code: private double doubleValue( double left, double right ) {
          switch( operator ) {
              case '+':
                  return left + right;
              case '-':
                  return left - right;
              case '*':
                  return left * right;
              case '/':
                  return left / right;
              default:
                  throw createException( "Not supported Oprator '" + operator + "' for Expression '" + toString() + '\'' );
          }
      }
docs: Calculate the number value of two operands if possible.
  
  @param left the left
  @param right the right
  @return the result.