当由另一个字段变量打印时，字段变量将恢复为初始化值

Question

下面的裁剪代码。

我正在使用Java和NetBeans。我已经声明了两个字符串变量，dbName 和 成功连接，作为字段。

已成功连接打印 数据库名称。

该行 JOptionPane.showMessageDialog（null， 成功连接）;打印"已成功连接到 null"而不是"已成功连接到数据库"。

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

我的问题是，为什么使用字段来存储字符串不起作用？为什么当我使用成功连接打印它时，dbName 会恢复为 null，而直接放在方法调用中时却不会？

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();
    }
}

Answer 1

dbName初始化时总是空的 successfullyConnected .

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

我认为你不需要静态字段.

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

如果 dbName 已初始化，则可以将 dbName 添加到消息中：

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

Answer 2

因为 您在用户输入之前初始化successfullyConnected文本，此时dbName null . 在用户输入后初始化它

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

Answer 3

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，并且没有用于检测更改的值的绑定。

 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);