Java Graphics - 如何在屏幕上绘制图像

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.net.URL;

import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class Main {
  public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
      @Override
      public void run() {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        Panel panel = new Panel();
        frame.setContentPane(panel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
      }
    });
  }
}

class Panel extends JPanel {
  BufferedImage image;

  public Panel() {
    try {
      image = ImageIO.read(new URL("http://www.w3cschool.cn/style/download.png"));
    } catch (Exception e) {
      e.printStackTrace();
    }
  }

  @Override
  protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.drawImage(image, 0, 0, this);

  }

  @Override
  public Dimension getPreferredSize() {
    return new Dimension(image.getWidth(), image.getHeight());
  }
}