java 反射 报错：Attempt to get java.lang.Integer field "..." with illegal data type conversion to int

类： Integer id;

反射时：

Field f = User.class.getDeclaredField("id");　

f.setAccessible(true);

int s = f.getInt(u);

这样会报错：

java.lang.IllegalArgumentException: Attempt to get java.lang.Integer field "..." with illegal data type conversion to int

改为：

int s = (Integer) f.get(u);