class Counter {
    private Integer currentValue = 0;
    public Integer next() {
         synchronized(currentValue) {
             return currentValue++;
         }
     }
}

class Worker implements Runnable {

    private boolean stop;

    public void run() {
         while(!stop) {
             // do something ...
         }
    }

    public void stop() {
         stop = true;
     }
}