How to find the difference of two timestamps in java?

Question

I have an ArrayList including several number of time-stamps and the aim is finding the difference of the first and the last elements of the ArrayList.

String a = ArrayList.get(0);
String b = ArrayList.get(ArrayList.size()-1);
long diff = b.getTime() - a.getTime();

I also converted the types to int but still it gives me an error The method getTime is undefined for the type String.

Additional info :

I have a class A which includes

String timeStamp = new SimpleDateFormat("ss S").format(new Date());

and there is a class B which has a method private void dialogDuration(String timeStamp)

and dialogueDuration method includes:

String a = timeSt.get(0); // timeSt  is an ArrayList which includes all the timeStamps
String b = timeSt.get(timeSt.size()-1);   // This method aims finding the difference of the first and the last elements(timestamps) of the ArrayList  (in seconds)

long i = Long.parseLong(a);
long j = Long.parseLong(b);

long diff = j.getTime()- i.getTime();

System.out.println("a: " +i); 
System.out.println("b: " +j); 

And one condition is that the statement(String timeStamp = new SimpleDateFormat("ss S").format(new Date());) wont be changed in class A. And an object of class B is created in class A so that it invokes the dialogueDuration(timeStamp) method and passes the values of time-stamps to class B.

My problem is this subtraction does not work, it gives an error cannot invoke getTime() method on the primitive type long. It gives the same kind of error also for int and String types?

Thanks a lot in advance!

Answer 1

Assuming you have Timestamp objects or Date Objects in your ArrayList you could do:

Timestamp a = timeSt.get(0);

Timestamp b = timeSt.get(timeSt.size()-1);

long diff = b.getTime() - a.getTime();

Answer 2

You can calculate the difference with the both following methods(also you can modify the mentioned methods to return difference as 'millisecond', 'day', 'month', etc by adding additional if statement or using switch case):

private Long calculateDifference(String date1, String date2, String value) {
        Timestamp date_1 = stringToTimestamp(date1);
        Timestamp date_2 = stringToTimestamp(date2);
        long milliseconds = date_1.getTime() - date_2.getTime();
        if (value.equals("second"))
            return milliseconds / 1000;
        if (value.equals("minute"))
            return milliseconds / 1000 / 60;
        if (value.equals("hours"))
            return milliseconds / 1000 / 3600;
        else
            return new Long(999999999);
    }

private Timestamp stringToTimestamp(String date) {
        try {
            SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            Date parsedDate = dateFormat.parse(date);
            return new Timestamp(parsedDate.getTime());
        } catch (Exception e) {
            return null;
        }
    }

For example:

calculateDifference("2021-10-20 10:00:01", "2021-10-20 10:15:01", "minute");

will return '-15'

or

calculateDifference("2021-10-20 12:00:01", "2021-10-20 10:15:01", "minute");

will return '105'