Calculator for Big Numbers

package example.biginteger;
 
import java.math.BigDecimal;
import java.util.Stack;
 
/** A trivial reverse-polish stack-based calculator for big numbers */
public class BigNumCalc {
 
    /** an array of Objects, simulating user input */
    public static Object[] testInput = { new BigDecimal("3419229223372036854775807.23343"), new BigDecimal("2.0"), "*", };
 
    public static void main(String[] args) {
	BigNumCalc calc = new BigNumCalc();
	System.out.println(calc.calculate(testInput));
    }
 
    Stack s = new Stack();
 
    public BigDecimal calculate(Object[] input) {
	BigDecimal tmp;
	for (int i = 0; i < input.length; i++) {
	    Object o = input[i];
	    if (o instanceof BigDecimal) {
		s.push(o);
	    } else if (o instanceof String) {
		switch (((String) o).charAt(0)) {
		// + and * are commutative, order doesn't matter
		case '+':
		    s.push(((BigDecimal) s.pop()).add((BigDecimal) s.pop()));
		    break;
		case '*':
		    s.push(((BigDecimal) s.pop()).multiply((BigDecimal) s.pop()));
		    break;
		// - and /, order *does* matter
		case '-':
		    tmp = (BigDecimal) s.pop();
		    s.push(((BigDecimal) s.pop()).subtract(tmp));
		    break;
		case '/':
		    tmp = (BigDecimal) s.pop();
		    s.push(((BigDecimal) s.pop()).divide(tmp, BigDecimal.ROUND_UP));
		    break;
		default:
		    throw new IllegalStateException("Unknown OPERATOR popped");
		}
	    } else {
		throw new IllegalArgumentException("Syntax error in input");
	    }
	}
	return (BigDecimal) s.pop();
    }
}