I know it's simple question, but in

ArrayList<ArrayList<String>> collection;
ArrayList<String> listOfSomething;

collection= new ArrayList<ArrayList<String>>();
listOfSomething = new ArrayList<String>();

listOfSomething.Add("first");
listOfSomething.Add("second");
collection.Add(listOfSomething);
listOfSomething.Clear();
listOfSomething.Add("first");
collection.Add(listOfSomething);

I want to take String from ArrayList of ArrayList, and I don't know how to do that. For example I go

ArrayList<String> myList = collection.get(0); 
String s = myList.get(0);

and it works! but:

Big update:

private List<S> valuesS;
private List<Z> valuesZ;


ArrayList<ArrayList<String>> listOfS;
ArrayList<String> listOfZ;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
        Zdatasource = new ZDataSource(this);
        Zdatasource.open();
        valuesZ = Zdatasource.getAllZ();
        
        Sdatasource = new SDataSource(this);
        Sdatasource.open();
        valuesS = Sdatasource.getAllS();

        List<Map<String, String>> groupData 
             = new ArrayList<Map<String, String>>();
        List<List<Map<String, String>>> childData 
             = new ArrayList<List<Map<String, String>>>();

        listOfS = new ArrayList<ArrayList<String>>();
        listOfZ = new ArrayList<String>();
        for (S i : valuesS) { // S is class
            for (Z j : valuesZ) { // Z is class
                if(j.getNumerS().equals(i.getNumerS())) {
                    listOfZ.add(j.getNumerZ());
                }
                else
                {
                    //listOfZ.add("nothing");
                }
            }
                listOfS.add(list