canvas java没有扩展

Question

我正在尝试做一个画布对象...是作业..但我想制作一个对象

Canvas obj=new Canvas();

但我无法画画..

package canvasobjeto;


import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
import java.awt.image.BufferedImage;

/**
 *
 * @author Robin
 */

public class CanvasObjeto {


    JFrame Frame=new JFrame();

    public CanvasObjeto() {

        Frame.setTitle("CanvasObjeto");
        Frame.setName("CanvasObjeto");
        Frame.setSize(300, 300);
        Frame.setResizable(false);
        Frame.setUndecorated(false);
        Frame.setLayout(null);
        Frame.setLocationRelativeTo(null);
        Frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        Frame.getContentPane().setBackground(Color.WHITE);
        Formato();
        Frame.setVisible(true);
    }


    private void Formato() {

        Canvas lien=new Canvas();
        Graphics g= lien.getGraphics();
        g.drawLine(0, 0, 20, 20);
        lien.setBounds(0, 0, 200, 200);
        lien.setBackground(Color.red);
        lien.repaint();
        Frame.add(lien);

    }



    public static void main(String[] args) {

        CanvasObjeto Ventana=new CanvasObjeto();
    }

}

这是错误......我不知道为什么说如果我做了obj ..

canvasobjeto.CanvasObjeto.Formato（CanvasObjeto.java:43）中线程“main”java.lang.NullPointerException中的异常