ELEMENTARY PROGRAMMING - Introduction to Java Programming: Comprehensive Version - page 52

2.12 Case Study: Displaying the Current Time
You can invoke System.currentTimeMillis() to return the current time.

Key
Point

The problem is to develop a program that displays the current time in GMT (Greenwich Mean
Time) in the format hour:minute:second, such as 13:19:8.
The currentTimeMillis method in the System class returns the current time in mil-
liseconds elapsed since midnight, January 1, 1970 GMT, as shown in Figure 2.2. This time
is known as the UNIX epoch. The epoch is the point when time starts, and 1970 was the year
when the UNIX operating system was formally introduced.

VideoNote

Use operators / and %

currentTimeMillis
UNIX epoch

Elapsed
time

Time

UNIX epoch
01-01-1970
00:00:00 GMT

Current time
System.currentTimeMillis()

F IGURE 2.2

The System.currentTimeMillis() returns the number of milliseconds
since the UNIX epoch.

You can use this method to obtain the current time, and then compute the current second,
minute, and hour as follows.

1. Obtain the total milliseconds since midnight, January 1, 1970, in totalMilliseconds
by invoking System.currentTimeMillis() (e.g., 1203183068328 milliseconds).

2. Obtain the total seconds totalSeconds by dividing totalMilliseconds by 1000
(e.g., 1203183068328 milliseconds / 1000 = 1203183068 seconds).

3. Compute the current second from totalSeconds % 60 (e.g., 1203183068 seconds
% 60 = 8 , which is the current second).

4. Obtain the total minutes totalMinutes by dividing totalSeconds by 60 (e.g.,
1203183068 seconds / 60 = 20053051 minutes).

5. Compute the current minute from totalMinutes % 60 (e.g., 20053051 minutes %
60 = 31 , which is the current minute).

6. Obtain the total hours totalHours by dividing totalMinutes by 60 (e.g., 20053051
minutes / 60 = 334217 hours).

7. Compute the current hour from totalHours % 24 (e.g., 334217 hours % 24 = 17 ,

which is the current hour).

Listing 2.7 gives the complete program.

L ISTING 2.7

ShowCurrentTime.java
1 public class ShowCurrentTime {
2 public static void main(String[] args) {

3 // Obtain the total milliseconds since midnight, Jan 1, 1970

4 long totalMilliseconds = System.currentTimeMillis(); totalMilliseconds

5
6 // Obtain the total seconds since midnight, Jan 1, 1970

7 long totalSeconds = totalMilliseconds / 1000 ;

totalSeconds

8
9 // Compute the current second in the minute in the hour

10 long currentSecond = totalSeconds % 60 ;

currentSecond