Class sederhana untuk menggambar kotak menggunakan Java

Main.java

import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;

import javax.swing.JComponent;
import javax.swing.JFrame;

public class Main extends JFrame {

    public Main(){
        initUI();
    }

    private void initUI() {
        add(new Canvas());
        setTitle("Simple Java 2D");
        setSize(800, 600);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);
    }

    public static void main(String[] args){
        EventQueue.invokeLater(new Runnable(){
            public void run() {
                Main ex = new Main();
                ex.setVisible(true);
            }
        });
    }
    
    public class Canvas extends JComponent {
    	
    	public void paint(Graphics g) {	
    		Graphics2D g2d = (Graphics2D) g;
    		g2d.setColor(Color.decode("#3498db"));
    		g2d.fillRect(100, 100, 200, 200);	
    				
    		g2d.dispose();
    		//---------------------------------------------------------------------------------------------------
    	}//end of paint(Graphics g) 
    }
}