Calculator.java

import java.text.*;
import java.util.*;

// Calculator will only accept numbers of type T
// A calculator knows how to parse a given string
public class Calculator <T extends Number> {
    Stack<T> st = new Stack<T>();
    List<IOperator<T>> operators = new LinkedList<IOperator<T>>();

    Calculator (List<IOperator<T>> ops) {
	operators.addAll(ops);
    }

    private IOperator<T> getOperator(String token) {
	for (IOperator<T> op : operators) {
	    if (op.match_name(token))
		return op;
	}
	return null;
    }

    public void read (String expr) {
	Scanner sc = new Scanner(expr);
	String token;
	IOperator<T> op;

	while (sc.hasNext()) {
	    token = sc.next();
	    if ((op = getOperator(token)) == null) {
		op.apply(st);
		continue;
	    }

	    // Read type T
	    try {
		st.push( (T)NumberFormat.getInstance().parse(token));
	    } catch (ParseException e) {
		System.err.println("Unrecognized token.");
	    }
	}
    }
}