LinkedList源码解析

1.概览

LinkedList是List的另一个实现类，底层以双向链表维持，因为不存在数组，故不会扩容。

2.主要流程

2.1底层定义

维护两个节点：头节点和尾节点，Node为双向链表

    /*** 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;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;}}

2.2初始化

LinkedList初始化可分为两种情况：无参构造、指定集合构造

无参构造

无参构造什么也不做，只初始化一个空list

    /*** Constructs an empty list.*/public LinkedList() {}

指定集合构造

首先初始化一个空的list，再把入参数集合中的元素添加进链表。

    /*** 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) {// 1.初始化空链表this();// 2.添加集合所有元素addAll(c);}

    /*** 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) {// 第一个参数表示插入位置，size为当前链表大小，表示从尾部插入return addAll(size, c);}

在将集合中的所有元素插入链表时，首先检查插入的位置是否合法，否则抛出IndexOutOfBoundsException

    /*** 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) {// 1.检查插入位置checkPositionIndex(index);// 2.集合转数组，空集合则直接return falseObject[] a = c.toArray();int numNew = a.length;if (numNew == 0)return false;// 3.构造前序节点，尾插法前序节点为last，否则去查找index位置的节点Node<E> pred, succ;if (index == size) {succ = null;pred = last;} else {succ = node(index);pred = succ.prev;}// 4.遍历插入for (Object o : a) {@SuppressWarnings("unchecked") E e = (E) o;Node<E> newNode = new Node<>(pred, e, null);if (pred == null)first = newNode;elsepred.next = newNode;pred = newNode;}if (succ == null) {last = pred;} else {pred.next = succ;succ.prev = pred;}// 5.更新size和modCountsize += numNew;modCount++;return true;}

2.3插入

默认插入（尾部）

默认是尾部插入，直接操作last节点

    /*** 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;}

头部插入

直接操作头节点

    /*** Inserts the specified element at the beginning of this list.** @param e the element to add*/public void addFirst(E e) {linkFirst(e);}

指定位置插入

会确认index是否合法，其次查找到index的节点，然后插入

    /*** 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);elselinkBefore(element, node(index));}

2.4删除

与插入类似

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

3.结论

LinkedList底层是双向链表的数据结构，因此不需要扩容
LinkedList不支持随机读写，读写的时间复杂度是O（n），但是不需要移动前后元素

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LinkedList源码解析

LinkedList源码解析

1.概览

2.主要流程

2.1底层定义

2.2初始化

无参构造

指定集合构造

2.3插入

默认插入（尾部）

头部插入

指定位置插入

2.4删除

3.结论

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