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

類： Integer id;

反射時：

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

f.setAccessible(true);

int s = f.getInt(u);

這樣會報錯：

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

改為：

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