public class Numbers
{
   /**
      Combines two numbers with a given operator.
      @param op one of "+", "-", "*", "/", or "^" (for power)
      @param first the first argument
      @param second the second argument
      @return the result of evaluating the operator with the arguments
   */
   public static double evaluate(String op, double first, double second)
   {
      switch(op)
      {
      case "+":
         return first + second;
      case "-":
         return first - second;
      case "*":
         return first * second;
      case "/":
         return first / second;
      case "^":
         return Math.pow(first, second);
      }
      return -1.0;
   }
}