Android studio如何在MainActivity.kt中调用java类？

我是android Studio的新手，我的MainActivity.kt中有此代码

如何在我的MainActivity.kt中调用java类？如何知道是否已连接到数据库？我的数据库是postgresql

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        btn_login.setOnClickListener{
            connectionDb()

            Log.i("connectinDb()", "Connected")
        }
    }
}

public class connectionDb {
    Connection connection=null;
    public Connection ConnectionDb(){
        try{
            Class.forName("org.postgresql.Driver");
            connection = DriverManager.getConnection("jdbc:postgresql://localhost:5432/mytransactiondb", "postgres", "root");

        } catch (Exception err) {
            System.err.println(err.getMessage());
        }
        return connection;
    }
    protected void close_connection(Connection con)throws Exception{
        con.close();
    }
}

我有一个java类名connectionDB.java，它连接到我的postgresql

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        btn_login.setOnClickListener{
            connectionDb()

            Log.i("connectinDb()", "Connected")
        }
    }
}

public class connectionDb {
    Connection connection=null;
    public Connection ConnectionDb(){
        try{
            Class.forName("org.postgresql.Driver");
            connection = DriverManager.getConnection("jdbc:postgresql://localhost:5432/mytransactiondb", "postgres", "root");

        } catch (Exception err) {
            System.err.println(err.getMessage());
        }
        return connection;
    }
    protected void close_connection(Connection con)throws Exception{
        con.close();
    }
}

---

提前感谢。

修改代码并创建连接类和get方法的对象

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        btn_login.setOnClickListener{
            //connectionDb()
        val objConnectionDb=connectionDb()
        Log.i("connectinDb()", "Connected")
        }
    }
}

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kotlin@user…mr@user，是的..错别字mr@user