/** Abstract class for all expressions */
abstract class Expr {
    abstract <T> T accept(Visitor<T> v);
}

class Number extends Expr {
    protected int n;

    public Number(int n) { this.n = n; }

    public <T> T accept(Visitor<T> v) {
	return v.visitNumber(this.n);
    }
}

class Sum extends Expr {
    protected Expr left, right;

    public Sum(Expr left, Expr right) {
	this.left = left;
	this.right = right;
    }

    public <T> T accept(Visitor<T> v) {
	return v.visitSum(left.accept(v), right.accept(v));
    }
}


/** Abstract class for all visitors */
abstract class Visitor<T> {
    abstract T visitNumber(int n);
    abstract T visitSum(T left, T right);
}

/** Example visitor to convert an Expr to a String */
class ToString extends Visitor<String> {
    public String visitNumber(int n) { 
	return "" + n;
    }
    public String visitSum(String left, String right) {
	return "(" + left + " + " + right + ")";
    }
}


public class ExprVisitor { 
    public static void main(String args[]) { 
	Expr e = new Sum(new Number(3), new Number(2)); 
	ToString printer = new ToString();
	String stringRep = e.accept(printer); 
	System.out.println(stringRep);
    }

}