Transfer an ArrayList<Character> or <Integer> from one class to the activity class

Question

I'm trying to transfer this code from one class to one of my activity classes in Android Studio as I am trying to use it for my Text View.

public class cartesian() {
    public ArrayList<Character> list1() {
        Random rand = new Random();
        int low = 3;
        int high = 5;
        int result = rand.nextInt(high - low) + low;

        ArrayList<Character> list1 = new ArrayList<>(result);

        for (int i = 0; i <= result; i++) {
            list1.add((char)(rand.nextInt(26) + 'a'));
        }

        return list1;
    }

    public ArrayList<Integer> list2() {
        Random rand = new Random();
        int low = 3;
        int high = 5;
        int result = rand.nextInt(high - low) + low;

        ArrayList<Integer> list2 = new ArrayList<>(result);

        for (int i = 0; i <= result; i++) {
            list2.add(rand.nextInt(100));
        }

        return list2;
    }
}

I have tried one of the answers from here, the one by "bastien pinaquy" but that didn't work either as it returns null each time. I also tried using a custom object but I'm not confident in my understanding of it to allow myself to make much use of it.

Custom Object Class

public class CustomObject {
    private List<Character> charList;
    private List<Integer> intList;

    public CustomObject(){
        // not sure if I need to put anything in here
    }

    public List<Character> getCharList() { return charList; }
    public void setCharList(List<Character> charList) { this.charList = charList; }
    public List<Integer> getIntList() { return intList; }
    public void setIntList(List<Integer> intList) { this.intList = intList; }
}

Activity Class

public class cartProdActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_cart_prod);
    
    ArrayList<Character> charList = list1();
    String charListStr = charList.toString();

    ArrayList<Integer> intList = list2();
    String intListStr = intList.toString();

    setTxt(charListStr, intListStr);
}

public void setTxt(String list1, String list2) {
    TextView list1View = (TextView) findViewById(R.id.list1);
    list1View.setText(list1);

    TextView list2View = (TextView) findViewById(R.id.list2);
    list2View.setText(list2);
}
// the list1 and list2 functions refer to the functions from the first code block
// they are there as I was testing whether the code was working or not

The function itself works as I have put it in the activity class to check but I want it in a separate class I need to work with the list itself for other purposes.