LinkedList

来源：

• extends:AbstractSequentialList
• implements:
  • List
  • Deque:deque 即双端队列。是一种具有队列和栈的性质的数据结构。双端队列中的元素可以从两端弹出，其限定插入和删除操作在表的两端进行。
  • Cloneable
  • java.io.Serializable

字段：

• transient int size = 0;
• transient Node<E> first:头节点
• transient Node<E> last:尾节点

    /**
     * Pointer to first node.
     * Invariant: (first == null && last == null) ||
     *            (first.prev == null && first.item != null)
     */
   transient Node<E> first;//头节点没有前驱
    /**
     * Pointer to last node.
     * Invariant: (first == null && last == null) ||
     *            (last.next == null && last.item != null)
     */
   transient Node<E> last;//尾节点没有后继
  /**
   * Node的定义
   */
    private static class Node<E> {
        E item;//元素
        Node<E> next;//后继
        Node<E> prev;//前驱啊S

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

方法：

• 构造方法：

/**
 * 构造空的list
 */
public LinkedList() {
}

/**
 * 按照给定集合的迭代器返回数据的顺序把所有元素添加到LinkedList中
 *
 * @param  c 集合
 * @throws NullPointerException 
 */
public LinkedList(Collection<? extends E> c) {
    this();
    addAll(c);
}

• 获取第一个元素

public E getFirst() {
    final Node<E> f = first;//上面预先定义的头节点
    if (f == null)
        throw new NoSuchElementException();
    return f.item;
}

类似的也有

• 获取最后一个元素

public E getLast() {
    final Node<E> l = last;//预定义的尾节点
    if (l == null)
        throw new NoSuchElementException();
    return l.item;
}

• 删除头节点:首节点前驱是空的，然后把原有的item、next置为空,然后让首节点指向原来的第二个节点first.next,然后size-1，返回首节点

//实现Deque接口中的方法
public E removeFirst() {
    final Node<E> f = first;
    if (f == null)
        throw new NoSuchElementException();
    return unlinkFirst(f);
}
private E unlinkFirst(Node<E> f) {
    // assert f == first && f != null;
    final E element = f.item;
    final Node<E> next = f.next;
    f.item = null;
    f.next = null; // help GC
    first = next;
    if (next == null)
        last = null;
    else
        next.prev = null;
    size--;
    modCount++;
    return element;
}

类似的，删除尾节点的方法也就能理解了

public E removeLast() {
    final Node<E> l = last;
    if (l == null)
        throw new NoSuchElementException();
    return unlinkLast(l);
}
private E unlinkLast(Node<E> l) {
    ...
    size--;
    modCount++;
    return element;
}

• 增加头节点:思路，新建一个Node,前驱为空，后继是原有首节点

public void addFirst(E e) {
    linkFirst(e);
}
private void linkFirst(E e) {
    final Node<E> f = first;
    final Node<E> newNode = new Node<>(null, e, f);
    first = newNode;
    if (f == null)
        last = newNode;
    else
        f.prev = newNode;
    size++;
    modCount++;
}

类似的新增尾节点也能理解

public void addLast(E e) {
        linkLast(e);
}
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++;
}

• 判断函数是否包含某个元素:查找索引，能查到就true，否则false

public boolean contains(Object o) {
    return indexOf(o) != -1;
}
public int indexOf(Object o) {
    int index = 0;
    if (o == null) {
        for (Node<E> x = first; x != null; x = x.next) {
            if (x.item == null)
                return index;
            index++;
        }
    } else {
        for (Node<E> x = first; x != null; x = x.next) {
            if (o.equals(x.item))
                return index;
            index++;
        }
    }
    return -1;
}

• 添加结点:设置增添的结点前驱为原尾结点，后继为空；尾结点的next是新节点

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

对于addAll(Collection c)无非就是多次操作，原理相同

• 删除结点：先寻找这个结点，再把该节点的pre的next指向该节点的next,该节点的next的pre指向该节点的pre.

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) {
        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;
}

• 清空clear:清除所有的结点是非必须的，但是有助于GC回收释放内存

public void clear() {
    for (Node<E> x = first; x != null; ) {
        Node<E> next = x.next;
        x.item = null;
        x.next = null;
        x.prev = null;
        x = next;
    }
    first = last = null;
    size = 0;
    modCount++;
}

• 查找索引位置的元素.先判断是否合规：如果索引超出范围，抛出角标越界异常，否则进行从头开始的查找

public E get(int index) {
    checkElementIndex(index);
    return node(index).item;
}
Node<E> node(int index) {
    // assert isElementIndex(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;
    }
}

• 向指定位置添加元素.如果该位置为末端，则直接在末端添加；否则在添加位置+1节点处添加。

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

    if (index == size)
        linkLast(element);
    else
        linkBefore(element, node(index));
}