Java Buffer strategy learning

I'm learning to use bufferstrategy with canvas, I coded this class which is then added to a JFrame in another class. I'm trying to draw a rectangle, but the canvas is empty. The console says

at java.awt.Component$FlipBufferStrategy.createBuffers(Unknown Source)
at java.awt.Component$FlipBufferStrategy.<init>(Unknown Source)
at java.awt.Component$FlipSubRegionBufferStrategy.<init>(Unknown Source)
at java.awt.Component.createBufferStrategy(Unknown Source)
at java.awt.Canvas.createBufferStrategy(Unknown Source)
at java.awt.Component.createBufferStrategy(Unknown Source)
at java.awt.Canvas.createBufferStrategy(Unknown Source)
at myPanel.draw(Pannello.java:72)
at myPanel.run(Pannello.java:59)
at java.lang.Thread.run(Unknown Source)

And here's the code. I've debugged it and it goes into every method that I've made. So basically now I don't know why it's not showing my rectangle.

public class myPanel extends Canvas implements Runnable {

    //FIELDS
    private static final long serialVersionUID = 1L;
    public static int WIDTH = 1024;
    public static int HEIGHT = WIDTH / 16 * 9;
    private boolean running;
    private Thread t1;





    public synchronized void start (){
        running = true;
        t1 = new Thread (this);
        t1.start(); // calls run()

        }


    //INIT
    public myPanel(){

        setPreferredSize(new Dimension(WIDTH, HEIGHT));
        setFocusable(true);
        requestFocus();
    }


    //Main runnable

    public void run(){
        while (running){   
           update();
           draw();
        }
    }


    public void update(){

    }


    public void draw(){
       BufferStrategy bs = getBufferStrategy();
       if (bs== null){
       createBufferStrategy(3);
       }

       Graphics g = bs.getDrawGraphics();
       g.setColor(Color.BLACK);
       g.fillOval(0, 0, 20, 20);
       g.dispose();
       bs.show();
    }

}

Here is my initial answer, this should fix your problem. I will explain your errors in my next edit:

import java.awt.Canvas;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.image.BufferStrategy;

import javax.swing.JFrame;

public class myPanel extends Canvas implements Runnable {

//FIELDS
private static final long serialVersionUID = 1L;
public static int WIDTH = 1024;
public static int HEIGHT = WIDTH / 16 * 9;
private JFrame frame;
private boolean running;
private Thread t1;


public myPanel(){
    Dimension size = new Dimension(WIDTH, HEIGHT);

    setPreferredSize(size);

    frame = new JFrame();
}


public synchronized void start (){
    running = true;
    t1 = new Thread (this);
    t1.start(); // calls run()
}


//Main runnable

public void run(){
    while (running){   
       update();
       draw();
    }
}


public void update(){

}


public void draw(){
   BufferStrategy bs = getBufferStrategy();
   if (bs== null){
   createBufferStrategy(3);
   return;
   }

   Graphics g = bs.getDrawGraphics();
   g.setColor(Color.BLACK);
   g.fillOval(0, 0, 20, 20);
   g.dispose();
   bs.show();
}

public static void main(String[] args){
    myPanel myPanel = new myPanel();

     myPanel.frame.setResizable(false);
     myPanel.frame.setTitle("My frame");
     myPanel.frame.add(myPanel);
     myPanel.frame.pack();
     myPanel.frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     myPanel.frame.setLocationRelativeTo(null);
     myPanel.frame.setVisible(true);

     myPanel.start();
}
}

Ok, first of all, it is convention in Java to capitalize your class names. I did not do it here, but I hope you'll remember.

Second of all, there were a few things missing from your code. You didn't make a JFrame, and there wasn't a main method which I'm sure you know that every Java program needs to know where to start. You also need a constructor to initialize the JFrame and to set the size of the frame.

Third, you need to return; after creating the buffer strategy.

I hope this helped.

EDIT:

Instead of adding a new myPanel() to your frame, add game instead. frame.add(game);