java - 将 JComboBox 与 SQL 链接

Question

我正在编写一段代码，使用数据库中的数据在 JComboBox 中创建一个列表。它实际上是针对 POS 系统的，我必须从数据库的列表中选择一个项目..

这是我一直在尝试的代码:(我确实在没有 while(itemsList != null) 的情况下尝试过..但它也不起作用

private class ButtonHandlerSales implements ActionListener 
{ 
    public final String userName = "root";
    private final String password = "";
    private final String serverName = "localhost";
    private final int portNumber = 3306;
    private final String dbName = "alphapos";



    public void actionPerformed(ActionEvent action) 
    {   
        Connection conn = null;
        try 
        {
            conn = this.getConnection();
        } 
        catch (SQLException e1) 
        {
            e1.printStackTrace();
        }
        //System.out.println("Connected to database");

        while(itemsList != null)
        {
            String[] list= null;
            String command = "SELECT itemName FROM item";

            try 
            {
                list = viewTable(conn, command);
            }
            catch (SQLException e) 
            {
                e.printStackTrace();
            }

            itemList = new JComboBox(list);


        }


    }

    private String[] viewTable(Connection con, String command) throws SQLException 
    {
        String list[] = null;
        Statement stmt = null;

        try 
        {
            stmt = con.createStatement();
            ResultSet rs = stmt.executeQuery(command);

            int i=0;
            while (rs.next()) 
            {
                list[i] = rs.getString("itemName");
                i++;
            }

        } 

        catch (SQLException e ) 
        {
            e.printStackTrace();
        } 

        finally
        {
            if (stmt != 
            null) { stmt.close(); }
        }

        return list;

    }

    private Connection getConnection() throws SQLException 
    {
        Connection conn = null;
        Properties connectionProps = new Properties();
        connectionProps.put("user", this.userName);
        connectionProps.put("password", this.password);

        conn = DriverManager.getConnection("jdbc:mysql://"
                + this.serverName + ":" + this.portNumber + "/" + this.dbName,
                connectionProps);

        return conn;
    }

}//end of class

我没有收到任何错误..代码编译..但我没有得到任何输出(列表为空)..我在这里做错了什么吗？

非常感谢任何帮助...(我不是在寻找直接代码)

Answer 1

问题 1

在您的viewTable方法中String list[] = null;。您永远不会使用 new String[..] 初始化它。因此，您将新建一个 NullPointerException。但最好使用 ArrayList，因为您可能不知道将返回多少个值

private List<String> viewTable(Connection con, String command) {
    List<String> list = new ArrayList<>();
    ...
    while (rs.next()) {
        list.add(rs.getString("itemName"));
    }
    return list;
}

问题2

您正在 ActionListener 中创建 JComboBox。因此，您无法在执行操作之前添加组合。对于组合框，在处理数据时，最好使用其模型，而不是组件。 JComboBox 有一个 ComboBoxModel。我们可以使用具体的DefaultComboBoxModel 。您可以将数组传递给其构造函数。 List 具有方法 toArray，我们可以调用该方法从 List 创建数组。然后只需将数组传递给 DefaultComboBoxModel 构造函数并调用组合框 setModel

List<String> list = null;
String command = "SELECT itemName FROM item";

try {
    list = viewTable(conn, command);
} catch (SQLException e) {
    e.printStackTrace();
}

ComboBoxModel model = new DefaultComboBoxModel(list.toArray());
itemList.setModel(model);

这样，您可以在执行操作之前初始化组合框。

<小时/>

更新

如果您希望在应用程序启动时加载组合框，我不明白问题是什么。只需使用 actionPerformed 中的代码在您的类中创建一个方法即可。然后只需调用该方法，就像在主构造函数或其他东西中一样。也许您可以执行如下所示的操作，其中方法返回 ComboBoxModel 并且您可以使用它来设置组合框的模型

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;
import java.util.Properties;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.ComboBoxModel;
import javax.swing.DefaultComboBoxModel;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JOptionPane;
import javax.swing.JPanel;

public class Test {

    private final JComboBox itemList;

    public Test() throws SQLException {
        itemList = new JComboBox(viewTable(getConnection(), command));
        JButton button = new JButton("Populate");
        button.addActionListener(new ButtonHandler());

        JPanel panel = new JPanel(new GridLayout(0, 1));
        panel.add(itemList);
        panel.add(button);

        JOptionPane.showMessageDialog(null, panel);
    }

    private class ButtonHandler implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent e) {
            try {
                ComboBoxModel model = viewTable(getConnection(), command);
                itemList.setModel(model);
            } catch (SQLException ex) {
                Logger.getLogger(Test.class.getName()).log(Level.SEVERE, null, ex);
            }
        }

    }

    public static void main(String[] args) throws SQLException {
        Test test = new Test();
    }

    private ComboBoxModel viewTable(Connection con, String command) throws SQLException {
        List<String> list = new ArrayList<>();
        try (Statement stmt = con.createStatement()) {
            ResultSet rs = stmt.executeQuery(command);

            while (rs.next()) {
                list.add(rs.getString("itemName"));
            }

        } catch (SQLException e) {
            e.printStackTrace();
        }

        return new DefaultComboBoxModel(list.toArray());

    }

    public final String userName = "root";
    private final String password = "";
    private final String serverName = "localhost";
    private final int portNumber = 3306;
    private final String dbName = "alphapos";
    private final String command = "select itemName from item";

    private Connection getConnection() throws SQLException {
        Connection conn = null;
        Properties connectionProps = new Properties();
        connectionProps.put("user", this.userName);
        connectionProps.put("password", this.password);

        conn = DriverManager.getConnection("jdbc:mysql://"
                + this.serverName + ":" + this.portNumber + "/" + this.dbName,
                connectionProps);

        return conn;
    }
}