sumtype on java way

_1.java

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);
}

_2.java

Base base = Base.createA(...);
Integer n = base.accept(new Base.Visitor<Integer> {
  public Integer visit(Base.A a) { return 1; }
  public Integer visit(Base.B b) { return 2; }
});