making a method thread safe even when method is called by makeing multiple instances of class

I want to have thread save method that returns a unique current Timestamp.Even when the method is called by same time i want to get a unique current datetime.Even if this method is called by making multiple instances of MyClass i want it be be thread 开发者_C百科safe always

class Myclass{

 Date getUniquetimeStam(){
   synchronized(Myclass.class){
    //return date here 
   }

}

Now if i make 2 instances of Myclass and call getUniqueStam at same ,is it gaurented to return uniue date time.

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You can't guarantee unique time for each call. But you can for instance increment it manually if it didn't change yet:

private AtomicLong lastTime = new AtomicLong();

long getUniquetimeStam() {
    while (true) { // way of working with atomics, but they are really fast
        long now = System.currentTimeMillis();
        long last = lastTime.get();
        if (last < now) {
            if (lastTime.compareAndSet(last, now))
                return now;
        } else
            return lastTime.incrementAndGet();
    }
}

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No, you are not guaranteed. If your computer is fast enough then both method calls can happen in same millisecond and produce identical Date object.

You have 2 options here:

1. Use UUID instead of date. Read more about it here . UUID by specification is guaranteed to be unique each time you generate it - so it's the safest and easiest option you can get.
2. Store the date object, synchronize on it, and check if it's the same. Here's an example:

.

class Myclass {

    //Static to make it possible to synchronize between instances of Myclass
    static Date lastDate;

    Date getUniqueTimeStamp() {
        synchronized (Myclass.lastDate) {
            Date newDate = new Date();
            if (newDate.getTime() <= Myclass.lastDate.getTime()) {
                newDate.setTime(Myclass.lastDate.getTime()+1);
            }
            Myclass.lastDate.setTime(newDate.getTime());
            return newDate;
        }
    }
}

---

Not cool though - you can add a delay and then create the Data.

class Myclass {

 Date getUniquetimeStam() {
   //in try catch
   Thread.sleep(100);
   //
   new Date();
  }

}

---

Could you just change it to return a new instance of Date on every call?

class Myclass {

 Date getUniquetimeStam() {
   new Date();
  }

}