Java容器源码LinkedList原理解析

public LinkedList() {
}

//从已有的一个容器创建一个LinkedList对象
public LinkedList(Collection<? extends E> c) {
  this();
  addAll(c);
}

public boolean addAll(Collection<? extends E> c) {
  return addAll(size, c);
}

public boolean addAll(int index, Collection<? extends E> c) {
  //检查index是否溢出
  checkPositionIndex(index);
  
  Object[] a = c.toArray();
  int numNew = a.length;
  if (numNew == 0)
    return false;
  //获取第index位置的node元素和node的前一个元素
  //succ：第index位置的node元素
  //pred：index位置前一个node元素
  Node<E> pred, succ;
  if (index == size) {
    succ = null;
    pred = last;
  } else {
    succ = node(index);
    pred = succ.prev;
  }

  //遍历，将元素插入链表中
  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;
}

public boolean add(E e) {
  //添加元素到队尾
  linkLast(e);
  return true;
}

public void add(int index, E element) {
  //检查index是否溢出
  checkPositionIndex(index);

  if (index == size)
    //index == size，直接添加到队尾
    linkLast(element);
  else
    //index != size，添加元素到index位置
    linkBefore(element, node(index));
}

void linkLast(E e) {
  final Node<E> l = last;
  //新建一个node，将其前一个元素指针指向原链表的最后一个元素
  final Node<E> newNode = new Node<>(l, e, null);
  //更新尾指针
  last = newNode;
  if (l == null)
    //若原last==null说明此时链表就一个元素
    first = newNode;
  else
    //更新原链表尾元素指针
    l.next = newNode;
  size++;
  modCount++;
}

void linkBefore(E e, Node<E> succ) {
  // assert succ != null;
  //获取指定位node元素的前一个元素pred
  final Node<E> pred = succ.prev;
  //新建一个node，将其前指针指向pred元素
  final Node<E> newNode = new Node<>(pred, e, succ);
  //将指定位置的node元素的前指针指向新元素，完成插入
  succ.prev = newNode;
  if (pred == null)
    first = newNode;
  else
    pred.next = newNode;
  size++;
  modCount++;
}

Node<E> node(int index) {
  // assert isElementIndex(index);
  //index > size/2时，说明在链表前半段，从前往后搜索
  if (index < (size >> 1)) {
    Node<E> x = first;
    for (int i = 0; i < index; i++)
      x = x.next;
    return x;
  //index < size/2时，从后往前搜索
  } else {
    Node<E> x = last;
    for (int i = size - 1; i > index; i--)
      x = x.prev;
    return x;
  }
}