public CellFormatter getFormatter(String columnId) {
        checkId(columnId);
        CellFormatter fmt = formatters.get(columnId);
        return fmt == null ? DEF_FMT : fmt;
    }
@Test
    public void defaultFormatter() {
        tm = new TableModel(FOO);
        fmt = tm.getFormatter(FOO);
        assertTrue("Wrong formatter", fmt instanceof DefaultCellFormatter);
    }