Question

Put simply: I want the following code to print “sub”:

Element e = new SubElement();
print(e);
... 

private static void print(Element e) {
    System.out.println("e");
}

private static void print(SubElement e) {
    System.out.println("sub");
}

and i dont want to change print(Element e). so nothing like

private static void print(Element e) {
    if (e instanceof SubElement) {
        print((SubElement) e);
    } else {
        System.out.println("e");
    }
}

what i would like to do is

print(e.getClass().cast(e));

to automatically cast it to the real subclass and force the system to enter print(SubElement e). is this somehow possible?

Answer 1

Yes. You can use Visitor pattern. However it’s suitable for stablished well-defined hierarchies, because the Visitor interface you have to define needs a method for each type.

interface ElementVisitor {
   visit(Element e);
   visit(SubElement se);
}

class ElementerPrinter implements ElementVisitor {
   visit(Element e) { System.out.println("e"); }
   visit(SubElement e) { System.out.println("sub"); }
}

class Element {
  // here's the trick, Element knows that this is Element
  // and childs have to implement it!
  // if the parent-most class is an interface it force you to implement!
  accept(ElementVisitor visitor) { visitor.visit(this); } 
}

class SubElement {
  // here's the trick, Element knows that this is SubElement
  accept(ElementVisitor visitor) { visitor.visit(this); }
}