Modifications of a SetUniqueList.subList() invalidate the parent list

The List returned by SetUniqueList.subList() is again a SetUniqueList. The contract for List.subList() says that the returned list supports all the operations of the parent list, and it is backed by the parent list.

We have a SetUniqueList uniqueList equal to

{"Hello", "World"}

. We get a subList containing the last element. Now we add the element "Hello", contained in the uniqueList but not in the subList, to the subList.

What should happen?

Should the subList behave like a SetUniqueList and add the element - meaning that it changes position in the uniqueList because at the old place it gets removed, so now uniqueList equals

{"World", "Hello"}

(which fails)?
Or should the element not be added, because it is already contained in the parent list, thereby violating the SetUniqueList-ness of the subList (which fails)?
I prefer the former behaviour, because modifications should only be made through the subList and not through the parent list (as explained in List.subList()).

What should happen if we replace (using set) the subList element "World" with "Hello" instead of adding an element?

The subList should contain only "Hello", and for the parent list, the old element 0 (now a duplicate of the just set element 1) should be removed (which fails).

And of course the parent list should know what happens to it (specifically, its uniqueness Set) (which fails in the current snapshot).

public void testSubListAddNew() {
List uniqueList = SetUniqueList.decorate(new ArrayList());
uniqueList.add("Hello");
uniqueList.add("World");
List subList = uniqueList.subList(1, 2);

subList.add("Goodbye");

List expectedSubList = Arrays.asList(new Object[]

{ "World", "Goodbye" }

);
List expectedParentList = Arrays.asList(new Object[]

{ "Hello", "World", "Goodbye" }

);
assertEquals(expectedSubList, subList);
assertEquals(expectedParentList, uniqueList);
assertTrue(uniqueList.contains("Goodbye")); // fails
}

public void testSubListAddDuplicate() {
List uniqueList = SetUniqueList.decorate(new ArrayList());
uniqueList.add("Hello");
uniqueList.add("World");
List subList = uniqueList.subList(1, 2);

subList.add("Hello");

List expectedSubList = Arrays.asList(new Object[]

{ "World", "Hello" });
List expectedParentList = Arrays.asList(new Object[] { "World", "Hello" }

);
assertEquals(expectedSubList, subList);
assertEquals(expectedParentList, uniqueList); // fails
}

public void testSubListSetDuplicate() {
List uniqueList = SetUniqueList.decorate(new ArrayList());
uniqueList.add("Hello");
uniqueList.add("World");
List subList = uniqueList.subList(1, 2);

subList.set(0, "Hello");

List expectedSubList = Arrays.asList(new Object[]

{ "Hello" });
List expectedParentList = Arrays.asList(new Object[] { "Hello" }

);
assertEquals(expectedSubList, subList);
assertEquals(expectedParentList, uniqueList); // fails
}