compare format of time string in java

Question

I have two time strings that can be in any format(12 hours or 24 hours, with or without timezone). How do I compare if their format is different in java and if data mismatch is there?

PS> I have prepared a list of regex expressions and matching string with those expressions to get the format, then checking for data differences using equals() method of string. problem with this approach is (20:01:02,20 01 01) return format difference whereas the expected result should be data difference. Please help, I am stuck here for a long time.

map of regex expressions-

private static final Map<String, String> TIME_FORMAT_REGEXPS = new HashMap<String, String>() {{
    put("^(1[0-2]|0?[1-9]):([0-5]?[0-9])(●?[AP]M)?$", "1");
    put("^(2[0-3]|[01]?[0-9]):([0-5]?[0-9])$", "2");
    put("^(1[0-2]|0?[1-9]):([0-5]?[0-9]):([0-5]?[0-9])(●?[AP]M)?$", "3");
    put("^(2[0-3]|[01]?[0-9]):([0-5]?[0-9]):([0-5]?[0-9])$", "4");
    put("^(2[0-3]|[01][0-9]):?([0-5][0-9])$", "5");
    put("^(?<hour>2[0-3]|[01][0-9]):?(?<minute>[0-5][0-9])$", "6");
    put("^(2[0-3]|[01][0-9]):?([0-5][0-9]):?([0-5][0-9])$", "7");
    put("^(?<hour>2[0-3]|[01][0-9]):?(?<minute>[0-5][0-9]):?(?<second>[0-5][0-9])$", "8");
    put("^(Z|[+-](?:2[0-3]|[01][0-9])(?::?(?:[0-5][0-9]))?)$", "9");
    put("^(2[0-3]|[01][0-9]):?([0-5][0-9]):?([0-5][0-9])(Z|[+-](?:2[0-3]|[01][0-9])(?::?(?:[0-5][0-9]))?)$", "10");
    put("^(?<hour>2[0-3]|[01][0-9]):?(?<minute>[0-5][0-9]):?(?<second>[0-5][0-9])(?<timezone>Z|[+-]"
                    + "(?:2[0-3]|[01][0-9])(?::?(?:[0-5][0-9]))?)$",
            "11");

}};

function to check format of string-

private String determineTimeFormat(String dateString) {
    for (String regexp : TIME_FORMAT_REGEXPS.keySet()) {
        if (dateString.toLowerCase().matches(regexp)) {
            return TIME_FORMAT_REGEXPS.get(regexp);
        }
    }
    return "100"; // Unknown format.
}

Once you’ve determined the format of a string, parse it into a LocalTime object using an appropriate DateTimeFormatter for that format. Then compare the two obtained LocalTime objects for equality. Use your search engine for the details. (You may also use the formatters for determining the format and skip the regex.) — Anonymous
– Anonymous, Commented May 25, 2020 at 16:51
Under what circumstances would you consider that two times are the same time if one has UTC offset and the other one hasn’t? And if they have different offsets? — Anonymous
– Anonymous, Commented May 25, 2020 at 16:54

Alessandro Scarlatti · Accepted Answer · 2020-05-27 02:37:59Z

You can try something like this:

// create a formatter
DateTimeFormatter formatter = new DateTimeFormatterBuilder()
        .appendPattern("[dd/MM/yyyy HH:mm:ss]")  // Add as many optional patterns as required
        .appendPattern("[dd-MM-yyyy HH:mm:ss]")
        .appendPattern("[dd-MM-yyyy[ [HH[:mm][:ss]]]]")   // nested optional tokens specified with square brackets []
        .appendOptional(DateTimeFormatter.ISO_DATE_TIME)  // can use standard Java DateTimeFormatters as well
        .parseDefaulting(ChronoField.HOUR_OF_DAY, 0)      // supply default values for missing fields
        .parseDefaulting(ChronoField.MINUTE_OF_HOUR, 0)
        .parseDefaulting(ChronoField.SECOND_OF_MINUTE, 0)
        .toFormatter();

// each of these will parse successfully
LocalDateTime time1 = LocalDateTime.parse("2020-02-01T01:02:00+00:00", formatter);
LocalDateTime time2 = LocalDateTime.parse("01-02-2020 01:02:00", formatter);
LocalDateTime time3 = LocalDateTime.parse("01-02-2020 01:02", formatter);
LocalDateTime time4 = LocalDateTime.parse("01-02-2020", formatter);


// use .equals() method to compare times
assert time1.equals(time2);  // true
assert time2.equals(time3);  // true
assert time3.equals(time4);  // true

Assistance from this related answer https://stackoverflow.com/a/39685202/7174786

See the JavaDocs for additional DateTimeFormatter details: https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html

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