Class type injection in Spring xml vs java config

Question

Class X{
   Class<B> object;
   public setObject( Class<B> a ){....}
}

Interface B{}
Interface C extends B {}

I instantiate X like this,

X x = new X();
x.setObject( C.class );

When i build the code it complains required Class<B> found Class<C>. Since C extends B, can i not use C.class? If so can some one explain why?

I i do the same thing using Spring`s XML based bean creating it works just fine. The bean definition would be

<bean id="dummy" class="X">
<property name"object" value="C">
</bean>

This works just fine. Im not getting why instantiating in java would fail.

Answer 1

The same way a List<Dog> is not assignable to List<Animal>, Class<C> is not assignable to Class<B>, even if C is a subtype of B.

Using XML configuration, you are ignoring all type safety rules. It's as if every parametrized type usage was erased, as if you were using raw types. A Class<C> becomes a Class and a Class<B> becomes a Class. A Class is assignable to a Class.

Answer 2

It's normal that setObject(Class<B>) won't take a Class<C> parameter, for the same reason that someMethod(List<Object>) won't take a List<Integer> parameter: List<Integer> is not a subclass of List<Object>, even though Integer is a subclass of Object.

To make it work, you can change your method signature to this:

public setObject(Class<? extends B> a) {....}

Answer 3

this is not valid Java code

It works because there is type erasure done for parameters <>

the java bytecode looks exactly the same as the one for

class X{
   Class object;
   public void setObject( Class a ){....}
}

interface B{}
interface C extends B {}

It looses the type checks information, it is only used at compile time.

You can use this to make it work in the code:

class X{
   Class<? extends B> object;
   public void setObject( Class<? extends B> a ){....}
}

interface B{}
interface C extends B {}