What is this error I am missing?

[ComputerJy]

Your code should be:

JOptionPane.showMessageDialog(null,
                                  "Your ending mileage," +
                                  Integer.toString(endMiles) +
                                  ", must be greater than your starting mileage," +
                                  Integer.toString(startMiles), "Error",
                                  JOptionPane.ERROR_MESSAGE);