Why is this class not thread safe?

Question

class ThreadSafeClass extends Thread
{
     private static int count = 0;

     public synchronized static void increment()
     {
         count++;
     }

     public          


        

Answer 1

1. decrement is locking on a different thing to increment so they do not prevent each other from running.
2. Calling decrement on one instance is locking on a different thing to calling decrement on another instance, but they are affecting the same thing.

The first means that overlapping calls to increment and decrement could result in a cancel-out (correct), an increment or a decrement.

The second means that two overlapping calls to decrement on different instances could result in a double decrement (correct) or a single decrement.