public interface MyCustomStringInterface {
    int countNumbers();
}

public class MyCustomString implements MyCustomStringInterface {
    private String string;



    @Override
    public int countNumbers() {
        while (string.length() != 0 ) {
            int count = 0;
            for (int i = 0; i < string.length(); i++) {
                if(Character.isDigit(string.charAt(i))) count++;
            }
            return count;
        }
        return 0;
}

public class MyCustomStringTest {
    private MyCustomStringInterface mycustomstring;

    @Test
    public void testCountNumbers1() {
       mycustomstring.setString("th1s strin8 shou1d hav3 numb3rs,
       righ7?");

       assertEquals(6, mycustomstring.countNumbers());
    }
}