How do I implement countdown in the GUI

I wrote a piece of code in Java about a countdown function in GUI, but I found that when I wanted to use timers and threads, time could not be refreshed in the GUI. Sleep () {I know this will be a function in the main thread, but later I ran it. It is not, and I don’t understand}. So how do I implement this countdown to refresh constantly in the GUI? The code is as follows,I only wrote a function to display the system time:

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;


public class CurrentTime {

    public String SystemDate(){
        //获取系统默认时区当前时间
        //Get the current time of the system default time zone
        LocalDateTime now_time = LocalDateTime.now();

        //对时间进行相应的格式化
        //Format the time accordingly
        DateTimeFormatter ofPattern = DateTimeFormatter.ofPattern("yyyy年MM月dd日  hh时mm分ss秒");

        //返回对应的信息
        //Return the corresponding information
        return now_time.format(ofPattern);
    }
}

----------------------------------------------------------------------------------------------------

import javax.swing.*;
import java.awt.*;


public class MainGUI {
    public void showGUI(){
        // 创建一个庄口JFrame名为：Target_Countdown
        //Create a portal JFrame named Target_ Countdown
        JFrame frame = new JFrame("Target_Countdown");

        //设置关闭窗口时的默认操作
        //Set the default action when closing the window
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //设置窗口标题
        //Set Window Title
        frame.setTitle("目标倒计时");

        //设置窗口尺寸
        //Set window size
        frame.setSize(600,400);

        //设置窗口初始显示位置
        //Set the initial display position of the window
        frame.setLocation(500,150);   //800  450

        //使窗口显示状态为 True
        //Make the window display state true
        frame.setVisible(true);



        //创建一个Jpanel面板组件
        //Create a Jpanel panel component
        JPanel jpanel = new JPanel();


        //从currentTime获得数据，并导入组件名为showArea的JTextfield中
        //Get the data from currentTime and import it into the JTextfield named showArea
        CurrentTime currentTime = new CurrentTime();

        String ymd_hms = currentTime.SystemDate();

        //使用JLabel来显示当前时间，可与JTextField替换
        //Use JLabel to display the current time, which can be replaced with JTextField
        JLabel showLabel = new JLabel(ymd_hms);
        showLabel.setFont(new Font("宋体",0,20));

        //将showLabel放置在jpanel的顶部
        //Place showLabel on the top of the jpanel
        jpanel.add(showLabel,BorderLayout.PAGE_START);

        frame.add(jpanel);
    }
}


----------------------------------------------------------------------------------------------------
    
import javax.swing.*;


public class Main {
    public static void main(String[] args) {
        //生成mainGUI实例
        //Generate mainGUI instance
        MainGUI mainGUI = new MainGUI();

        //使用SwingUtilities工具调用createAndShowGUI()方法
        //调用mainGUI中的showGUI方法   此处如果不想实例化，此处可以把mainGUI 加 static修饰 或者把showGUI()方法加static修饰
        
        //Use the SwingUtilities tool to call the createAndShowGUI () method
        //Call showGUI method in mainGUI. If you don't want to instantiate, you can add static decoration to            mainGUI or static decoration to showGUI () method
        SwingUtilities.invokeLater(mainGUI::showGUI);

    }
}

    

I hope to see a function of continuous countdown in the GUI, hope mogul can solve my problems ,sincerely say thanks for you!

Answer

Oracle has a helpful tutorial, Creating a GUI With Swing. Skip the Learning Swing with the NetBeans IDE section. Pay particular attention to the How to Use Swing Timers section.

I added a Swing Timer to your MainGUI class. The code in the actionPerformed method is executed every 200 milliseconds, which should be accurate enough for a clock that displays seconds.

Here’s the complete runnable code. I made the additional classes inner classes so I could post this code as one block.

import java.awt.BorderLayout;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

public class CountdownTimerGUI {
    public static void main(String[] args) {
        //生成mainGUI实例
        //Generate mainGUI instance
        MainGUI mainGUI = new CountdownTimerGUI().new MainGUI();

        //使用SwingUtilities工具调用createAndShowGUI()方法
        //调用mainGUI中的showGUI方法   此处如果不想实例化，此处可以把mainGUI 加 static修饰 或者把showGUI()方法加static修饰
        
        //Use the SwingUtilities tool to call the createAndShowGUI () method
        //Call showGUI method in mainGUI. If you don't want to instantiate, you can add static decoration to            mainGUI or static decoration to showGUI () method
        SwingUtilities.invokeLater(mainGUI::showGUI);
    }
    
    public class MainGUI {
        public void showGUI(){
            // 创建一个庄口JFrame名为：Target_Countdown
            //Create a portal JFrame named Target_ Countdown
            JFrame frame = new JFrame("Target_Countdown");

            //设置关闭窗口时的默认操作
            //Set the default action when closing the window
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

            //设置窗口标题
            //Set Window Title
            frame.setTitle("目标倒计时");

            //设置窗口尺寸
            //Set window size
            frame.setSize(600,400);

            //设置窗口初始显示位置
            //Set the initial display position of the window
            frame.setLocation(500,150);   //800  450

            //使窗口显示状态为 True
            //Make the window display state true
            frame.setVisible(true);



            //创建一个Jpanel面板组件
            //Create a Jpanel panel component
            JPanel jpanel = new JPanel();


            //从currentTime获得数据，并导入组件名为showArea的JTextfield中
            //Get the data from currentTime and import it into the JTextfield named showArea
            CurrentTime currentTime = new CurrentTime();

            String ymd_hms = currentTime.SystemDate();

            //使用JLabel来显示当前时间，可与JTextField替换
            //Use JLabel to display the current time, which can be replaced with JTextField
            JLabel showLabel = new JLabel(ymd_hms);
            showLabel.setFont(new Font("宋体",0,20));

            //将showLabel放置在jpanel的顶部
            //Place showLabel on the top of the jpanel
            jpanel.add(showLabel,BorderLayout.PAGE_START);

            frame.add(jpanel);
            
            Timer timer = new Timer(200, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent event) {
                    String ymd_hms = currentTime.SystemDate();
                    showLabel.setText(ymd_hms);
                }
            });
            timer.start();
        }
    }
    
    public class CurrentTime {

        public String SystemDate(){
            //获取系统默认时区当前时间
            //Get the current time of the system default time zone
            LocalDateTime now_time = LocalDateTime.now();

            //对时间进行相应的格式化
            //Format the time accordingly
            DateTimeFormatter ofPattern = DateTimeFormatter.ofPattern("yyyy年MM月dd日  hh时mm分ss秒");

            //返回对应的信息
            //Return the corresponding information
            return now_time.format(ofPattern);
        }
    }
}