Difference between start and run method in Thread

Main difference is that when program calls start() method a new Thread is created and code inside run() method is executed in new Thread while if you call run() method directly no new Thread is created and code inside run() will execute on current Thread.

If you want to perform time consuming task than always call start() method otherwise your main thread will stuck while performing time consuming task if you call run() method directly.

Another difference between start vs run in Java thread is that you can not call start() method twice on thread object. once started, second call of start() will throw IllegalStateException in Java while you can call run() method twice.