code_coverage_13.java

public class CalculatorImpl implements Calculator {
  public CalculatorImpl() {
  }
  
  @Override
  public double process(String expression) {
    String[] tokens = expression.split(" ");
    Deque<String> operators = new ArrayDeque<>();
    Deque<Double> numbers = new ArrayDeque<>();
    try {
      for (String token : tokens) {
        switch (token) {
          case "+":
          case "-":
          case "/":
          case "*":
            while (shouldEvaluate(token, operators.peekFirst())) {
              evaluate(operators, numbers);
            }
            operators.push(token);
            break;
          case "(":
            operators.push(token);
            break;
          case ")":
            for (String op = operators.peekFirst(); !op.equals("("); op = operators.peekFirst()) {
              evaluate(operators, numbers);
            }
            operators.pop();
            break;
          default:
            double d = Double.parseDouble(token);
            numbers.push(d);
            break;
        }
      }
      for (String op = operators.peekFirst(); op != null; op = operators.peekFirst()) {
        evaluate(operators, numbers);
      }
    } catch (Exception e) {
      throw new CalculatorException("Invalid expression: " + expression, e);
    }
    double result = numbers.pop();
    if (numbers.size() > 0) {
      throw new CalculatorException("Invalid expression: " + expression);
    }
    return result;
  }

  private void evaluate(Deque<String> operators, Deque<Double> numbers) {
    String op = operators.pop();
    double second = numbers.pop();
    double first = numbers.pop();
    double result = getResult(op, second, first);
    numbers.push(result);
  }

  private double getResult(String op, double second, double first) {
    double result;
    switch (op) {
      case "+":
        result = first + second;
        break;
      case "-":
        result = first - second;
        break;
      case "*":
        result = first * second;
        break;
      case "/":
        result = first / second;
        break;
      default:
        throw new CalculatorException("Unexpected operator " + op);
    }
    return result;
  }

  private boolean shouldEvaluate(String newOp, String topOp) {
    if (topOp == null || topOp.equals("(")) {
      return false;
    }
    // with 4 standard operators, the only time you don't evaluate is
    // when the new operator is a * or / and the top operator is a + or -
    // topOp newOp shouldEvaluate
    // - - - - - - - - - - - - - 
    // +, - +, - true
    // *, / +, - true
    // +, - *, / false
    // *, / *, / true
    if ((topOp.equals("+") || topOp.equals("-")) && (newOp.equals("*") || newOp.equals("/"))) {
      return false;
    }
    return true;
  }
}