public void putSingleInstance(RootFragment fragment){
  ArrayList<RootFragment> frags=new ArrayList<>();
  frags.add(fragment);
  stackList.add(frags);
}

public void putSingleInstance ( RootFragment fragment ) { ArrayList < RootFragment > frags = new ArrayList < > ( ) ; frags . add ( fragment ) ; stackList . add ( frags ) ; }
singleinstance mode , create a new task stack at a time .
