Java 当由另一个字段变量打印时，字段变量恢复为初始化值

下面是裁剪过的代码

我在Java和NetBeans工作。我已经声明了两个字符串变量，dbName和successfullyConnected，作为字段

成功连接打印dbName

行JOptionPane.showMessageDialog（空，成功连接）；打印“成功连接到空”而不是“成功连接到数据库”

将文本直接放入参数JOptionPane.showMessageDialog（null，“成功连接到”+dbName）；，打印所需的文本

我的问题是，为什么不使用字段来存储字符串？为什么在使用成功连接的打印时，而不是直接放置在方法调用中时，dbName会恢复为null

public class ABMain extends JFrame implements ActionListener {
    //Field
    ...
    private static String dbName;
    private static String successfullyConnected = "Successfully connected to " +dbName;


    //Constructor
    public ABMain() {
        ...
    }

    //Listener
    public void actionPerformed (ActionEvent e) {
        if (e.getSource() == item1) {
            dbName = JOptionPane.showInputDialog(requestDBName);
            try {
                stmt = ABDerbyConnection.createCon(dbName);
                JOptionPane.showMessageDialog(null, successfullyConnected); //echo for debugging
            } catch (SQLException ex) {
                ...
            }
            ...
        }  
    }

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

---

因为您正在用户输入之前成功初始化

已连接的

文本，此时

dbName

为

null

。在用户输入后初始化它

dbName = JOptionPane.showInputDialog(requestDBName);
successfullyConnected = "Successfully connected to " + dbName;

---

初始化后，除非修改，否则这些值不会更改。它们不是动态变量，并且没有检测更改值的绑定

 private static String dbName;
 private static String successfullyConnected = "Successfully connected to " +dbName;

你需要这样做

private static String dbName;
private static String successfullyConnected = "Successfully connected to ";

stmt = ABDerbyConnection.createCon(dbName);
successfullyConnected = successfullyConnected +dbName; // See this line is added 
JOptionPane.showMessageDialog(null, successfullyConnected);

---

As

dbName

在初始化

成功连接时始终为空

private static String dbName;
private static String successfullyConnected = "Successfully connected to " +dbName;

我想你不需要静态场

private String dbName;
private String successfullyConnected = "Successfully connected to ";

如果dbName为initialize，则可以将dbName添加到消息：

    dbName = JOptionPane.showInputDialog(requestDBName);
    if(dbName!=null){
      OptionPane.showMessageDialog(null, successfullyConnected + dbName);
    } else {
      OptionPane.showMessageDialog(null, "Please choise one database");
    }