Question

Is the following java implementation of the visitor pattern using generics, general enough to be useful? (I suppose it is).

Could it be improved in some way? It’s important to be easily call-able using anonymous classes. Thanks.

(Example of use):

Vector<Number> numbers = new Vector<Number>();

        numbers.add(new Double(1.2));
        numbers.add(new Float(-1.2));
        numbers.add(new Double(4.8));
        numbers.add(new Float(-3.4));
        numbers.add(new Long(123456));
        numbers.add(new Short("14"));

        For.each(numbers, new Visitor<Number>() {
            public void doIt(Double n) {
                System.out.println("doIt() for double: " + n);
            }
            public void doIt(Float n) {
                System.out.println("doIt() for float: " + n);
            }
            public void doIt(Number n) {
                System.out.println("doIt() for Number: " + n);
            }
        });

        Visitor<Number> visi =  new Visitor<Number>() {
            private StringBuffer  all = new StringBuffer ();
            public void doIt(Number n) {
                System.out.println("doIt() for Number: " + n);
                all.append(n.toString() + " ");
            }
            public Object getResult () {
                return all;
            }
        };

        For.each(numbers, visi);

        System.out.println ("all -> " + visi.getResult());

Definitions:

//............................................
abstract class Visitor<T> {
    public void visit(T n) {
        try {
            this.getClass().getDeclaredMethod("doIt", n.getClass()).invoke(this, n);
        } catch (Exception ex) {
            doIt((T) n);
        }
    }
    public void doIt(T n) {
        System.out.println("doIt() for base " + n);
    }
    public Object getResult() {
        return null;
    }
} // class

//............................................
class For {
    public static <T> void each (Collection<T> c, Visitor<T> f) {
        for (T v : c) {
            f.visit(v);
        }
    } // ()
} // class

Answer 1

Thanks to the answer of donroby about my initial code not implementing the visitor pattern I came to this new version.

I suppose it now implements the visitor pattern without the need of modifying the visited element with an accept() method. Anyway, it is able to call the right method depending on the element type (I guess that’s the mission for the accept()), thanks to reflection.

First, an example of use:

Vector<Number> numbers = new Vector<Number>();

    numbers.add(new Double(1.2));
    numbers.add(new Float(-1.2));
    numbers.add(new Double(4.8));
    numbers.add(new Float(-3.4));
    numbers.add(new Long(123456));
    numbers.add(new Short("14"));

    For.each(numbers, new Visitor<Number>() {
        public void doIt(Double n) {
            System.out.println("doIt() for double: " + n);
        }
        public void doIt(Float n) {
            System.out.println("doIt() for float: " + n);
        }
        public void doIt(Number n) {
            System.out.println("doIt() for Number: " + n);
        }
    });

That produces this output

doIt() for double: 1.2
doIt() for float: -1.2
doIt() for double: 4.8
doIt() for float: -3.4
doIt() for Number: 123456
doIt() for Number: 14

And finally the code

abstract class Visitor<T> {
public void visit(T n) {
    try {
        this.getClass().getDeclaredMethod("doIt", n.getClass()).invoke(this, n);
    } catch (Exception ex) {
        doIt((T) n);
    }
}
public void doIt(T n) {
    System.out.println("doIt() for base " + n);
}
public Object getResult() {
    return null;
}

}

class For {
public static <T> void each (Collection<T> c, Visitor<T> f) {
    for (T v : c) {
        f.visit(v);
    }
} // ()

}