public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable ## 成员变量

 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
   transient Object[] elementData; // non-private to simplify nested class access
    private int size;

//显然是一个空数组
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
} ## size
    public int size() {
        //先检查一下修改次数
        checkForComodification();
        return this.size;
    } ## add
public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}

private void ensureCapacityInternal(int minCapacity) {
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));

private static int calculateCapacity(Object[] elementData, int minCapacity) {
    //如果是默认的空数组
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        //返回10
        return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    //返回传进来的数组容量
    return minCapacity;
}

 private void ensureExplicitCapacity(int minCapacity) {
       //记录修改次数
        modCount++;

        // overflow-conscious code
        //如果传进来的值大于底层数组的长度，扩容
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    } ## grow方法
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:
    //显然，要进行拷贝
    elementData = Arrays.copyOf(elementData, newCapacity);
}

 @SuppressWarnings("unchecked")
    public static <T> T[] copyOf(T[] original, int newLength) {
        return (T[]) copyOf(original, newLength, original.getClass());
    }
    
   public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
    @SuppressWarnings("unchecked")
    T[] copy = ((Object)newType == (Object)Object[].class)
        ? (T[]) new Object[newLength]
        : (T[]) Array.newInstance(newType.getComponentType(), newLength);
        //性能优良，建议多多使用
    System.arraycopy(original, 0, copy, 0,
                     Math.min(original.length, newLength));
    return copy;
}

//native方法，c++实现
    public static native void arraycopy(Object src,  int  srcPos,
                                        Object dest, int destPos,
                                        int length);