ArrayList<String> string2 = new ArrayList<> ( ); // to check odd or even for(int i= 0 ; i < list.size (); i++){ String s = list.get ( i ); string2.add ( s ); } for(int i = 0 ; i < string2.size();i++){ if(string2.get ( i ).length ()%2 == 0){ string2.get ( i ).concat( string2.get ( i ) ); System.out.println (string2.get ( i ) ); }else{ string2.get ( i ).concat ( string2.get ( i ) ); string2.get(i).concat ( string2.get ( i ) ); System.out.println (string2.get ( i ) ); } } System.out.println(string2); }