JDK容器学习之LinkedList：底层存储&读写逻辑

transient int size = 0;

/**
 * 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;
    }
}

public E get(int index) {
    // 越界判断
    checkElementIndex(index);
    return node(index).item;
}

Node<E> node(int index) {
    if (index < (size >> 1)) { // 从前往后找
        Node<E> x = first;
        for (int i = 0; i < index; i++)
            x = x.next;
        return x;
    } else { // 从后往前找
        Node<E> x = last;
        for (int i = size - 1; i > index; i--)
            x = x.prev;
        return x;
    }
}

public void add(int index, E element) {
    // 越界判断
    checkPositionIndex(index);

    if (index == size)
        linkLast(element);
    else {
      // 先获取到index索引对应的节点，然后在该节点之前插入新的元素
        linkBefore(element, node(index));
    }
}

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


void linkBefore(E e, Node<E> succ) {
  // assert succ != null;
  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++;
}

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;
}

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) { // 删除链表头
      first = next;
  } else {
      prev.next = next;
      x.prev = null;
  }

  if (next == null) { // 删除链表尾
      last = prev;
  } else {
      next.prev = prev;
      x.next = null;
  }

  x.item = null;
  size--;
  modCount++;
  return element;
}