[hardwired]

All user-defined interfaces are subtypes of the type Interface.
Interfaces
Using an Interface as a Type
A method call is sent to an object and the method of the actual type is executed.
Defining Methods
A data model specifies the user interface.
Models are usually designed to function independently of user interfaces.
If classes B and C are subclasses of A, then any state variable of A is also a state variable of B and C.
Controlling Access to Members of a Class

Code:

public class InheritanceTest {
    public static void main(String[] args) {
        A a = new A();
        System.out.println("a = " + a);
        B b = new B();
        System.out.println("b = " + b);
        C c = new C();
        System.out.println("c = " + c);
    }
}

class A {
    public    String fieldOne   = "field one";
    protected String fieldTwo   = "field two";
    private   String fieldThree = "field three";
              String fieldFour  = "field four";

    public String toString() {
        String s = getClass().getName();
        return s + "[fieldOne:"   + fieldOne +
                   " fieldTwo:"   + fieldTwo +
                   " fieldThree:" + fieldThree +
                   " fieldFour:"  + fieldFour + "]";
    }
}

class B extends A {
    public String toString() {
        return "B[fieldOne:"   + fieldOne +
                " fieldTwo:"   + fieldTwo +
//                " fieldThree:" + fieldThree +
                " fieldFour:"  + fieldFour + "]";
    }
}

class C extends A {
    public String toString() {
        return super.toString();
    }
}