Trying To Implement Interface - The Method DoSomething Is Undefined For The Type B

Let's say we have situation like this:

abstract class A
class B extends A
class C extends B
class D extends C implements SomeInterface

I'm trying to implement a method "doSomething" declared in SomeInterface in class D. While trying to call doSomething in main I get the error message ”The method doSomething is undefined for the type B”

This is my code i main:

B container = new D("1,2,3,4,5,6,7,8");
System.out.println(container.doSomething());

I need container to be an object of type B, because it goes later into a list of type B. According to what I've been told, the only file I need to edit to make this work is class D.