ArrayList<String> resultStrings = new ArrayList<String>(); for (int i = 0; i < strings.size(); i++) { if( strings.get(i).length()%2==0) resultStrings.add(i,strings.get(i) + " " + strings.get(i)); else resultStrings.add(i,strings.get(i) + " " + strings.get(i)+ " " +strings.get(i)); }