Worker.java

import java.util.Date;

public class Worker implements Runnable{

    long executionMillis = -1;


    public void run() {
        for(int i=0; i<100; i++) {

            executionMillis = (new Date()).getTime();
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public long getIdleMillis() {
        if (executionMillis == -1) {
            return 0;
        }
        return (new Date()).getTime() - executionMillis;
    }
}