Expr. Visitor for calculations

public class EvalVisitor implements ExprVisitor<Integer>{
        private int res;
    private int x1;
    public EvalVisitor(int x) {
        this.x1 = x;
    }

    public int eval (Expr<Integer> e) {
        e.accept(this);         //recognize subexpression
        return res;         //update result of it on level above
    }

    public void visitNeg(Neg<Integer> e) {
        res = -eval(e.a());
    }

    public void visitBinary(Binary<Integer> e) {
        if (e.operation() == '+') {
            res = eval(e.a()) + eval(e.b());    //dig into operand
        } else if (e.operation() == '-') {          //to check if its subexpr
                   res = eval(e.a()) - eval(e.b());
               } else if (e.operation() == '*') {
                          res = eval(e.a()) * eval(e.b());
                      } else if (e.operation() == '/') {
                                 res = eval(e.a()) / eval(e.b());
                             }
    }

    public void visitVar(Var<Integer> e) {
        res = x1;   //cant dig deeper => get and return variable
    }

    public void visitConst(Const<Integer> e) {
        res = e.value();    // -||- => get and ret constant
    }
}

//========================================================================================

public class Test
{
        public static void main(String[] args)
        {
                ExprFactory<Integer> f = new ExprFactory<Integer>();

                Expr<Integer> e =
                        f.newBinary(
                                f.newBinary(
                                        f.newVar(),
                                        f.newConst(10),
                                        ’+’
                                ),
                                f.newVar(),
                                ’*’
                        );

                EvalVisitor v = new EvalVisitor(5);
                System.out.println(v.eval(e));
        }
}