Interface List method add(int index, E element); Why i have unsupportedOperationEception?

I'm make unit test Interface List. Simple code but i can't understand, why testAdd() throws UnsupportedOperationException but test Set() do not throws this exception.

public class testList {
    private static List<Integer> testList = new ArrayList<>();

    public static void main(String[] args) {
    init();
    testGet();
    testSet();
    testAdd();
}

private static void init() {
    testList = Arrays.asList(0, 1, 2, 3, 1, 2, 5, 4);
}

private static void testGet() {
    assertEquals(Integer.valueOf(2), testList.get(2));
}

private static void testSet() {
    testList.set(6, 5);
    assertEquals(new Integer[]{0, 1, 2, 3, 1, 2, 5, 4}, testList.toArray());
}

private static void testAdd() {
    testList.add(0, 1);
    assertEquals(new Integer[]{1, 0, 2, 2, 3, 3, 4, 5, 4}, testList.toArray());
  }
}

This is from AbstractList