how to convert seconds into hhmmss

Using the old java date api (not recommended, see comments):

int sec = .... //
Date d = new Date(sec * 1000L);
SimpleDateFormat df = new SimpleDateFormat("HH:mm:ss"); // HH for 0-23
df.setTimeZone(TimeZone.getTimeZone("GMT"));
String time = df.format(d);

See also SimpleDateFormat.

Note: as per comments, if the number of seconds exceeds the number of seconds in a day (86400) that won't work as expected. In that case a different approach must be taken.

EDIT: If you're using JDK 8 you can write:

int sec = ...
Duration duration = Duration.ofSeconds(sec);

This duration object better represents what you're talking about. I am trying to find out how to format it as you want to but I have had no luck so far.

EDIT 2: prior to JDK 8 you can use the Joda API:

int sec = ...
Period period = new Period(sec * 1000L);
String time = PeriodFormat.getDefault().print(period); // you can customize the format if this one doesn't cut it

That's probably the most elegant solution. See also this.

EDIT 3: As per comments, I explicitly added the time zone.

Just in case you're looking to write your own algorithm for this:

Let's say we have 1000 seconds.

We know that there're 3600 seconds in an hour, so when we format this time as hh:mm:ss, the hh field will be 00. Now let's say we're given a time of 3700 seconds. This time interval is slightly larger than 1 hour, so we know that the hh field will show 01.

So to calculate the number for the hh field, simply divide the provided seconds amount by 3600.

int hours = seconds / 3600

Note that when we have a seconds amount greater than 3600, the result is truncated, so we're left with an integer amount for the hours.

Moving on to the mm field. Again, let's assume we're given a time interval of 3700 seconds. We already know that 3700 seconds is slightly more than 1 hour - we've stored the number of hours in the hour field. To calculate the number of minutes, we'll subtract the hours times 3600 from the provided seconds input:

int minutes = (seconds - hours * 3600) / 60

So if we have a provided time of 3700 seconds, the above code translates to (3700 - 3600) / 60 - we divide by 60 because we want to convert from seconds to minutes.

Finally, the ss field. We use a similar technique as above to calculate the number of seconds.

int seconds = (seconds - hours * 3600) - minutes * 60

public static String formatSeconds(int timeInSeconds)
{
    int hours = timeInSeconds / 3600;
    int secondsLeft = timeInSeconds - hours * 3600;
    int minutes = secondsLeft / 60;
    int seconds = secondsLeft - minutes * 60;

    String formattedTime = "";
    if (hours < 10)
        formattedTime += "0";
    formattedTime += hours + ":";

    if (minutes < 10)
        formattedTime += "0";
    formattedTime += minutes + ":";

    if (seconds < 10)
        formattedTime += "0";
    formattedTime += seconds ;

    return formattedTime;
}

    public static void formatSeconds(Integer result){

      System.out.print(String.format("%02d",result/3600)+":");
      System.out.print(String.format("%02d",result/60%60)+":");
      System.out.println(String.format("%02d",result%60));

     }