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ArryList实现接口List, RandomAccess, Cloneable, java.io.Serializable
基础的数据结构是数组，根据存储数据的多少动态扩容。

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/**
 * Default initial capacity.
 */
private static final int DEFAULT_CAPACITY = 10;

/**
 * Shared empty array instance used for empty instances.
 */
private static final Object[] EMPTY_ELEMENTDATA = {};

/**
 * Shared empty array instance used for default sized empty instances. We
 * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
 * first element is added.
 */
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

/**
 * The array buffer into which the elements of the ArrayList are stored.
 * The capacity of the ArrayList is the length of this array buffer. Any
 * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
 * will be expanded to DEFAULT_CAPACITY when the first element is added.
 */
transient Object[] elementData
/**
 * The size of the ArrayList (the number of elements it contains).
 *
 * @serial
 */
private int size;

elementData是实际存储数据的数组。DEFAULT_CAPACITY是默认的容量，如果没有指定数组的大小。DEFAULTCAPACITY_EMPTY_ELEMENTDATA 与EMPTY_ELEMENTDATA都是 空的初始数组，DEFAULTCAPACITY_EMPTY_ELEMENTDATA是用来判断是不是通过默认方法创建的空数组，而不是删除后才空的。如果指定了数组大小，就用EMPTY_ELEMENTDATA作为默认空数组。

扩容是在添加元素的时候进行，这样不会初始化没有数据的情况下就占据空间。

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/**
 * Appends the specified element to the end of this list.
 *
 * @param e element to be appended to this list
 * @return <tt>true</tt> (as specified by {@link Collection#add})
 */
public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}

添加方法很简单，先去内部扩容，然后在最后一个添加值，看扩容的内部实现。

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private void ensureCapacityInternal(int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
    }

    ensureExplicitCapacity(minCapacity);
}

private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}
/**
 * Increases the capacity to ensure that it can hold at least the
 * number of elements specified by the minimum capacity argument.
 *
 * @param minCapacity the desired minimum capacity
 */
private void grow(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length;
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    if (newCapacity - minCapacity < 0)
        newCapacity = minCapacity;
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
    // minCapacity is usually close to size, so this is a win:
    elementData = Arrays.copyOf(elementData, newCapacity);
}

ensureCapacityInternal 先是一个判断数组是怎么创建，如果是默认空参数的构造器创建，就使用默认的初始大小10作为最小容量，这样就能避免不能手动创建更小数组的问题。
主要扩容是grow方法，可以看出 newCapacity是扩容0.5倍大小，到原来的1.5倍。
还有就是容器上下限的判断。扩容就是迁移数组newCapacity的大小的数组里。
归根到底，就是将数组扩容1.5倍，旧数组搬迁到新数组。
里面还有几个内部类，一个Itr，实现了Iterator接口，

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private class Itr implements Iterator<E> {
    int cursor;       // index of next element to return
    int lastRet = -1; // index of last element returned; -1 if no such
    int expectedModCount = modCount;

ListItr 实现了 ListIterator，可以前后访问遍历

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private class ListItr extends Itr implements ListIterator<E> {
    ListItr(int index) {
        super();
        cursor = index;
    }

ArrayList使用遍历接口，都是使用内部类实现的，重新记录状态，这部分还有些不太懂的，后面继续补充。

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