为什么创建JPanel类对象时会自动调用paintComponent方法？

Question

This is the java code about JPanel :

class Battle_field extends JPanel{
    public List<Image_Obj> pics_to_be_drawn;    

    @Override   
    public void paintComponent(Graphics g){     
            super.paintComponent(g);                 
            g.drawImage(pics_to_be_drawn.get(0).Get_the_buf_img() , 41, 41, 59, 59, Color.black, null);
    }       
}

As I set up the GUI:

added_panel= new Battle_field();
added_panel.setBorder(new LineBorder(SystemColor.activeCaption, 3));
added_panel.setBounds(27, 10, 397, 630);
added_panel.setBackground(Color.white);
this.getContentPane().add(added_panel);

I found that the creation of Battle_field() object above will call the paintComponent automatically.

But here, I didn't initialize the variable "pics_to_be_drawn" yet,so if it is called, it would cause compiler error. Is this design unavoidable?
As you do so, it's necessary to happen? I wanna know this very much and of course the solution.

ps: According to the official document,only if I call repaint() , it will call paintComponent() . So I can write my customized code within paintComponent .

Answer 1

Realize that you do not have control over when or if paint(...) or paintComponent(...) are called, that repaint() only suggests the repaint manager that the component should be painted, but that this doesn't always happen, that painting will occur as soon as the component is rendered , and then accept this and gear your code to work around this. Consider:

• Initialize pics_to_be_drawn to null (so you initialize it to something)
• Then check that it's not null before drawing it.

Change this:

class Battle_field extends JPanel{
    public List<Image_Obj> pics_to_be_drawn;    

    @Override   
    public void paintComponent(Graphics g){     
            super.paintComponent(g);                 
            g.drawImage(pics_to_be_drawn.get(0).Get_the_buf_img() , 41, 41, 59, 59, Color.black, null);
    }       
}

to this:

class Battle_field extends JPanel{
    public List<Image_Obj> pics_to_be_drawn;    

    @Override   
    public void paintComponent(Graphics g){     
            super.paintComponent(g);          
            if (pics_to_be_drawn != null) { 
               g.drawImage(pics_to_be_drawn.get(0).Get_the_buf_img() , 41, 41, 59, 59, Color.black, null);
            }
    }       
}

Simple

Answer 2

Swing calls the paintComponent() method when the panel must be painted. I don't think it will call it until the panel is made visible.

That said, your panel should be in a paintable state as soon as it's added to the GUI. So the paintComponent() method should handle the case where the pictures have not been added yet, by simply checking that the list if not null (and not empty):

@Override   
public void paintComponent(Graphics g){     
    super.paintComponent(g);                 
    if (pics_to_be_drawn != null && !pics_to_be_drawn.isEmpty()) {
        g.drawImage(pics_to_be_drawn.get(0).Get_the_buf_img() , 41, 41, 59, 59, Color.black, null);
    }
}

Side note: I would initialize the list to Collections.emptyList() or to new ArrayList<>() , which would avoid the check for null. I would also respect the Java naming conventions: no underscore in class and variable names, camelCasing.

Answer 3

Jpanel's paintComponent is automatically called when a window becomes visible (uncovered or deminimized) or is resized. Here's some more info about when paintComponent can be called.