Java: thread safe id generator (synchronization issue)

Question

Let's suppose I have a very simple id generator class which can be use my many threads simultaneously. Will the following code work fine - I mean it will always return unique ids?

class Generator{
private int counter=0;

public int getId(){
 return counter++;//the key point is here
}
}

Answer 1

No. It's not thread safe at all, you would need to synchronize your getId() method.

However it's better to use AtomicInteger.incrementAndGet() instead.

public class Generator {
   private final static AtomicInteger counter = new AtomicInteger();

   public static int getId() {
      return counter.incrementAndGet();
   }
}

Answer 2

You need to make counter as static variable. And get method as static synchronize .

Answer 3

class Generator{
    private static int counter=0;

    public static synchronized int getId(){
        return counter++;
    }
}