Postfix expression evaluation

Algorithm:

for each expression token:
  if the token is a number:
    push it
  else (the token is an operator, OP):
    second = pop();
    first = pop();
    compute: result = first OP second
    push the result on the stack

when there are no more tokens, pop the answer off the stack

4 2 3 - -

token    stack
-----    -----
         []
  4      [4]
  2      [2, 4]
  3      [3, 2, 4]
  -      [-1, 4]
  -      [5]
 END     []    ⇒ answer = 5

public static Double evaluatePostfix(String rpn) {
		char[] exp = rpn.toCharArray();
		Stack<Double> stack = new Stack<Double>();

		int len = exp.length;
		for (int i = 0; i < len; i++) {
			if (!isOperator(exp[i])) {
				stack.push(Double.parseDouble("" + exp[i]));
			} else {
				double b = stack.pop();
				double a = stack.pop();
				stack.push(evaluate(a, b, exp[i]));
			}
		}
		double result = stack.pop();
		System.out.println("Result:" + result);
		return result;
	}

	private static Double evaluate(double a, double b, char operator) {
		double result = 0.0;
		switch (operator) {
		case '/':
			result = a / b;
			break;
		case '*':
			result = a * b;
			break;
		case '+':
			result = a + b;
			break;
		case '-':
			result = a - b;
			break;
		}
		return result;
	}