LinkedList源码阅读笔记

初始化

• 无参的

public LinkedList() {
    }
           

• 初始化的同时添加一个Collection

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

• addAll(c)方法待会在add的时候会讲。

成员变量们

transient int size = ;
transient Node<E> first;
transient Node<E> last;
           

Node类

他是LinkedList的内部private类。

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;
        }
    }
           

看得出他可以向前指，也可以向后指。

增加

有 add，addAll，addFirst，addLast

add（E e）

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

直接默认添加在link尾部。

• linkLast(E e)

/**
     * Links e as last element.
     */
    void linkLast(E e) {
        final Node<E> l = last;//final域指向last节点
        final Node<E> newNode = new Node<>(l, e, null);//构造新节点作为最后一个节点，next = null
        last = newNode;//设置新节点为最后一个节点
        if (l == null)//如果这是一个空链表（null==first==last）
            first = newNode;
        else//不是空链表，让原last的next指针指向新节点
            l.next = newNode;
        size++;
        modCount++;//增加list结构改变次数，用于规避并发产生的错误
    }
           

add(int index, E element)

根据索引添加，跟ArrayList差不多，index从0开始。

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

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

• 首先调用checkPositionIndex检查是否越界。

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

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

就是很平常的跟size和0比较。但是请注意，index是可以 == size的。

• 然后如果index == size ，表示我想在最后一个元素的后面添加e。那就直接调用刚刚的linkLast就可以了。
• 如果不是就需要调用node(index)和void linkBefore(E e, Node succ) {}了。
  • node(int index)

  /**
   * Returns the (non-null) Node at the specified element index.
   */
  Node<E> node(int index) {
      // assert isElementIndex(index);
  
      if (index < (size >> )) {
          Node<E> x = first;
          for (int i = ; i < index; i++)
              x = x.next;
          return x;
      } else {
          Node<E> x = last;
          for (int i = size - ; i > index; i--)
              x = x.prev;
          return x;
      }
  }
             

  通过双向节点的好处和分为两段遍历减少遍历量提高性能。注意这个地方用不成二分法（二分法需要数组的下标来和查询的index比较）。

  如果是前半段就从0往后遍历，直到index，返回即可。

  如果是后半段就从后往前遍历，直到index，返回即可。

  • linkBefore(E e, Node succ)

  /**
   * Inserts element e before non-null Node succ.
   */
  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++;
  }
             

  • 需要四步：pred.next = newNode;nowNode.prev=pred;newNode.next=succ;succ.prev=newNode;
  • 其中关于设置newNode的前后指针的在new Node<>(pred,e,succ);完成。

批量增加 boolean addAll(Collection

/**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the specified
     * collection's iterator.  The behavior of this operation is undefined if
     * the specified collection is modified while the operation is in
     * progress.  (Note that this will occur if the specified collection is
     * this list, and it's nonempty.)
     *
     * @param c collection containing elements to be added to this list
     * @return {@code true} if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }
           

批量增加 boolean addAll(int index, Collection

/**
     * Inserts all of the elements in the specified collection into this
     * list, starting at the specified position.  Shifts the element
     * currently at that position (if any) and any subsequent elements to
     * the right (increases their indices).  The new elements will appear
     * in the list in the order that they are returned by the
     * specified collection's iterator.
     *
     * @param index index at which to insert the first element
     *              from the specified collection
     * @param c collection containing elements to be added to this list
     * @return {@code true} if this list changed as a result of the call
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        checkPositionIndex(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew == )
            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;//J2SE 提供的最后一个批注是 @SuppressWarnings。该批注的作用是给编译器一条指令，告诉它对被批注的代码元素内部的某些警告保持静默。 
            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;
    }
           

• 总体思路就是检查越界，被插入节点succ，被插入节点的上一个节点pred，注意要考虑到在link尾部插入的情况（当index==size就没有succ了），然后遍历c.toArray()，让一个个数据链接到pred.next（注意要考虑到在空link中插入collection），遍历完成后把succ和最新的pred链接起来。

删除

返回并删除第一个元素poll()

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

• unlinkFirst(f)

/**
     * Unlinks non-null first node 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;
    }
           

从此列表所表示的堆栈处弹出一个元素pop()

/**
     * Pops an element from the stack represented by this list.  In other
     * words, removes and returns the first element of this list.
     *
     * <p>This method is equivalent to {@link #removeFirst()}.
     *
     * @return the element at the front of this list (which is the top
     *         of the stack represented by this list)
     * @throws NoSuchElementException if this list is empty
     * @since 1.6
     */
    public E pop() {
        return removeFirst();
    }
           

返回并删除第一个元素

- removeFirst():

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

remove == removeFirst

删除第一个出现的元素removeFirstOccurrence(Object o) == remove(Object o)

• remove(Object o)

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;
    }
           

可以看出LinkedList是支持元素为空的。

为null和非null分开unlink()

- unlink(Node node)

/**
     * Unlinks non-null node x.
     */
    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;
    }
           

移除最后一次出现的指定元素（从头部到尾部遍历列表时）removeLastOccurrence

public boolean removeLastOccurrence(Object o) {
        if (o == null) {
            for (Node<E> x = last; x != null; x = x.prev) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = last; x != null; x = x.prev) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }
           

因为有prev指针的存在，所以跟removeFirstOccurrence差不多。

修改 set(int index, E element)

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

检查越界，通过node(int index)找到该节点，修改该节点。

查询 get

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

emmmmmmm 还是node(int index)