How to resolve 'Implicit super constructor classA() is not visible. Must explicitly invoke another constructor'?

Change the constructor visibility of ClassA from private to protected.

Constructors always begin by calling a superclass constructor. If the constructor explicitly contains a call to a superclass constructor, that constructor is used. Otherwise the parameterless constructor is implied. If the no-argument constructor does not exist or is not visible to the subclass, you get a compile-time error.

I would suggest composition instead of inheritance (maybe that's what the designer of ClassA intended for class usage. Example:

public class ClassB {
   private ClassA classA;

   ClassB() {
       // init classA
       ...
   }

   public ClassA asClassA() {
       return classA;
   }

   // other methods and members for ClassB extension
}

You can delegate methods from ClassB to ClassA or override them.

Java will implicitly create a constructor with no parameters for ClassB, which will call super(). In your case the constructor in ClassA is not visible, hence the error you are getting. Changing the visibility to public or protected will resolve the error.