Java: 从数据库中检索并显示标签中的值

Question

我正在使用 Java Swing 构建一个 Java 桌面应用程序。

这是select服务方式

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());
        }
    }

我想在标签中显示这个结果

    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());
       
    }

出现错误 jLabel1.setText(numberOfDays.select(1)); 说 'void type not allowed here'.

我做错了什么？感谢有关此事的任何意见。谢谢。

Answer 1

如错误所述，'void type not allowed here' 因此您应该在“select”方法中添加 return。

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();
    }
}

Answer 2

你的select方法定义

public void select(int id)

这个return秒void

查看 Java 文档 JLabel: setText(String str) 此方法需要一个字符串 DataType

因此将您的方法 select 更改为 return 字符串，即

public String select(int id)

也在你的方法中存储准备好的语句的结果，如下所示

    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;

Answer 3

这是我想出的解决方案：

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;
    }

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