JavaFX中用于FXML comboBox的函数是什么

Question

我已经在javaFX FXML文件中定义了组合框现在我想定义它的函数来获取组合框的值并在我的代码中使用它。

是FXML文件

CREATE INDEX ON fp USING btree(session_id); --maybe cover license_key as well, but doubt it

WITH s AS (SELECT session_id FROM sd WHERE sd.device_hash = 'hash'),
     f AS (SELECT user_id, session_id
             FROM fp
            WHERE fp.license_key = 'license'
              AND session_id = ANY((SELECT array_agg(session_id) FROM s)::int[]))
SELECT COUNT (DISTINCT(user_id)) 
FROM f
JOIN s ON f.session_id = s.session_id; --your left join is not left join anyway

现在任何人都可以告诉我如何编写一个函数来获取组合框的值并以字符串形式使用它