获取list的某个字段

方法一：

例子：

List<Shoppingcart> list = shoppingcartDao.selList(userId);

Integer[] arr = new Integer[list.size()];//声明数组
 if (list != null && list.size() > 0) {
    for (int z = 0; z < arr.length; z++) {
       arr[z] = list.get(z).getCommodityId();
    }
    for (int z = 0; z < arr.length; z++) {
	System.out.println(arr[z]);
    }
}List<Shoppingcart> list = shopping