Java System.out.Println() influencing the code

Question

This is a part of a thread. It didn't seem to go in to the if statement, except when I put the print function before the if statement. Then suddenly everything works perfectly. How is that even possible?

 while(RUNNING) {
        // WITHOUT THIS LINE DOESN'T GO IN if ->  System.out.println("");
        if (Process.MOUSE_DOWN) {
         //   System.out.println("in if");
            fireNext(mouseDownX);
            try {
              System.out.println("Sleeping");
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }