Question

In the code snippet below, why does 1 not produce a run-time Exception as I am trying to cast Class>B> to Class>A>?

package example;

Class A {
   public A() {
   }
}

Class B extends A {
   public B() {
   }
}

public static void main() {

    Class<A> c = null;

    //1. Does not produce exception at run-time even though I cast Class<B> to Class<A>
    try {
        c = (Class<A>) Class.forName("example.B");
    } catch (ClassNotFoundException e) {
    }

    //2. Compile time error: Cannot Cast Class<B> to Class<A>
    c = (Class<A>) B.class; //Error
}

Answer 1

Class.forName() returns Class<?>, which is roughly equivalent to Class (without generics).

The second version will also compile if you add another non-generic cast inbetween:

c = (Class<A>)(Class) B.class; // compiles

However, this can’t be right, so the compiler saves you from this error. In the first version, it can’t do that.