SimplePaint

package test;

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

public class SimpleDraw {

    public static void main(String[] args) {
        JFrame frame = new JFrame("Draw something");
        frame.setBounds(30, 30, 500, 500);
        frame.setLayout(null);

        JPanel panel = new SimplePanel();
        panel.setBounds(30, 20, 350, 350);
        panel.setBackground(Color.YELLOW);

        frame.add(panel);

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

}

class SimplePanel extends JPanel {

    public void paint(Graphics g) {
        super.paint(g); //důležité třeba kvůli vykreslení pozadí

        g.setColor(Color.blue);
        g.fillOval(50, 20, 70, 90);

        g.setColor(Color.green);
        g.drawRect(80, 20, 60, 70);

        g.setColor(Color.red);
        g.drawLine(10, 20, 30, 40);
    }

}