which method to call to remove default layout manager ( flow layout )
My advice is don't do this. There are very few occasions where you need to manually position components - in all my years of programming in Java I never yet had a situation where I can't get a layout manager to do the job for me (admittedly I had to write my own layout manager on a couple of occasions but it's not that hard to do).

Using layout managers may seem a little complicated at first but once you get the hang of them the benefits are more than worth the effort.