LinkedList源码分析

LinkedList是基于双链表实现的。

相对与普通链表，多了个prev 前驱的指针。用于指向前一个元素。

基于虚拟头节点。统一为中间插入

基于虚拟头节点。统一为中间删除

通过IDEA生成LinkedList类图。LinkedList实现了List(线性表)、Queue(队列)、Deque（双端队列）接口。

• AbstractList

    protected transient int modCount = 0;

• LinkedList

    // 节点个数。冗余存储，防止遍历
    transient int size = 0;

    
    transient Node first;

    
    transient Node last;
    
    // 序列化id,用于反序列化验证。无需关系
    private static final long serialVersionUID = 876323262645176354L;

• AbstractCollection

    protected AbstractCollection() {
    }

• AbstractList

 
    protected AbstractList() {
    }

• AbstractSequentialList

 
    protected AbstractSequentialList() {
    }

• LinkedList

 
    public LinkedList() {
    }

    
    public LinkedList(Collection c) {
        this();
        addAll(c);
    }

1. 作为List
   

尾插
由于没有虚拟头节点。尾插，分为两种情况(first == null , first ！= null)。

     
    public boolean add(E e) {
        linkLast(e);
        return true;
    }

        
    void linkLast(E e) {
        final Node l = last;
        final Node newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
        modCount++;
    }

按索引插入
定位节点需要遍历，性能差。

    
    public void add(int index, E element) {
        checkPositionIndex(index);

        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }
    
    void linkBefore(E e, Node succ) {
        // assert succ != null;
        final Node pred = succ.prev;
        final Node newNode = new Node<>(pred, e, succ);
        succ.prev = newNode;
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        size++;
        modCount++;
    }
    
    Node node(int index) {
        // assert isElementIndex(index);

        if (index < (size >> 1)) {
            Node x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {
            Node x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }
    private void checkPositionIndex(int index) {
        if (!isPositionIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
     
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }

指定位置
定位节点需要遍历，性能差。

    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }
 
    E unlink(Node x) {
        // assert x != null;
        final E element = x.item;
        final Node next = x.next;
        final Node 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;
    }
     
    Node node(int index) {
        // assert isElementIndex(index);

        if (index < (size >> 1)) {
            Node x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {
            Node x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }
    private void checkElementIndex(int index) {
        if (!isElementIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    
    private boolean isElementIndex(int index) {
        return index >= 0 && index < size;
    }

指定值
定位节点需要遍历，性能差。

    public boolean remove(Object o) {
        if (o == null) {
            for (Node x = first; x != null; x = x.next) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node x = first; x != null; x = x.next) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }

定位节点需要遍历，性能差。

 
    public E set(int index, E element) {
        checkElementIndex(index);
        Node x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }

遍历

    
    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 x = first; x != null; ) {
            Node next = x.next;
            x.item = null;
            x.next = null;
            x.prev = null;
            x = next;
        }
        first = last = null;
        size = 0;
        modCount++;
    }

查询大量依赖遍历，性能差
// 定位节点需要遍历，性能差。
public boolean contains(Object o)；
// 定位节点需要遍历，性能差。
public E get(int index);
// 定位节点需要遍历，性能差。
public int indexOf(Object o);
// 定位节点需要遍历，性能差。
public int lastIndexOf(Object o);
// 冗余变量，不需要遍历
public int size();