java - 组合框未显示

Question

所以，我的要求是使用 Java 中的 JTable 显示数据库中的记录。我提到了this link 。但错误是，当我在 Eclipse 中运行它时，我看不到理想情况下应该出现的组合框。如何解决？

 import java.awt.*;
    import java.awt.event.*;
    import java.sql.*;
    import java.util.Vector;

    import javax.swing.*;
    import javax.swing.table.DefaultTableModel;

    public class DisplayEmpData extends JFrame implements ActionListener {

        JFrame frame1;
        JLabel l0, l1, l2;
        JComboBox c1;
        JButton b1;
        Connection con;
        ResultSet rs, rs1;
        Statement st, st1;
        PreparedStatement pst;
        String ids;
        static JTable table;
        String[] columnNames = {"User name", "Email", "Password", "Country"};
        String from;

        DisplayEmpData() {

            l0 = new JLabel("Fatching Employee Information");
            l0.setForeground(Color.red);
            l0.setFont(new Font("Serif", Font.BOLD, 20));
            l1 = new JLabel("Select name");
            b1 = new JButton("submit");

            l0.setBounds(100, 50, 350, 40);
            l1.setBounds(75, 110, 75, 20);
            b1.setBounds(150, 150, 150, 20);
            b1.addActionListener(this);

            setTitle("Fetching Student Info From DataBase");
            setLayout(null);
            setVisible(true);
            setSize(500, 500);
            setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

            add(l0);
            add(l1);;
            add(b1);
            try {

                String url="jdbc:odbc:Driver={Microsoft Access Driver (*.mdb, *.accdb)};DBQ=" + "C:\\users\\ppreeti\\emp_details.accdb";
                Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
                con=DriverManager.getConnection(url,"","");
                /*con = DriverManager.getConnection("jdbc:oracle:thin:@mcndesktop07:1521:xe", "sandeep", "welcome");*/
                st = con.createStatement();
                rs = st.executeQuery("select uname from emp");
                Vector v = new Vector();
                while (rs.next()) {
                    ids = rs.getString(1);
                    v.add(ids);
                }
                c1 = new JComboBox(v);
                c1.setBounds(150, 110, 150, 20);

                add(c1);
                st.close();
                rs.close();
            } catch (Exception e) {
            }
        }

        public void actionPerformed(ActionEvent ae) {
            if (ae.getSource() == b1) {
                showTableData();
            }

        }

        public void showTableData() {

            frame1 = new JFrame("Database Search Result");
            frame1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame1.setLayout(new BorderLayout());
    //TableModel tm = new TableModel();
            DefaultTableModel model = new DefaultTableModel();
            model.setColumnIdentifiers(columnNames);
    //DefaultTableModel model = new DefaultTableModel(tm.getData1(), tm.getColumnNames());
    //table = new JTable(model);
            table = new JTable();
            table.setModel(model);
            table.setAutoResizeMode(JTable.AUTO_RESIZE_ALL_COLUMNS);
            table.setFillsViewportHeight(true);
            JScrollPane scroll = new JScrollPane(table);
            scroll.setHorizontalScrollBarPolicy(
                    JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
            scroll.setVerticalScrollBarPolicy(
                    JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED);
            from = (String) c1.getSelectedItem();
    //String textvalue = textbox.getText();
            String uname = "";
            String email = "";
            String pass = "";
            String cou = "";

            try {
                pst = con.prepareStatement("select * from emp where UNAME='" + from + "'");
                ResultSet rs = pst.executeQuery();
                int i = 0;
                if (rs.next()) {
                    uname = rs.getString("uname");
                    email = rs.getString("umail");
                    pass = rs.getString("upass");
                    cou = rs.getString("ucountry");
                    model.addRow(new Object[]{uname, email, pass, cou});
                    i++;
                }
                if (i < 1) {
                    JOptionPane.showMessageDialog(null, "No Record Found", "Error", JOptionPane.ERROR_MESSAGE);
                }
                if (i == 1) {
                    System.out.println(i + " Record Found");
                } else {
                    System.out.println(i + " Records Found");
                }
            } catch (Exception ex) {
                JOptionPane.showMessageDialog(null, ex.getMessage(), "Error", JOptionPane.ERROR_MESSAGE);
            }
            frame1.add(scroll);
            frame1.setVisible(true);
            frame1.setSize(400, 300);
        }

        public static void main(String args[]) {
            new DisplayEmpData();
        }
    }

Answer 1

首先，我的建议是在代码中为方法和字段使用更有意义的名称:现在对您来说可能看起来很清晰，但对于外部读者和您自己来说，几个月后代码的作用就赢了一点也不明显。

也就是说，您可能最好使用 LayoutManager，它可以确保正确的 Component 放置和平台上的可见性(正如 @Andrew Thompson 已经指出的那样)。

基本上来说，第一步是创建一个 JPanel 并将其添加到您的 JFrame 中:

JPanel framePanel;
framePanel = new JPanel();
this.add(framePanel);

当 JPanel() 构造函数中未指定任何参数时，布局将自动设置为 BorderLayout。此布局有 5 个可以放置组件的区域:北、南、西、东、中心。

您现在可以将其他 JPanel 添加到您的 framePanel 中，如下所示:

JPanel northPanel;
JPanel southPanel;
JPanel westPanel;
JPanel eastPanel;
JPanel centerPanel;

northPanel = new JPanel();
southPanel = new JPanel();
westPanel = new JPanel();
eastPanel = new JPanel();
centerPanel = new JPanel();

framePanel.add(northPanel, BorderLayout.NORTH);
framePanel.add(southPanel, BorderLayout.SOUTH);
framePanel.add(westPanel, BorderLayout.WEST);
framePanel.add(eastPanel, BorderLayout.EAST);
framePanel.add(centerPanel, BorderLayout.CENTER);

每个面板可能包含另一个 JPanel 或您的一个组件。 GridLayout 是可用于放置多个组件的最有用和基本的 LayoutManager 之一，它允许您指定网格大小(行数和列数):每次您添加一个组件，将其放置在从左上角开始的第一个空闲槽中。

如果您要向 northPanel 添加 3 个组件，您需要编写代码

northPanel.setLayout(new GridLayout(1, 3));

northPanel.add(component1);
northPanel.add(component2);
northPanel.add(component3);

这些只是示例，如果您搜索 LayoutManager，您可以在 Java API 中找到更多信息。只是一些快速提示:

• 您可以直接在 JPanel 构造函数中指定 Layout:

  JPanel testPanel = new JPanel(new GridLayout(行、列));

• 您可以通过在网格位置插入空 JPanel 来添加空格:

  JPanel whiteSpacePanel = new JPanel();   
  
  testPanel.add(component1);
  testPanel.add(whiteSpacePanel);
  testPanel.add(component2);
  

• 您可以在 Layout 构造函数中指定 Component 之间的一些基本边框

  JPanel panelWithBorders = new JPanel(new GridLayout(rows, columns, xBorder, yBorder));

引用文献:

• LayoutManager

• BorderLayout

• GridLayout

• JPanel