一段时间后如何在面板上重新绘制符号？

Question

I started making a small app on swing and ran into problems. I do not know how to draw a character (string) in a "for" loop on the panel, AFTER a CERTAIN AMOUNT of TIME(5 seconds), and so that the characters do not overlap each other on the panel, but so that the program does not stop working, but stops!

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

public class TestClass {

    private static JFrame frame;
    private static JPanel panel;

    public static void main(String[] args) {

        frame = new JFrame();

        panel = new JPanel() {

            @Override
            protected void paintComponent(Graphics g) {
                super.paintComponent(g);

                g.setColor(Color.GREEN);
                g.setFont(new Font("ms mincho", Font.ROMAN_BASELINE, 20));

                for (char c = '\u30A0'; c <= '\u30FF'; c++)
  //Here the program should stop for a while, and the next character should not overlap the previous one.
  // I tried using Thread.sleep() and IT DOESN't WORK AS it SHOULD, because the program is interrupted and the panel disappears.
                g.drawString(String.valueOf(c), 0, 20);
            }
        };
        panel.setBackground(Color.BLACK);

        frame.add(panel);

        frame.setSize(600, 800);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}

If you solve these problems, there is another one. In the loop, I will have to re-create the JPanel object each time and draw on it. And that's not good.

Thanks)

Answer 1

(I am not Swing guru so below answer may not be optimal, any improvements are welcome)

From what I remember Swing has its own dedicated thread which is responsible for drawing components and handling events . So if you place too much logic (loops, etc) in paintComponent method it will make that thread unresponsive until it will finish handling that logic which will make it look like your application stopped working.

To solve that kind of problem make paintComponent print single character (that is simple logic which swing should be able to handle quickly) but at the same time create separate thread which will periodically update variable which holds that character (and after updating character will suggest frame or panel to repaint itself again).

So your code can look something like:

class TestClass {

    private static JFrame frame;
    private static JPanel panel;

    private static volatile char charToPrint = '\u30A0';

    public static void main(String[] args) {
        frame = new JFrame();
        panel = new JPanel() {
            Font ms_mincho = new Font("ms mincho", Font.ROMAN_BASELINE, 20);

            @Override
            protected void paintComponent(Graphics g) {
                super.paintComponent(g);
                g.setColor(Color.GREEN);
                g.setFont(ms_mincho);
                g.drawString(String.valueOf(charToPrint), 0, 20);
            }
        };
        panel.setBackground(Color.BLACK);
        frame.add(panel);
        frame.setSize(600, 800);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setVisible(true);

        ScheduledExecutorService scheduledExecutorService = Executors.newSingleThreadScheduledExecutor();
        scheduledExecutorService.scheduleAtFixedRate(() -> {
            if (charToPrint == '\u30FF')//if last character in range
                charToPrint = '\u30A0'; //start over from first
            else
                charToPrint++;          //else set next character

            frame.repaint();            //suggest frame to repaint its component
                                        //probably can be change to panel.repaint()
        }, 0, 100, TimeUnit.MILLISECONDS);
    }
}

Answer 2

Here's my take on a solution. You have to create some sort of thread to count down the time for you. I used the one in javax.swing:

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import java.util.List;
import java.util.ArrayList;
import javax.swing.Timer;

public class TestClass {

    private static JFrame frame;
    private static JPanel panel;
    private static List<Character> chars = new ArrayList<>();
    private static int currentIndex = 0;

    public static void main(String[] args) {

      // create an array of characters up front...
      for (char c = '\u30A0'; c <= '\u30FF'; c++) {
        chars.add(Character.valueOf(c));
      }

      // delayedPaint() called when timer fires. Updates the index
      // into the character array; resets when necessary. Calls
      // repaint() to redraw the screen...
      ActionListener delayedPaint = new ActionListener() {
        public void actionPerformed(ActionEvent e) {
          currentIndex++;
          if (currentIndex == 95) {
            currentIndex = 0;
          }
          panel.repaint();
        }
      };

      // Timer setup...
      Timer timer = new Timer(500, delayedPaint);
      timer.setInitialDelay(1000);
      timer.setDelay(500);
      timer.start();

        frame = new JFrame();

        panel = new JPanel() {

            @Override
            protected void paintComponent(Graphics g) {
                super.paintComponent(g);

                g.setColor(Color.GREEN);
                g.setFont(new Font("ms mincho", Font.ROMAN_BASELINE, 20));

                // did you want these in a row? I wasn't sure...
                int x = 10;
                int y = 20;
                for (int i = 0; i < currentIndex; i++) {
                  g.drawString(String.valueOf(chars.get(i)), x, y);
                  x += 20;
                  // bumps y to the next row...
                  if (x > panel.getWidth()) {
                    x = 0;
                    y += 20;
                  }
                }
            }
        };
        panel.setBackground(Color.BLACK);

        frame.add(panel);

        frame.setSize(600, 800);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}