How to check if time format is correct in Java (with Exception)

Question

I'm working on a program that converts 24 hour time stamps into 12 hour time stamps. I managed to complete the conversion and loop it, but I need to code in an input validation that checks for incorrect inputs. An example of an incorrect input would be: "10:83" or "1):*2". Can someone show me how I can go about this using an Exception method? So far I have this:

public class conversion {

        public static void timeChange() throws Exception {
            System.out.println("Enter time in 24hr format");
            Scanner sc = new Scanner(System.in);
            String input1 = sc.nextLine();
            DateFormat df = new SimpleDateFormat("HH:mm");
            DateFormat df2 = new SimpleDateFormat ("hh:mm a");
            Date date = null;
            String timeOutput = null;

            date = df.parse(input1);
            timeOutput = df2.format(date);

            System.out.println("in 12 hour format: " + timeOutput);

            decision();
        }

        public static void decision() throws Exception {
            System.out.println("Would you like to enter another time?");

            Scanner sc2 = new Scanner(System.in);
            String userChoice = sc2.nextLine();

            while (userChoice.equalsIgnoreCase("Y")) {
                timeChange();
            }
            System.exit(0);
        }

        public static void main(String[] args) throws Exception {
            timeChange();       
    }
}

Answer 1

Use java.time the modern Java date and time API for this.

For a slightly lenient validation:

    String inputTimeString = "10:83";
    try {
        LocalTime.parse(inputTimeString);
        System.out.println("Valid time string: " + inputTimeString);
    } catch (DateTimeParseException | NullPointerException e) {
        System.out.println("Invalid time string: " + inputTimeString);
    }

This will accept 09:41, 09:41:32 and even 09:41:32.46293846. But not 10:83, not 24:00 (should be 00:00) and not 9:00 (requires 09:00).

For stricter validation use an explicit formatter with the format you require:

    DateTimeFormatter strictTimeFormatter = DateTimeFormatter.ofPattern("HH:mm")
            .withResolverStyle(ResolverStyle.STRICT);

And pass it to the parsemethod:

        LocalTime.parse(inputTimeString, strictTimeFormatter);

Now also 09:41:32 gets rejected.

Question: Can I use java.time with my Java version?

If using at least Java 6, you can.

• In Java 8 and later the new API comes built-in.
• In Java 6 and 7 get the ThreeTen Backport, the backport of the new classes (ThreeTen for JSR 310, where the new API was forst described).
• On Android, use the Android edition of ThreeTen Backport. It’s called ThreeTenABP, and there’s a thorough explanation in this question: How to use ThreeTenABP in Android Project.

For learning to use java.time, see the Oracle tutorial or find other resoureces on the net.