code: public Integer next() {
  		for(int i = currentIndex; i < t.size(); i++){
  			if(i+timelag>=t.size()){
  				return null;
  			}
  			if((t.get(i) != null) && (t.get(i+timelag) != null)){
  				if(overlap){
  					currentIndex = i+1;
  				}
  				else{
  					currentIndex = i+timelag;
  				}
  				return i;
  			}
  		}
  		
  		return null;
  	}
docs: Give next index i where i and i+timelag is valid