Question

I am trying to run a line of code every second by using System.currentTimeMillis();.

The code:

     while(true){
           long var = System.currentTimeMillis() / 1000;
           double var2 = var %2;

           if(var2 == 1.0){

               //code to run

           }//If():

        }//While

The code which I want to run, runs multiple times because var2 is set to 1.0 multiple times due to the infinite whole loop. I just want to run the code line when var2 is first set to 1.0, and then every time again when var2 becomes 1.0 after 0.0.

Answer 1

If you want to busy wait for the seconds to change you can use the following.

long lastSec = 0;
while(true){
    long sec = System.currentTimeMillis() / 1000;
    if (sec != lastSec) {
       //code to run
       lastSec = sec;
    }//If():
}//While

A more efficient approach is to sleep until the next second.

while(true) {
    long millis = System.currentTimeMillis();
    //code to run
    Thread.sleep(1000 - millis % 1000);
}//While

An alternative is to use a ScheduledExecutorService

ScheduledExecutorService ses = Executors.newSingleThreadScheduledExecutor();

ses.scheduleAtFixedRate(new Runnable() {
    @Override
    public void run() {
        // code to run
    }
}, 0, 1, TimeUnit.SECONDS);

// when finished
ses.shutdown();

The advantage of this approach is that

• you can have a number of tasks with different periods sharing the same thread.
• you can have non-repeating delay or asynchronous tasks.
• you can collect the results in another thread.
• you can shutdown the thread pool with one command.