Doc of focusPrevious:

"If the current focus owner is the first row, or is -1 (representing that there is no current focus owner), calling this method will have no result."

mapped to a test method:

    @Test
    public void focusPreviousFromNoFocus() {
        focusModel.focus(-1);
        assertEquals("sanity", -1, focusModel.getFocusedIndex());
        focusModel.focusPrevious();
        assertEquals(-1, focusModel.getFocusedIndex());
    }
    
fails for all implementations - actually, already in the abstract base class (and overriding subs more or less just copied):

    public void focusPrevious() {
        if (getFocusedIndex() == -1) {
            focus(0);
        } else if (getFocusedIndex() > 0) {
            focus(getFocusedIndex() - 1);
        }
    }

either implementation or specification must be changed to keep in sync