Weeeeeeee. My first red circle! Im so happy.
Code:
   import javax.swing.*;
   import java.awt.*;
   import java.awt.event.*; 
  
  public class Shape{  
    public static void main(String[] args){
    new Circle();
  }
 }
  class Circle extends Canvas{

   public Circle(){

   JFrame jf = new JFrame("Circle!");
    jf.addWindowListener(new WindowAdapter(){
       public void windowClosing(WindowEvent e){
          System.exit(0);
       }
    });

     Dimension size = jf.getSize();
     Insets i = jf.getInsets(); 
     int height = size.height - i.top - i.bottom;
     int width = size.width - i.left - i.right;

    this.setSize(height,width);
    jf.getContentPane().add(this);
    jf.setSize(400,300);
    jf.setVisible(true);
  }
 
   public void paint(Graphics g){
    g.setColor(Color.red);
    g.fillOval(50,50,50,50); 
   }
  }