public CellFormatter getFormatter(String columnId) {
        checkId(columnId);
        CellFormatter fmt = formatters.get(columnId);
        return fmt == null ? DEF_FMT : fmt;
    }
@Test(expected = IllegalArgumentException.class)
    public void formatterBadColumn() {
        tm = new TableModel(FOO);
        fmt = tm.getFormatter(BAR);
    }