如何在没有在main方法中调用的情况下运行paint()？

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

public class MyApp extends JPanel {
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        // 在这里编写paint()方法的内容
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }

    private static void createAndShowGUI() {
        JFrame frame = new JFrame("MyApp");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(new MyApp());
        frame.pack();
        frame.setVisible(true);
    }
}

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class MyApp extends Application {
    @Override
    public void start(Stage primaryStage) {
        StackPane root = new StackPane();
        Scene scene = new Scene(root, 300, 275);
        primaryStage.setTitle("MyApp");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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

import java.awt.*;
import java.awt.event.*;

public class MyApp extends Frame implements WindowListener {
    public MyApp() {
        setSize(300, 200);
        setTitle("MyApp");
        addWindowListener(this);
        setVisible(true);
    }

    @Override
    public void paint(Graphics g) {
        super.paint(g);
        // 在这里编写paint()方法的内容
    }

    public static void main(String[] args) {
        MyApp app = new MyApp();
    }

    // 实现WindowListener接口的其他方法
    public void windowOpened(WindowEvent e) {}
    public void windowClosing(WindowEvent e) {System.exit(0);}
    public void windowClosed(WindowEvent e) {}
    public void windowIconified(WindowEvent e) {}
    public void windowDeiconified(WindowEvent e) {}
    public void windowActivated(WindowEvent e) {}
    public void windowDeactivated(WindowEvent e) {}
}