JDK1.8 源码 java.util.LinkedList

前言

今天我们来看下java.util.LinkedMap.

从上图可以看出. LinkedList和ArrayList同属List <-AbstractList流程下的不同实现.
此外, 因为链表的关系, LinkedList主要有2层继承关系.

• Collection <-- List <-- AbstractList <-- AbstractSequentialList. 此为第一段继承关系.
• Collection <-- Queue <- Deque <-- LinkedList. 此为第二段继承关系.

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正文

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成员变量&方法

• 主要成员变量

    transient int size = 0;
    transient Node first;
    transient Node last;

由于LinkedList是用链表进行实现. 所以, 其成员变量多于链表数据结构相关. Node头指针, Node 尾指针, size长度.

    private static class Node {
        E item;
        Node next;
        Node prev;

        Node(Node prev, E element, Node next) {
            this.item = element;
            this.next = next;
            this.prev = prev;
        }
    }

局部类Node, 可以看出, 实现是一个双向链表.

• 主要方法
  • 构造函数 public LinkedList() / public LinkedList(Collection c)
  • 增 public boolean add(E e) / public boolean addAll(int index, Collection c)
  • 删 public E remove(int index)/public boolean remove(Object o)
  • 查 public E get(int index)
  • 查下标 public int indexOf(Object o) / public int lastIndexOf(Object o)
  • 改 public E set(int index, E element)
• 对于头尾节点操作
  • 增 public void addFirst(E e) / public void addLast(E e)
  • 删 public E removeFirst()/public E removeLast()
  • 查 public E getFirst()/public E getLast()
• 队列接口相关操作
  • public E peek() / public E peekFirst() / public E peekLast()
  • public E poll() / public E pollFirst() / public E pollLast()
  • public void push(E e)
  • public E pop()
• 其余方法
  • toArray()
  • clone()
  • toString()
  • equals()
  • compareTo()
  • writeObject/readObject() 序列化&反序列化

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类型声明

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

其中 LinkedList <-- AbstractSequentialList <-- AbstractList <-- List <-- Collection. 在上章已经讲过. 这章只记录下继承关系.

public abstract class AbstractSequentialList extends AbstractList {

public abstract class AbstractList extends AbstractCollection implements List {

public abstract class AbstractCollection implements Collection {

其中 Cloneable和Serializable分别是拷贝和序列化接口.

public interface Deque extends Queue {
    void addFirst(E e);
    void addLast(E e);
    boolean offerFirst(E e);
    boolean offerLast(E e);
    E removeFirst();
    E removeLast();
    E pollFirst();
    E pollLast();
    E getFirst();
    E getLast();
    E peekFirst();
    E peekLast();
    boolean removeFirstOccurrence(Object o);
    boolean removeLastOccurrence(Object o);
    boolean add(E e);
    boolean offer(E e);
    E remove();
    E poll();
    E element();
    E peek();
    void push(E e);
    E pop();
    boolean remove(Object o);
    boolean contains(Object o);
    public int size();
    Iterator iterator();
    Iterator descendingIterator();
}

• Queue 接口

public interface Queue extends Collection {
    boolean add(E e);
    boolean offer(E e);
    E remove();
    E poll();
    E element();
    E peek();
}

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构造函数

• 空构造函数

    public LinkedList() {
    }

• 含参构造函数

	//	 将Collection对象放入List内
   public LinkedList(Collection c) {
        this();
        addAll(c);
    }
    public boolean addAll(Collection c) {
        return addAll(size, c);
    }
public boolean addAll(int index, Collection c) {
        checkPositionIndex(index);
		// 转换为数组
        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew == 0)
            return false;

        Node pred, succ;
        if (index == size) {
        	// index是开始位置 index是指向的链表尾部
        	// 从尾部插入
            succ = null;
            pred = last;
        } else {
        	// 非从尾部进行插入
            succ = node(index);
            pred = succ.prev;
        }

        for (Object o : a) {
            @SuppressWarnings("unchecked") E e = (E) o;
            Node newNode = new Node<>(pred, e, null);
            if (pred == null)
                first = newNode;
            else
                pred.next = newNode;
            pred = newNode;
        }

		// 链表队尾收尾操作
        if (succ == null) {
            last = pred;
        } else {
            pred.next = succ;
            succ.prev = pred;
        }

        size += numNew;
        modCount++;
        return true;
    }

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• 增 - 队尾增加元素

    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++;
    }    

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• 删除

 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;
    }

  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;
    }

删除方法. 因为是双向链表. 所以略微繁琐一点, unlink()方法的逻辑:

• 节点前节点. 节点当前节点. 节点后节点.
• 节点前节点.next = 节点后节点 .
• 节点后节点.head = 节点前节点.

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查

• public E get(int index)

    public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }
    
   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;
        }
    }

这边有一个节点的优化. 判断index是偏向头还是尾巴. 为了搜索的速率更快一点.

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• 改

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

直接修改节点的值. 也就是node.item. 返回旧的数据oldVal.

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Dequeue接口相关操作

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增

• 增 public void addFirst(E e) / public void addLast(E e)

    public void addFirst(E e) {
        linkFirst(e);
    }
    public void addLast(E e) {
        linkLast(e);
    }

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删

• 删 public E removeFirst()/public E removeLast()

   public E removeFirst() {
        final Node f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }
    public E removeLast() {
        final Node l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }

其中unlinkFirst()和unLinklast()都是前面unlink()方法的变种方法. 这边不在赘述.

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查

• 查 public E getFirst()/public E getLast()

    public E getFirst() {
        final Node f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }
    public E getLast() {
        final Node l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
    }

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• ToArray()

    public Object[] toArray() {
        Object[] result = new Object[size];
        int i = 0;
        for (Node x = first; x != null; x = x.next)
            result[i++] = x.item;
        return result;
    }

LinkedList的toArray()返回的是一个Object[].

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Others

• for循环和迭代器循环 效率问题?

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Q

Q1: 什么是Dqueue?

Q2: LinkedList的实现原理是什么？ 具有什么优势？
链表. 增删快, 查询慢.

Q3: 链表如何增删节点？

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Reference

[1]. JDK1.8源码(六)——java.util.LinkedList 类