public boolean add(E e) {
 ensureCapacity(size + 1); // 增加元素，判断是否能够容纳。不能的话就要新建数组 
 elementData[size++] = e;
 return true;
}
 public void ensureCapacity(int minCapacity) {
 modCount++; 
 int oldCapacity = elementData.length;
 if (minCapacity > oldCapacity) {
  Object oldData[] = elementData; // 此行没看出来用处，不知道开发者出于什么考虑 
  int newCapacity = (oldCapacity * 3)/2 + 1; // 增加新的数组的大小 
  if (newCapacity < minCapacity)
  newCapacity = minCapacity;
   // minCapacity is usually close to size, so this is a win: 
   elementData = Arrays.copyOf(elementData, newCapacity);

 }

}

private void ensureCapacityHelper(int minCapacity) {
 int oldCapacity = elementData.length;
 if (minCapacity > oldCapacity) {
  Object[] oldData = elementData;
  int newCapacity = (capacityIncrement > 0) ?
  (oldCapacity + capacityIncrement) : (oldCapacity * 2);
  if (newCapacity < minCapacity) {
  newCapacity = minCapacity;
  }
   elementData = Arrays.copyOf(elementData, newCapacity);
 }
}