public abstract class Base {
  public static class A extends Base { ... }
  public static class B extends Base { ... }

  public static Base createA(...) { return new A(...); }
  public static Base createB(...) { return new B(...); }

  public static interface Visitor<T> {
    T visit(A a);
    T visit(B b);
  }

  public abstract <T> T accept(Visitor<T> visitor);
}