Endless loop in for-statement

With the list.add in line 23 you insert the existing word at the place the original eord was before, so the old word is shifted to j+1. So in case of incertion you have to also adjust j

public class Solution { public static void main(String[] args) throws Exception { ArrayList<String> liste = new ArrayList<String>(); liste.add("Rose"); // 0 liste.add("Liebe"); // 1 liste.add("Leier"); // 2 korrigieren(liste); for (String s : liste) { System.out.println(s); } } public static ArrayList<String> korrigieren(ArrayList<String> list) { for (int i = 0; i<list.size();){ if ((list.get(i).contains("r")||list.get(i).contains("R"))&&(!(list.get(i).contains("l")||list.get(i).contains("L")))){ list.remove(i); } else if ((list.get(i).contains("r")||list.get(i).contains("R"))&&((list.get(i).contains("l")||list.get(i).contains("L")))){ i++; } else if (!(list.get(i).contains("r")||list.get(i).contains("R"))&&((list.get(i).contains("l")||list.get(i).contains("L")))){ list.add(i+1,list.get(i)); i++; i++; }else i++; } return list; }

I managed to fix the program through content, which I learned later in this level from the professor! You need to use an Iterator, but not the normal one, instead use a ListIterator! What´s the difference? The Iterator only has three functions, hasNext(), next(), remove(). So with the normal Iterator, you can NOT ADD elements! That´s where the ListIterator comes into play. You can move between elements (e.g. previous()) and you can add elements via add(). This will add an element just before the current element and as a parameter you give him the String, which you saved at the very beginning of the while loop. Have a look here:

public static ArrayList<String> korrigieren(ArrayList<String> liste) {
        ListIterator<String> listIterator = liste.listIterator();
        while (listIterator.hasNext()) {
            String check = listIterator.next();

           // please insert your conditions for testing, whether check contains "r" or "r" and "l" here.

            } else if (check.contains("l") || check.contains("L")) {
                listIterator.add(check);
            }
        }
        return liste;
    }