问题

我正在使用Java Swing构建Java桌面应用程序。

这是选择服务方法：

```java
public void select(int id){
    String sql = "SELECT num_of_working_days FROM working_days_and_hours WHERE id = ?";

     try {
        connection = SQLite_Connection.connect();
        preparedStatement = connection.prepareStatement(sql);

        preparedStatement.setInt(1, id);

        preparedStatement.execute();
        preparedStatement.close();

        System.out.println("DB status 1: " + preparedStatement.toString());

    } catch (Exception ex) {
        System.out.println(ex.toString());
    }
}

我想在标签中显示此结果：

private void jTextField1ActionPerformed(java.awt.event.ActionEvent evt) {                                            
    // TODO add your handling code here:
    jLabel1.setText(String.valueOf(numberOfDays.select(1)));     
    // numberOfDays.select(1);
    System.out.println("Entered: " + jTextField1.getText());

}

在jLabel1.setText(numberOfDays.select(1));这一行出现错误，提示“在此处不允许使用void类型”。

我做错了什么？感谢有关此问题的任何输入。谢谢。

<details>
<summary>英文:</summary>

I am building a Java desktop app using Java Swing.

This is the select service method

public void select(int id){
String sql = "SELECT num_of_working_days FROM working_days_and_hours WHERE id = ?";

     try {
        connection = SQLite_Connection.connect();
        preparedStatement = connection.prepareStatement(sql);
        
        preparedStatement.setInt(1, id);
        
        preparedStatement.execute();
        preparedStatement.close();

        System.out.println("DB status 1: "+ preparedStatement.toString());

    } catch (Exception ex) {
        System.out.println(ex.toString());
    }
}

I would like to display the result of this in a label

private void jTextField1ActionPerformed(java.awt.event.ActionEvent evt) {                                            
    // TODO add your handling code here:
    jLabel1.setText(numberOfDays.select(1));     
    // numberOfDays.select(1);
    System.out.println("Entered: " + jTextField1.getText());
   
} 

There is an error at 
```        jLabel1.setText(numberOfDays.select(1));     ```
Saying 'void type not allowed here'.

What am I doing wrong? Appreciate any input regarding this matter. Thank you.



</details>


# 答案1
**得分**: 0

根据错误提示，'void类型在此不允许'，因此您应该在您的“select”方法中添加返回值。

```java
public String select(int id){
    String sql = "SELECT num_of_working_days FROM working_days_and_hours WHERE id = ?";
    
     try {
        connection = SQLite_Connection.connect();
        preparedStatement = connection.prepareStatement(sql);
        
        preparedStatement.setInt(1, id);
        
        preparedStatement.execute();
        preparedStatement.close();

        return "DB状态 1：" + preparedStatement.toString();

    } catch (Exception ex) {
       return ex.toString();
    }
}

public String select(int id){
    String sql = "SELECT num_of_working_days FROM working_days_and_hours WHERE id = ?";
    
     try {
        connection = SQLite_Connection.connect();
        preparedStatement = connection.prepareStatement(sql);
        
        preparedStatement.setInt(1, id);
        
        preparedStatement.execute();
        preparedStatement.close();

        return "DB status 1: "+ preparedStatement.toString();

    } catch (Exception ex) {
       return ex.toString();
    }
}

答案2

得分: 0

你的选择方法定义如下：

public void select(int id)

这将返回 void。

查看 Java 文档中的 JLabel: setText(String str) 方法，该方法需要一个字符串数据类型。

因此，请将你的方法 select 修改为返回一个字符串，即：

public String select(int id)

另外，在你的方法中，像下面这样存储预处理语句的结果：

ResultSet rs = preparedStatement.executeQuery();  // 从查询中获取结果表
while (rs.next()) {  // 将光标定位到结果上
  String result = rs.getInt(1);  // 如果你的列是整数类型
}
preparedStatement.close();
return result;

public void select(int id)

public String select(int id)

    ResultSet rs = preparedStatement.executeQuery();        // Get the result table from the q
    while (rs.next()) {               // Position the cursor to the result 
      String result = rs.getInt(1); //If ur Column is of int type
    }
preparedStatement.close();
return result;

答案3

得分: 0

这是我提出的解决方案：

public ResultSet retrieveData(){
    String sql = "SELECT num_of_working_days FROM working_days_and_hours";
    try {
        connection = SQLite_Connection.connect();
        preparedStatement = connection.prepareStatement(sql);
        resultSet = preparedStatement.executeQuery();
    } catch (Exception ex) {
        System.out.println(ex.toString());
    }finally {		
          //Services.colsedConnections();
    }
    return resultSet;
}

public ResultSet retrieveData(){
        String sql = "SELECT num_of_working_days FROM working_days_and_hours";
        try {
                connection = SQLite_Connection.connect();
                preparedStatement = connection.prepareStatement(sql);
                resultSet = preparedStatement.executeQuery();
            } catch (Exception ex) {
                System.out.println(ex.toString());
            }finally {		
                      //Services.colsedConnections();
            }
        return resultSet;
    }