LinkedList源码分析

LinkedList内部类，维持了一个节点的数据结构：

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;
	}
    }

    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;
    }

    private E remove(Entry<E> e) {
	if (e == header)
	    throw new NoSuchElementException();

        E result = e.element;
	e.previous.next = e.next;
	e.next.previous = e.previous;
        e.next = e.previous = null;
        e.element = null;
	size--;
	modCount++;
        return result;
    }

 其中，维持了该节点的前驱和后继。

 linkedList的构造方法：

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

    /**
     * Constructs an empty list.
     */
    public LinkedList() {
        header.next = header.previous = header;
    }

 创建了一个链表的头指针。

 我们在添加元素时，调用其add方法：

 代码如下：

public boolean add(E e) {
	addBefore(e, header);
        return true;
    }

 将header头指针传递给entry内部类的addbefore方法，其目的是使得将插入的新数据，放置在链表最后，并且将新数据的后继，指向头指针。

 

 另外，还存在一个内部类，封装了Entry这一数据结构的相关操作：

private class ListItr implements ListIterator<E> {
	private Entry<E> lastReturned = header;
	private Entry<E> next;
	private int nextIndex;
	private int expectedModCount = modCount;

	ListItr(int index) {
	    if (index < 0 || index > size)
		throw new IndexOutOfBoundsException("Index: "+index+
						    ", Size: "+size);
	    if (index < (size >> 1)) {
		next = header.next;
		for (nextIndex=0; nextIndex<index; nextIndex++)
		    next = next.next;
	    } else {
		next = header;
		for (nextIndex=size; nextIndex>index; nextIndex--)
		    next = next.previous;
	    }
	}

	public boolean hasNext() {
	    return nextIndex != size;
	}

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

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

	public boolean hasPrevious() {
	    return nextIndex != 0;
	}

	public E previous() {
	    if (nextIndex == 0)
		throw new NoSuchElementException();

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

	public int nextIndex() {
	    return nextIndex;
	}

	public int previousIndex() {
	    return nextIndex-1;
	}

	public void remove() {
            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) {
	    checkForComodification();
	    lastReturned = header;
	    addBefore(e, next);
	    nextIndex++;
	    expectedModCount++;
	}

	final void checkForComodification() {
	    if (modCount != expectedModCount)
		throw new ConcurrentModificationException();
	}
    }