サンプル

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class CustomSwing {
  public static void main(String[] args) {
    JFrame frame = new JFrame("BorderLayout Demo");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    JPanel panel = new JPanel() {
      private static final long serialVersionUID = 1L;

      @Override
      protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.DARK_GRAY);
        g.fillRect(10, 10, 100, 100);
        g.setColor(Color.BLUE);
        g.setFont(new Font("Serif", Font.BOLD, 36));
        g.drawString("Hello World!", 10, 50);
      }

    };

    frame.add(panel);
    frame.pack();
    frame.setVisible(true);
  }
}

参考文献