For the first time I have to play with Threads in java.Basically, my code is a simple FIFO job queue.The main thread regularly puts jobs in the queue wich need to be executed.The code below is a striped down version of what I am doing :public class JobPerformer implements Runnable {    private final ArrayList<Job> jobList = new ArrayList<Job>();    private final AtomicBoolean threadStop = new AtomicBoolean(false);    private Thread thread;    public JobExecuter(){       thread = new Thread(this)       thread.start();    }    public void perform(Job job, bool async){        if(async){            jobList.add(job);            jobList.notifyAll();        }        else{            job.perform();        }    }    public void perform(Job job){        this.perform(job, false);    }    public void shutdown(){         threadStop.set(true);         if(thread.getState() == Thread.State.WAITING){             thread.interrupt();         }    }    public void run(){        while(! threadStop.get()){            if(jobList.isEmpty()){                try{                    jobList.wait();                } catch (InterruptedException ex) {}            }            jobList.get(0).perform();            jobList.remove(0);        }        // Finish the jobs still in the queue        while(!jobList.isEmpty()){             jobList.get(0).perform();             jobList.remove(0);        }    }}I have two questions :I use wait() and notifyAll() to avoid polling. But nowhere do I synchronize. Since it's a FIFO queue, threads are just adding jobs at the end or reading at the start. I don't see any conflict that would make me say Hey, I have to make sure that during this op. only one thread can be working on this object, is there a possible conflict I am overseeing that would require me to synchronize anyway ?I am using an Atomic boolean because of this question : Multithreading correctly done? What was wrong with using a normal boolean ? 
Simple FIFO Job list in Java
java;multithreading;thread safety
Why don't you use the classes provided in java.util.concurrent that can handle these requirements? Or is this written for practice?There is still problem if you don't synchronize, because the ArrayList.add() method is not thread-safe thus it can happen that two threads trying to add a new job into the queue nearly the same instant (to the same slot in the array), then it is possible that the one adding later will overwrite the job added by the first thread.Using normal boolean does not guarantee, that if one thread writes the value, and another accesses immediately after that then it will see the new value. You can use volatile with boolean as well, as in mentioned in the answer you linked. See an explanation for volatileOr refer to Java Concurrency in Practice, pretty good book.