Java集合002 --- LinkedList源码解析

前言

LinkedList内部实现是一个双链表，linkedList除了实现了list相关的接口外，还实现了Queue、Dequeue接口，所以它有着双端队列、list、栈的功能

 注意LinkedList没有实现RandomAccess接口，这意味着LinkedList没有提供快速随机访问功能

属性

// 链表数据长度
transient int size = 0;

// 链表首指针
transient Node<E> first;

// 链表尾指针
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;
    }
}

构造器

public LinkedList() {
}

public LinkedList(Collection<? extends E> c) {
    this();
    addAll(c);
}

public boolean addAll(Collection<? extends E> c) {
    // 复用了在指定索引批量添加元素的方法, 只不过这里size在构造器调用时为0
    return addAll(size, c);
}

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;
}

添加单个元素方法

1、添加单个元素到尾部（返回值boolean类型，表示是否添加成功）

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

void linkLast(E e) {
    final Node<E> l = last;
    // 因为是在尾部添加节点, 所以新节点前一个节点为last, 后一个节点为null
    final Node<E> newNode = new Node<>(l, e, null);
    last = newNode;// 新节点置为尾结点S
    if (l == null) // 双链表为空, 新节点即为首节点
        first = newNode;
    else
        l.next = newNode; // 否则将原来尾结点的next置为新节点
    size++; // 长度加一
    modCount++; // 序列化时使用
}

2、在指定索引添加一个元素

public void add(int index, E element) {
    // 校验索引, 为啥这里索引可以取值为size呢？这是因为可以在尾部添加节点
    checkPositionIndex(index);

    if (index == size)
        linkLast(element); // 尾部添加节点, 同上
    else
        linkBefore(element, node(index));// 指定索引添加节点
}

private void checkPositionIndex(int index) {
    if (!isPositionIndex(index))
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

private boolean isPositionIndex(int index) {
    return index >= 0 && index <= size;
}

Node<E> node(int 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;
    }
}

// succ由node(index)保证在链表中存在, 并且不为空
// 相当于在succ节点前添加节点
void linkBefore(E e, Node<E> succ) {
    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++;
}

删除单个元素方法

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;
}

public E remove(int index) {
    checkElementIndex(index);
    return unlink(node(index));
}

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;
}

两个集合的并集

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;
}

作为栈的push和pop方法

push为入栈，pop为出栈

peek和element方法 

功能都是获取首节点元素，区别是peek在首节点为null时，返回null；而element方法在首节点为null时抛出NoSuchElementException异常

public E peek() {
    final Node<E> f = first;
    return (f == null) ? null : f.item;
}

public E element() {
    return getFirst();
}

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

poll和remove方法

功能都是删除首节点，区别是poll在首节点为null时，返回null；而remove方法在首节点为null时抛出NoSuchElementException异常

public E poll() {
    final Node<E> f = first;
    return (f == null) ? null : unlinkFirst(f);
}

public E remove() {
    return removeFirst();
}

public E removeFirst() {
    final Node<E> f = first;
    if (f == null)
        throw new NoSuchElementException();
    return unlinkFirst(f);
}

序列化和反序列化

和ArrayList类似，LinkedList序列化方式也是通过提供私有无返回值的writeObject和readObject方法，按需序列化

private void writeObject(java.io.ObjectOutputStream s)
    throws java.io.IOException {
    // Write out any hidden serialization magic
    s.defaultWriteObject();

    // Write out size
    s.writeInt(size);

    // Write out all elements in the proper order.
    for (Node<E> x = first; x != null; x = x.next)
        s.writeObject(x.item);
}

@SuppressWarnings("unchecked")
private void readObject(java.io.ObjectInputStream s)
    throws java.io.IOException, ClassNotFoundException {
    // Read in any hidden serialization magic
    s.defaultReadObject();

    // Read in size
    int size = s.readInt();

    // Read in all elements in the proper order.
    for (int i = 0; i < size; i++)
        linkLast((E)s.readObject());
}