Java; casting base class to derived class

Question

Why can't I cast a base class instance to a derived class?

For example, if I have a class B which extends a class C, why can't I do this?

B b=(B)(new C()); 

or this?

C c=new C(); B b=(B)c; 

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Alright let me be more specific as to what I'm trying to do. Here's what I have:

public class Base(){     protected BaseNode n;     public void foo(BaseNode x){         n.foo(x);     } }   public class BaseNode(){     public void foo(BaseNode x){...} } 

Now I want to create a new set of classes which extend Base and Basenode, like this:

public class Derived extends Base(){     public void bar(DerivedNode x){         n.bar(x);//problem is here - n doesn't have bar     } }  public class DerivedNode extends BaseNode(){     public void bar(BaseNode){         ...     } } 

So essentially I want to add new functionality to Base and BaseNode by extending them both, and adding a function to both of them. Furthermore, Base and BaseNode should be able to be used on their own.

I'd really like to do this without generics if possible.

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Alright so I ended up figuring it out, partly thanks to Maruice Perry's answer.

In my constructor for Base, n is instantiated as a BaseNode. All I had to do was re-instantiate n as a DerivedNode in my derived class in the constructor, and it works perfectly.

Answer 1

because if B extends C, it means B is a C and not C is a B.

rethink what you are trying to do.