macOS Java graphics question

[smb]

I'm going through the book called Big Java by Cay Horstmann and I have a question about a graphics example in Chapter 2. My question is how is the RectangleComponent object "component" drawn out onto the frame without the "paintComponent" method being called? I understand that "Rectangle component = new RectangleComponent();" creates the new component object...does the paintComponent method just get called automatically when the object is created? I run the program it displays the two rectangles on the frame. Here is the code...

RectangleComponent.java

01: import java.awt.Graphics;
02: import java.awt.Graphics2D;
03: import java.awt.Rectangle;
04: import javax.swing.JComponent;
05: 
06: /**
07:    A component that draws two rectangles.
08: */
09: public class RectangleComponent extends JComponent
10: {  
11:    public void paintComponent(Graphics g)
12:    {  
13:       // Recover Graphics2D
14:       Graphics2D g2 = (Graphics2D) g;
15: 
16:       // Construct a rectangle and draw it
17:       Rectangle box = new Rectangle(5, 10, 20, 30);
18:       g2.draw(box);
19: 
20:       // Move rectangle 15 units to the right and 25 units down
21:       box.translate(15, 25);
22: 
23:       // Draw moved rectangle
24:       g2.draw(box);
25:    }
26: }

RectangleViewer.java

01: import javax.swing.JFrame;
02: 
03: public class RectangleViewer
04: {
05:    public static void main(String[] args)
06:    {
07:       JFrame frame = new JFrame();
08: 
09:       frame.setSize(300, 400);
10:       frame.setTitle("Two rectangles");
11:       frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
12: 
13:       RectangleComponent component = new RectangleComponent();
14:       frame.add(component);
15: 
16:       frame.setVisible(true);
17:    }
18: }

 

[Matthew Yohe]

paintComponent is called automatically anytime the RectangleComponent "component" needs to be displayed on the screen (ie nothing is actually drawn until you call frame.setVisible)

You don't call paintComponent directly since it is event triggered (covering/uncovering the component with another window) but in some cases, you may need to have paintComponent called. Here you would use the repaint method from the Component class.

 

[smb]

That makes sense and the book actually said this (I must've skimmed over it too quick). Thanks for your help.