Question

Recently, I have been trying to do some Java work with graphics. Now the Graphics class is abstract, meaning it itself cannot be instantiated but a subclass of it might be able to be instantiated. So what is going on in the following code. (That does successfully create a line)

import javax.swing.*;
import java.awt.*;

public class line extends JPanel {
    public void paintComponent(Graphics g) {
        g.drawLine(0, 0, 90, 90);
    }

    public static void main(String[] args) {
        JFrame jf = new JFrame();
        jf.add(new line());
        jf.setSize(500, 500);
        jf.setVisible(true);
    }
}

Okay now a few things don’t make sense to me. First, when line is called it must enact the paintComponent method in the line class. That’s a property of static methods (being called with the class name). But the paintComponent method doesn’t have a static modifier.

Since this program works though we have to assume that paintComponent does get called and expects a Graphics object as an argument. When it is called though it has no arguments. Why doesn’t this create an error? I ran some tests with passing in a String and then nothing into a method that was expecting an Integer object. I got errors on both programs.

So somehow a graphics object called g is created and the uses the drawline method. But Graphics is abstract.

Okay so here are my questions:

1. Why is paintComponent being automatically called when it’s class line is called when paintComponent isn’t static?

2. Why does paintComponent accept empty arguments when it expects a Graphics object?

3. How is the Graphics object g then made as the Graphics class is abstract and cannot be instantiated?

Help me Stack Overflow, you’re my only hope.

EDIT:
Okay, I think this question is pretty well answered, thanks!

Answer 1

1. paintComponent is called when AWT decides it is time to paint the window (in most cases because the OS sends it a message telling it to do so). When it paints the JFrame it traverses the tree of objects that are inside it. Among which is your instance of line because you added it with the jf.add(new Line()) call. On each of these the paintComponent method is called.

2. It doesn’t accept empty arguments.

3. When AWT calls paintComponent it passes an instance of a concrete class that inherits from Graphics.

The signature paintComponent(Graphics g) not so much says the parameter should be exactly of type Graphics. But actually says it should be of type Graphics or any class inheriting from Graphics. Another way to look at it is to say that any class that inherits from Graphics is also of type Graphics.