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

Others' answers are pretty good explained the reason. I just add something to summarize synchronized:

public class A {
    public synchronized void fun1() {}

    public synchronized void fun2() {}

    public void fun3() {}

    public static synchronized void fun4() {}

    public static void fun5() {}
}

A a1 = new A();

synchronized on fun1 and fun2 is synchronized on instance object level. synchronized on fun4 is synchronized on class object level. Which means:

1. When 2 threads call a1.fun1() at same time, latter call will be blocked.
2. When thread 1 call a1.fun1() and thread 2 call a1.fun2() at same time, latter call will be blocked.
3. When thread 1 call a1.fun1() and thread 2 call a1.fun3() at same time, no blocking, the 2 methods will be executed at same time.
4. When thread 1 call A.fun4(), if other threads call A.fun4() or A.fun5() at same time, latter calls will be blocked since synchronized on fun4 is class level.
5. When thread 1 call A.fun4(), thread 2 call a1.fun1() at same time, no blocking, the 2 methods will be executed at same time.