我可以在Java中将扩展类中的可变类型强制为子类型吗？

Question

Having this:

public class A{}
public class B extends A{}
public class C{
    protected A x;
}

Is there any way to create a class D which extends C such that the type of x must be B in all D's instances? Note that C is a class I'm writing, so I could edit it on purpose to accomplish D needs.

Answer 1

Use generic type

public class C<T>{
    protected T x;
}

public class D extends C<SomeConcreteType>{
     //x will be SomeConcreteType
}