Time without date

Question

I get time with time zone (without date component) from a PostgreSQL server in json like this { "time": "03:00:00+01" }. How do I handle this in Android? Is there any structure which can hold just time without date? Or converting it to the epoch Date representation i.e. Thu Jan 01 03:00:00 GMT+01:00 1970 is the only good solution?

Answer 1

OffsetTime from java.time and ThreeTenABP

An OffsetTime is a time of day without date and with an offset from UTC. It thus very precisely models the information in your string from JSON. So I would clearly prefer it over Date. Also because the Date class is poorly designed and long outdated.

    String timeStringFromJson = "03:00:00+01";
    DateTimeFormatter timeFormatter = DateTimeFormatter.ofPattern("HH:mm:ssX");
    OffsetTime parsedTime = OffsetTime.parse(timeStringFromJson, timeFormatter);
    System.out.println("Parsed time: " + parsedTime);

Output from this snippet is:

Parsed time: 03:00+01:00

As a detail that may or may not matter to you, the offset from the string is retained, contrary to what Date can do because a Date hasn’t got a time zone or offset.

Question: Doesn’t java.time require Android API level 26?

java.time works nicely on both older and newer Android devices. It just requires at least Java 6.

• In Java 8 and later and on newer Android devices (from API level 26) the modern API comes built-in.
• In non-Android Java 6 and 7 get the ThreeTen Backport, the backport of the modern classes (ThreeTen for JSR 310; see the links at the bottom).
• On (older) Android use the Android edition of ThreeTen Backport. It’s called ThreeTenABP. And make sure you import the date and time classes from org.threeten.bp with subpackages.

Links

• Oracle tutorial: Date Time explaining how to use java.time.
• Java Specification Request (JSR) 310, where java.time was first described.
• ThreeTen Backport project, the backport of java.time to Java 6 and 7 (ThreeTen for JSR-310).
• ThreeTenABP, Android edition of ThreeTen Backport
• Question: How to use ThreeTenABP in Android Project, with a very thorough explanation.

Answer 2

You can parse your date into a Date object using the SimpleDateFormat with the following pattern:

SimpleDateFormat format = new SimpleDateFormat("HH:mm:ss");

try
    {
    Date date = format.parse("03:00:00+01", Locale.getDefault());
    Log.d( "debugDate", " date is: " + date );
    }
catch (ParseException e)
    {
    e.printStackTrace();
    }

Output:

Thu Jan 01 03:00:00 GMT+01:00 1970

In case your date structure become "03:00:00+01:00", you need to use this pattern :

SimpleDateFormat format = new SimpleDateFormat("HH:mm:ssZ", Locale.getDefault());

Output:

Thu Jan 01 03:00:00 GMT+01:00 1970

Answer 3

You can convert the String timestamp to the milliseconds since Epoch as a long value, then store/retrieve the long value and convert it back to a String timestamp as done below in main.

I'm assuming the +01 in your example is for Central European Time.

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Locale;
import java.util.TimeZone;

public class SDFTest {

    /**
    Global SimpleDateFormat
    */
    private static SimpleDateFormat sdf = null;

    public static void main(String[] args) {
        // Initialize the Simple Date Format;
        sdf = new SimpleDateFormat("HH:mm:ssX");
        // Ignore this if you need timestamps converted to your local TimeZone;
        sdf.setTimeZone(TimeZone.getTimeZone("CET"));

        // Say this is the supplied time String;
        String suppliedDate = "03:00:00+01";

        // Convert from String timestamp to long;
        long convertedToTimeStamp = getTimeStamp(suppliedDate);
        System.out.println(suppliedDate + " => " + convertedToTimeStamp + "ms");

        // Convert from long timestamp to String;
        String convertedBackToString = getDateString(convertedToTimeStamp);
        System.out.println(convertedToTimeStamp + "ms => " + convertedBackToString);
    }


    /**
    Converts String timestamp to the number of milliseconds since Epoch.
    */
    private static long getTimeStamp(String date_str) {
        try {
            Date date = sdf.parse(date_str);
            return date.getTime();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return 0;
    }

    /**
    Converts long timestamp to String
    */
    private static String getDateString(long timestamp) {
        try {
            Date date = new Date(timestamp);
            return sdf.format(date);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

}

Output:

As the Date component is not present in the suppliedDate it won't ever matter.