// Illustrate covariant return type
public class Test {

  public static void main(String[] args) {

    B b = new B();
    System.out.println(b.m()); // I'm a Sub instance
  }
}

class Base { // a base class
}

class Sub extends Base { // a sub class

  public String toString() {
    return "I'm a Sub instance";
  }
}

class A {

  Base m() {  // covariant return type
    return new Base();
  }
}

class B extends A {

  Sub m() { // sub is 'narrower' than base
    return new Sub();
  }

}