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

As explained in other answers, your code is not Thread safe since static method increment() locks Class monitor and non-static method decrement() locks Object monitor.

For this code example, better solution exists without synchronzed keyword usage. You have to use AtomicInteger to achieve Thread safety.

Thread safe using AtomicInteger:

import java.util.concurrent.atomic.AtomicInteger;

class ThreadSafeClass extends Thread {

    private static AtomicInteger count = new AtomicInteger(0);

    public static void increment() {
        count.incrementAndGet();
    }

    public static void decrement() {
        count.decrementAndGet();
    }

    public static int value() {
        return count.get();
    }

}