multithreading - Can I keep a counter for Java Threads in the run method? -

i have class variable, sum. every time start new thread, want sum increment. seems run being called once, , can't find better info tell me more it. there way accomplish locks? here simple code:

public class myclass implements runnable{     static int sum = 0;     public static void main(string[] args) throws interruptedexception {         for(int = 0; < 5; ++i){             thread t = new thread(new myclass());             t.start();             t = null;         }     }     @override         public synchronized void run() {         ++sum;         system.out.println(sum);     } } 

keeping mutable state in static variables bad practice, how fix work:

public class myclass implements runnable {      static atomicinteger counter = new atomicinteger(0);      public static void main(string[] args) throws interruptedexception {         (int = 0; < 5; ++i) {             thread t = new thread(new myclass());             t.start();             t = null;         }     }      @override     public void run() {         int sum = counter.incrementandget();         system.out.println(sum);     } }