为什么没有在PreparedStatement中使用JComboBox的值？

Question

我已将主题定义为

public String[]subjects= {"English","Maths","Science","Social","Programming","Production_Project","20","ENG_MARKS"};

我在准备好的声明中使用了以下代码。但它在oracle中给我无效的列错误。

JComboBox comboBox = new JComboBox(subjects);
    comboBox.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent arg0) {
        Selected_subject = (String)comboBox.getSelectedItem();
        }
    });

准备好的声明代码

JButton btnNewButton_2 = new JButton("Upload");
    btnNewButton_2.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent arg0) {
        String up_assignments = "UPDATE student_assignments set ? = ? WHERE student_id = 2";
        try {
        PreparedStatement ps = connection.prepareStatement(up_assignments);
        ps.setString(1, Selected_subject);
        ps.setInt(2, 25);
        ps.execute();
        }catch(Exception ex)
        {
        ex.printStackTrace();
        }

        }
    });