ArrayList(int initialCapacity)：指定容量
ArrayList()：构造一个默认容量为10的ArrayList
ArrayList(Collection<? extends E> c)：构造一个指定Collection的ArrayList

 // 添加指定元素到list末尾
  public boolean add(E e) {
    // 检查是否需要扩容
    ensureCapacityInternal(size + 1); // Increments modCount!!
    // 赋值
    elementData[size++] = e;
    return true;
  }
  private void ensureCapacityInternal(int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
      // 比较默认的容量10和传入的容量，返回大点的数
      minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    //扩容
    ensureExplicitCapacity(minCapacity);
  } 
  private void ensureExplicitCapacity(int minCapacity) {
    // 记录修改次数
    modCount++;
    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
      grow(minCapacity);
  }
  private void grow(int minCapacity) {
    // overflow-conscious code
    // 记录当前list的容量
    int oldCapacity = elementData.length;
    // 扩展为原来的1.5倍
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    // 如果扩展1.5倍还不能满足，直接扩展为需求值
    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);
  }

  public E get(int index) {
    rangeCheck(index);
    return elementData(index);
  }
  public E set(int index, E element) {
    rangeCheck(index);
    E oldValue = elementData(index);
    elementData[index] = element;
    return oldValue;
  }

public E remove(int index) {
  rangeCheck(index);
  modCount++;
  E oldValue = elementData(index);
  int numMoved = size - index - 1;
  if (numMoved > 0)
    System.arraycopy(elementData, index+1, elementData, index,
             numMoved);
  elementData[--size] = null; // clear to let GC do its work
  return oldValue;
}