JDK源码学习系列05----LinkedList

                                         JDK源码学习系列05----LinkedList

1.LinkedList简介

LinkedList是基于双向链表实现的，它也可以被当作堆栈、队列或双端队列进行操作。

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable

LinkedList继承了AbstractSequentialList<E>，并实现了List<E>, Deque<E>, Cloneable, java.io.Serializable等接口。AbstractSequentialList 实现了get(int
index)、set(int index, E element)、add(int index, E element) 和 remove(int index)这些函数.

2.LinkedList的成员变量

 private transient Entry<E> header = new Entry<E>(null, null, null);
    private transient int size = 0;

LinkedList有两个成员变量，表头 header 和长度size.

3.LinkedList的构造函数

    public LinkedList() {//构造一个空链表
        header.next = header.previous = header;
    }

    public LinkedList(Collection<? extends E> c) {//将一个数据类型相同的集合添加到LinkedList的尾部
	this();
	addAll(c);
    }

4.LinkedList的内部类。

a.Entry<E>是LinkedList的节点类，节点类包含：当前节点的值,前一节点，后一节点。该节点类是一个静态内部类：当内部类对象不需要访问外围类对象时，应该声明为静态内部类。

 private static class Entry<E> {
	E element;
	Entry<E> next;
	Entry<E> previous;

	Entry(E element, Entry<E> next, Entry<E> previous) {
	    this.element = element;
	    this.next = next;
	    this.previous = previous;
	}
    }

b.ListItr 是 LinkedList 的迭代器类

 private class ListItr implements ListIterator<E> {
	private Entry<E> lastReturned = header;//上一次返回的节点
	private Entry<E> next;//下一节点
	private int nextIndex;//下一节点的索引
	private int expectedModCount = modCount;//!!!!!!期望的改变次数~ Java的 fail-fast 机制。

	ListItr(int index) {
	    if (index < 0 || index > size)
		throw new IndexOutOfBoundsException("Index: "+index+
						    ", Size: "+size);
	    if (index < (size >> 1)) {//size>>1是右移一位，即size/2 ,若索引值小于 size/2则从前开始
		next = header.next;
		for (nextIndex=0; nextIndex<index; nextIndex++)
		    next = next.next;
	    } else {//否则从后开始
		next = header;
		for (nextIndex=size; nextIndex>index; nextIndex--)
		    next = next.previous;
	    }
	}

	public boolean hasNext() {//是否存在下一个元素
	    return nextIndex != size;//通过下一节点索引值是否等于size来判断是否到了最末尾
	}

	public E next() {
	    checkForComodification();
	    if (nextIndex == size)//!!
		throw new NoSuchElementException();

	    lastReturned = next;
	    next = next.next;
	    nextIndex++;
	    return lastReturned.element;
	}

	public boolean hasPrevious() {//是否存在上一个
	    return nextIndex != 0;//通过下一节点的索引值是否等于0来判断是否在最前面即头节点，由此来判断是否有前节点
	}

	public E previous() {//取得上一元素
	    if (nextIndex == 0)
		throw new NoSuchElementException();

	    lastReturned = next = next.previous;  //??????
	    nextIndex--;
	    checkForComodification();
	    return lastReturned.element;
	}

	public int nextIndex() {
	    return nextIndex;
	}

	public int previousIndex() {//上一元素的索引
	    return nextIndex-1;
	}

	public void remove() {//删除当前节点！！
            checkForComodification();
            Entry<E> lastNext = lastReturned.next;
            try {
                LinkedList.this.remove(lastReturned);
            } catch (NoSuchElementException e) {
                throw new IllegalStateException();
            }
	    if (next==lastReturned)
                next = lastNext;
            else
		nextIndex--;
	    lastReturned = header;
	    expectedModCount++;
	}

	public void set(E e) {
	    if (lastReturned == header)
		throw new IllegalStateException();
	    checkForComodification();
	    lastReturned.element = e;
	}

	public void add(E e) {//讲e添加到当前节点前面
	    checkForComodification();
	    lastReturned = header;
	    addBefore(e, next);
	    nextIndex++;
	    expectedModCount++;
	}

	final void checkForComodification() {//!!!!!判断 modCount是否等于 expectedModCount来实现fail-fast机制。
	    if (modCount != expectedModCount)
		throw new ConcurrentModificationException();
	}
    }

c.

 private class DescendingIterator implements Iterator {
        final ListItr itr = new ListItr(size());
	public boolean hasNext() {
	    return itr.hasPrevious();
	}
	public E next() {
            return itr.previous();
        }
	public void remove() {
            itr.remove();
        }
    }

5.LinkedList的成员函数

由于LinkedList的成员函数很多，就不单独每一个做为一部分，把一些类似的函数放在一起，经常被调用的比较复杂的函数再单独介绍。

a.

