public void addSubSession(SubSession sub){
  if (subSessions == null) {
    subSessions=new ArrayList<SubSession>(INITIAL_SUB_SESSION_NUMBER);
    subSessions.add(sub);
  }
 else {
    subSessions.add(sub);
  }
}

public void addSubSession ( SubSession sub ) { if ( subSessions == null ) { subSessions = new ArrayList < SubSession > ( INITIAL_SUB_SESSION_NUMBER ) ; subSessions . add ( sub ) ; } else { subSessions . add ( sub ) ; } }
add sub session into main session .
