一 ArrayList

         1.  arraylist里面是通过数组实现的

 /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer.
     */
    private transient Object[] elementData;

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

public ArrayList(int initialCapacity) {
	super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
	this.elementData = http://www.mamicode.com/new Object[initialCapacity];>
         3. arraylist添加元素的时候，需要判断存放元素的数组是否需要扩容（扩容大小是原来大小的1/2+1）
public boolean add(E e) {
	ensureCapacity(size + 1);  // Increments modCount!!
	elementData[size++] = e;
	return true;
    }

 public void ensureCapacity(int minCapacity) {
	modCount++;
	int oldCapacity = elementData.length;
	if (minCapacity > oldCapacity) {
	    Object oldData[] = elementData;
	    int newCapacity = (oldCapacity * 3)/2 + 1;
    	    if (newCapacity < minCapacity)
		newCapacity = minCapacity;
            // minCapacity is usually close to size, so this is a win:
            elementData = http://www.mamicode.com/Arrays.copyOf(elementData, newCapacity);>
        4. arraylist 在指定位置添加元素或者移除指定位置元素，需要移动比较多的数据
public void add(int index, E element) {
	if (index > size || index < 0)
	    throw new IndexOutOfBoundsException(
		"Index: "+index+", Size: "+size);

	ensureCapacity(size+1);  // Increments modCount!!
	System.arraycopy(elementData, index, elementData, index + 1,
			 size - index);
	elementData[index] = element;
	size++;
    }

    public E remove(int index) {
	RangeCheck(index);

	modCount++;
	E oldValue = http://www.mamicode.com/(E) elementData[index];>
二.  LinkedList
          1. linkedlist是通过链表实现的，一个节点是一个对象，包含了自己前一个节点的和后一个节点的地址
private transient Entry<E> header = new Entry<E>(null, null, null);
    private transient int size = 0;

 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;
	}
    }
         2. 添加元素是通过移动链表指针
public boolean add(E e) {
	addBefore(e, header);
        return true;
    }
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;
    }
         3. 删除元素也是通过移动指针，所以理论上linkedlist比arraylist更适合添加和删除操作（不需要判断是否需要扩容，不需要移动大批的元素）
 public boolean remove(Object o) {
        if (o==null) {
            for (Entry<E> e = header.next; e != header; e = e.next) {
                if (e.element==null) {
                    remove(e);
                    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();

        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;
    }
         4. 获取元素需要遍历链表，比arraylist要慢
public E get(int index) {
        return entry(index).element;
    }
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)) {//size 右移一位表示除以2，其实就是看从前遍历快还是从后遍历快
            for (int i = 0; i <= index; i++)
                e = e.next;
        } else {
            for (int i = size; i > index; i--)
                e = e.previous;
        }
        return e;
    }
          5. linkedlist 迭代器，支持向前向后迭代，在迭代过程中移除元素。如果要遍历linkedlist，建议使用迭代器，因为如果使用get方法效率没迭代器高。（从get方法的源码可以看出）
 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) { //支持从指定位置开始迭代
	    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() {//是否有下一个
	    return nextIndex != size;
	}

	public E next() { //下一个
	    checkForComodification();
	    if (nextIndex == size)
		throw new NoSuchElementException();

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

	public boolean hasPrevious() { //是否有前一个
	    return nextIndex != 0;
	}

	public E previous() { //前一个
	    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() { //删除元素
            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();
	}
    }




java ArrayList与LinkedList知识点