保存元素，添加到其中的元素都会被包装成节点，每个节点包含两个其他节点的引用，一个指向前一个节点，另一个指向后一个节点。因为是一个双向链表，因此可以在链表的任意位置插入或删除元素，下面我们一起来看看LinkedList是如何实现数据的管理功能的。

LinkedList的节点Node的定义如下：

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

1、LInkedList的初始化

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); } ...... }

首先，LinkedList的默认大小为0，内部通过头部节点first和尾部节点last来维护双向链表；它有两个构造函数，一个无参构造函数，另一个是传入一个Collection，传入的Collection中的元素将逐个被包装成Node然后被添加到链表中，这将在下面的元素添加中介绍。

2、LinkedList的数据添加

2.1、LinkedList的数据添加之add系列

/** * 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. * * <p>This method is equivalent to {@link #add}. * * @param e the element to add */ public void addLast(E e) { linkLast(e); } ...... /** * Appends the specified element to the end of this list. * * <p>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; } ...... /** * 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)); } ...... /** * 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; } ...... /** * 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; } }

可以看到add添加一个数据的接口最后都调到link系列方法中了-link系列方法见2.2，而add Collection时是将collection中的元素取出来包装成新的节点newNode并按顺序挨个连起来，其中collection中的第一个元素生成的节点其前一个节点指向index位置处的节点，若不存在该节点，则collection中的第一个元素将为头节点。

2.2、LinkedList的数据添加之link系列

/** * 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++; }

从link这几个接口的名字可以看到各个接口的功能-linkFirst是将元素包装成节点然后放入链表的头部，若头节点为空，则该节点为头节点；linkLast是将元素包装成节点后放入链表的尾部，并且该节点成为尾部节点，如果头节点为空的话，则该节点同样也是头节点--即在该情况下该节点即是头节点也是尾节点；linkBefore是将元素包装成节点并放到指定的节点succ之前，若succ.prev为空即succ为头节点，则插入的该节点也将成为头节点。

2.3、LinkedList的数据添加之offer系列

/** * 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; }

offer系列调用的是add系列方法，而add系列调用的是link系列，见2.2

2.4、LinkedList的数据添加之push系列

/** * Pushes an element onto the stack represented by this list. In other * words, inserts the element at the front of this list. * * <p>This method is equivalent to {@link #addFirst}. * * @param e the element to push * @since 1.6 */ public void push(E e) { addFirst(e); }

同offer系列，最终走的也是link方法。

总的来说，LinkedList添加元素的方法有add、offer、push和link，但是add、offer和push最终调用的都是link方法，在头部添加元素调用linkFirst，尾部添加元素调用linkLast，中间添加元素调用linkBefore。

3、LinkedList的数据移除

3.1、LinkedList的数据移除之remove

/** * 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(); } ...... /** * 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)); } ...... /** * 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; } ...... /** * 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); } ...... /** * 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; }

可以看到remove系列最终调用的unlink。

3.2、LinkedList的数据移除之poll

...... /** * 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 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); }

可以看到poll系列最终也是调用的unlink。

3.3、LinkedList的数据移除之pop

/** * Pops an element from the stack represented by this list. In other * words, removes and returns the first element of this list. * * <p>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(); } ......

可以看到pop系列调用的remove，而remove调用的是unlink，因此pop最终也是调用的unlink。

3.4、LinkedList的数据移除之unlink

...... /** * 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; }

可以看到，同添加元素一样，LinkedList移除元素的方法有remove,poll,pop,unlink四种，但是前三种最终调用的都是最后一个unlink方法。移除头部元素调用unlinkFirst，移除尾部元素调用unlinkLast，移除中间元素调用unlink。

４、LInkedList的数据更新

/** * 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; }

linkedList的数据更新是通过调用set(int index, E element)来实现的，代码很简单就不介绍了。

５、LInkedList的数据获取

/** * 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; } 　　...... /** * 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; } ...... // 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; } ...... /** * 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 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; } ......

数据获取的方式有几种，其中getFirst和peekFirst、peek是获取头节点的数据，geLast和peekLast是获取尾节点的数据, get（int index）还可以获取指定节点处的数据；另外使用indexOf和lastIndexOf可以查询object在LInkedList中的位置。

６、LinkedList的fail-fast机制

同ArrayList一样，LinkedList是非线程安全的，也是采用fail-fast机制来控制对Iterator的使用，具体控制方法请参考我的另一篇文章java数据结构之ArrayList。