why is a new Graphics object not passed to paintComponent method in Java swing?

Question

I am reading swing tutorial. From what is mentioned there:

The paintComponent method of JComponent is automatically called 
when a frame is drawn/re-drawn. 

But, what I do not understand is what is the Graphics Object that is passed to it. I do not see any new object of Graphics type being instanstiated and passed. so how does all this happen?

public void paintComponent(Graphics g){
              Graphics2D g2 = (Graphics2D)g;
              g2.drawimage(image,x,y,null);
              //
            }

I guess it is a similar situation with actionlisteners. But in this case, The actionPerformed is called automatically when event occurs such as button click and the events gets passed to the actionPerformed. There is no need to separately call this method and pass the Actionevent object. But I do not understand how it happens with paintComponent method.

button.addActionListener(new ActionListener()
   {
     public void actionPerformed(ActionEvent event){
              //do something
     } 
   });

Thanks

Answer 1

The Graphics object is created by the paint sub system.

You should never be calling paintComponent yourself and should simply allow the paint system deal with it. Even if you wanted to capture or print the component using your Graphics context (from something like BufferedImage), you should be using print or printAll

Take a look at Painting in AWT and Swing

Answer 2

They are similar issues. The Graphics object is created by the Swing library at the request o the JVM and passed into the paintComponent(Graphics g) method when the JVM makes this method call. Because you yourself are not directly calling this method, you never see the object being created.