原文出處https://www.cnblogs.com/x110/p/4239585.html

我在原文的基礎上做了優化，使動畫看起來更流暢，效果如下

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.image.MemoryImageSource;
import java.util.Arrays;
 
import java.util.Random;
import javax.swing.JDialog;
import javax.swing.JPanel;
import javax.swing.Timer;

public class Rain extends JDialog{

    private Random random = new Random();
    private Dimension screenSize;
    private JPanel graphicsPanel;
    //行高,列寬
    private final static int gap = 20;
    
 
//存放雨點頂部的位置資訊(marginTop)
    private int[] posArr;

    public Rain() {
        initComponents();
    }

    private void initComponents() {
        setLayout(new BorderLayout());
        graphicsPanel = new GraphicsPanel();
        add(graphicsPanel, BorderLayout.CENTER);
        //設定游標不可見
        Toolkit defaultToolkit = Toolkit.getDefaultToolkit();
        Image image 
 
= defaultToolkit.createImage(new MemoryImageSource(0, 0, null, 0, 0));
        Cursor invisibleCursor = defaultToolkit.createCustomCursor(image, new Point(0, 0), "cursor");
        setCursor(invisibleCursor);
        
        //ESC鍵退出
        KeyPressListener keyPressListener = new KeyPressListener();
        this.addKeyListener(keyPressListener);
        this.setAlwaysOnTop(false);
        
        //去標題欄
        // this.setUndecorated(true);
        
        //全屏
        this.getGraphicsConfiguration().getDevice().setFullScreenWindow(this);
        this.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
        setVisible(true);

        screenSize = Toolkit.getDefaultToolkit().getScreenSize();

        posArr = new int[screenSize.width / gap];
        
        for (int i = 0; i < posArr.length; i++) {
            posArr[i] = random.nextInt(screenSize.height);
        }
        
        //每秒10幀
        new Timer(100, new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                 graphicsPanel.repaint();
                
            }
        }).start();
    }

    /**
     * @return 隨機字元
     */
    private char getChr() {
        return (char) (random.nextInt(94) + 33);
    }

    private class GraphicsPanel extends JPanel {

        @Override
        public void paint(Graphics g) {
            Graphics2D g2d = (Graphics2D) g;
            g2d.setFont(getFont().deriveFont(Font.BOLD));
            g2d.setColor(Color.BLACK);
            g2d.fillRect(0, 0, screenSize.width, screenSize.height);
           
            for (int x = 0; x < screenSize.width / gap; x++) {
                int endPos = posArr[x];
                int X = x * gap;
                int Y = posArr[x];
                g2d.setColor(Color.CYAN);
                g2d.drawString(getChr()+"", X, Y);
                int cg = 255;
                
                for (int j = endPos - 15; j < endPos; j++) {
                    //顏色漸變
                    cg -= 20;
                    if (cg <= 0) {
                        cg = 0;
                    }
                    
                    g2d.setColor(new Color(0, cg, 0));
                    g2d.drawString(getChr()+"", X, Y -= gap);
                }
                
                //每放完一幀，當前列上雨點的位置下移15個畫素
                posArr[x] += 15;
                
                //當雨點位置超過螢幕高度時，重新產生一個隨機位置
                if (posArr[x] - (15 * gap) > screenSize.height) {
                    posArr[x] = random.nextInt(screenSize.height / gap);
                }
            }
        }
    }

    private class KeyPressListener extends KeyAdapter {
        @Override
        public void keyPressed(KeyEvent e) {
            if (e.getKeyCode() == KeyEvent.VK_ESCAPE) {
                System.exit(0);
            }
        }
    }

    public static void main(String[] args) {
        new Rain();
    }
}

程式碼複製直接執行即可