Question

下面的裁剪代码。

我在Java和NetBeans工作。我已将两个String变量 dbName 和 successfulConnected 声明为字段。

成功连接打印 dbName 。

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

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

我的问题是，为什么使用字段存储字符串不起作用？当我使用 successfulConnected 打印时，为什么 dbName 会恢复为null，而不是直接放在方法调用中？

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

因为您在用户输入之前初始化了successfullyConnected文字，当时dbName为null。在用户输入后初始化

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

Answer 2

初始化后，除非您被修改，否则这些值不会更改。它们不是动态变量，并且没有绑定来检测更改的值。

 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);

Answer 3

初始化dbName时，successfullyConnected始终为空。

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是初始化，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");
    }

当由另一个字段变量打印时，字段变量将恢复为初始化值

3 个答案: