Trying to create 1 interface and 2 concrete classes inside a Parent class. This will qualify the enclosing classes to be Inner classes.
public class Test2 {
Here the inner class is not static, so you need to create an instance of outer class and then invoke new,
A a = new Test2().new C();
But in this case, you can make the inner class static,
static class C extends B implements A
then it's ok to use,
A a = new C()