为什么paintComponent无法接受Graphics2D对象？

Question

public class MyDrawPanel extends JPanel {
    public void paintComponent(Graphics g){

        Graphics2D gd2 = (Graphics2D) g;
        GradientPaint gradient = new GradientPaint(70,70,Color.blue,150,150,Color.red);


    }
}

Why is this valid but not this:

public class MyDrawPanel extends JPanel {
    public void paintComponent(Graphics2D g){

        GradientPaint gradient = new GradientPaint(70,70,Color.blue,150,150,Color.red);

        g.setPaint(gradient);
        g.fillOval(70,70,100,100);
    }
}

First one renders, but the second one renders no graphics other than the frame. I noticed that paintComponent() requires a Graphics object, but if Graphics2D is a subclass of the Graphics object why can I not call a subclass of Graphics?

Is there some concept I am not picking up as to why this is?

Answer 1

It says that you should implement it this way, because Graphics2D is Graphics, while Graphics is not Graphics2D.

If you find casting disturbing, you can always create your own eg. MyJPanel that extends JPanel, define your own method, and subclass it in the future, overriding your defined method.

public class MyJPanel extends JPanel {

    @Override
    protected void paintComponent(Graphics g) {
        paintComponent((Graphics2D) g);
    }

    protected void paintComponent(Graphics2D g) {
    }
}

Answer 2

Basically, when you override a method, you can be equally or less specific.

Think about this:

JPanel p = new MyPanel();
p.paintComponent(someGraphicsInstance);

A reference to a JPanel is expected to be able to accept a Graphics reference as a parameter to the paintComponent method. Your method, however, violates that requirement as it will not accept a Graphics instance, but only a Graphics2D .

More information about this can be found https://stackoverflow.com/a/9950538/567864