   public E getFirst() {//取得第一个节点的值
	if (size==0)
	    throw new NoSuchElementException();//时刻注意特殊情况的考虑
	return header.next.element;
    }

    public E getLast()  {
	if (size==0)
	    throw new NoSuchElementException();
	return header.previous.element;//获得最后一个是 header.previous.element
    }

    public E removeFirst() {//移除第一个节点
	return remove(header.next);//remove函数下面单独介绍
    }

    public E removeLast() {//移除最后一个节点
	return remove(header.previous);
    }

    public void addFirst(E e) {//在
	addBefore(e, header.next);
    }

    public void addLast(E e) {
	addBefore(e, header);
    }

    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }

    public int size() {
	return size;
    }

    public boolean add(E e) {//在最末尾添加值为e的节点，添加在header前即最末尾
	addBefore(e, header);
        return true;
    }

b. boolean remove(Object o)    /    E remove()     /     E remove(Entry<E> e)

    public boolean remove(Object o) {
        if (o==null) {//即使是null也要查找到然后再移除
            for (Entry<E> e = header.next; e != header; e = e.next) {
                if (e.element==null) {
                    remove(e);//调用下面的方法
                    return true;
                }
            }
        } else {
            for (Entry<E> e = header.next; e != header; e = e.next) {
                if (o.equals(e.element)) {
                    remove(e);
                    return true;
                }
            }
        }
        return false;
    }
   private E remove(Entry<E> e) {
	if (e == header)
	    throw new NoSuchElementException();//考虑头指针的特殊情况

        E result = e.element;
	e.previous.next = e.next;//!!!
	e.next.previous = e.previous;//!!!
        e.next = e.previous = null;//  ???不是特别理解
        e.element = null;
	size--;
	modCount++;
        return result;
    }

c.boolean addAll(Collection<? extends E> c)  /    boolean addAll(int index, Collection<? extends E> c)

d.

 public void clear() {//清空LinkedList
        Entry<E> e = header.next;
        while (e != header) {
            Entry<E> next = e.next;
            e.next = e.previous = null;
            e.element = null;
            e = next;
        }
        header.next = header.previous = header;
        size = 0;
	modCount++;
    }

    public E get(int index) {//获得某索引对应的节点值
        return entry(index).element;
    }

    public E set(int index, E element) {//设置某索引的节点值
        Entry<E> e = entry(index);
        E oldVal = e.element;
        e.element = element;
        return oldVal;
    }

    public void add(int index, E element) {
        addBefore(element, (index==size ? header : entry(index)));
    }

    public E remove(int index) {//移除节点
        return remove(entry(index));
    }

e.Entry<E> entry(int index)

此方法得到某索引对应的节点对象

private Entry<E> entry(int index) {
        if (index < 0 || index >= size)
            throw new IndexOutOfBoundsException("Index: "+index+
                                                ", Size: "+size);
        Entry<E> e = header;
        if (index < (size >> 1)) {//若小于size/2，则从头遍历
            for (int i = 0; i <= index; i++)
                e = e.next;
        } else {//否则从尾遍历
            for (int i = size; i > index; i--)
                e = e.previous;
        }
        return e;
    }

f.

 public E peek() {//获取第一个元素的值
        if (size==0)
            return null;
        return getFirst();
    }

    public E element() {//获取第一个元素的值
        return getFirst();
    }

    public E poll() {//移除第一个元素
        if (size==0)
            return null;
        return removeFirst();
    }

    public E remove() {//移除第一个元素
        return removeFirst();
    }

    public boolean offer(E e) {
        return add(e);
    }

    public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }

    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }

    public E peekFirst() {
        if (size==0)
            return null;
        return getFirst();
    }

    public E peekLast() {
        if (size==0)
            return null;
        return getLast();
    }

    public E pollFirst() {
        if (size==0)
            return null;
        return removeFirst();
    }

    public E pollLast() {
        if (size==0)
            return null;
        return removeLast();
    }

    public void push(E e) {
        addFirst(e);
    }

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

g.Entry<E> addBefore(E e, Entry<E> entry)

 private Entry<E> addBefore(E e, Entry<E> entry) {
	Entry<E> newEntry = new Entry<E>(e, entry, entry.previous);
	newEntry.previous.next = newEntry;
	newEntry.next.previous = newEntry;
	size++;
	modCount++;
	return newEntry;
    }

h.

   public Object[] toArray() {
	Object[] result = new Object[size];
        int i = 0;
        for (Entry<E> e = header.next; e != header; e = e.next)
            result[i++] = e.element;
	return result;
    }

    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            a = (T[])java.lang.reflect.Array.newInstance(
                                a.getClass().getComponentType(), size);
        int i = 0;
	Object[] result = a;
        for (Entry<E> e = header.next; e != header; e = e.next)
            result[i++] = e.element;

        if (a.length > size)
            a[size] = null;

        return a;
    }

i.

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 (Entry e = header.next; e != header; e = e.next)
            s.writeObject(e.element);
    }

    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();

        // Initialize header
        header = new Entry<E>(null, null, null);
        header.next = header.previous = header;

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

5.方法归类

a.LinkedList可以作为FIFO(先进先出)的队列，作为FIFO的队列时，下表的方法等价：

队列方法       等效方法

add(e)        addLast(e)

offer(e)      offerLast(e)

remove()      removeFirst()

poll()        pollFirst()

element()     getFirst()

peek()        peekFirst()

b.LinkedList可以作为LIFO(后进先出)的栈，作为LIFO的栈时，下表的方法等价：

栈方法        等效方法

push(e)      addFirst(e)

pop()        removeFirst()

peek()       peekFirst()

6.总结

a.LinkedList是以双链表的形式实现的。

b.LinkedList即可以作为链表，还可以作为队列和栈。

c.LinkedList是 非 线程安全的。