What does "Graphics2D g2d = (Graphics2D) g;" mean and what is the significance of this line in the following code segment?

Question

public void paint(Graphics g) 
{
    Graphics2D g2d = (Graphics2D) g;
    g2d.setColor(Color.RED);
    g2d.fillOval(0, 0, 30, 30);
    g2d.drawOval(0, 50, 30, 30);        
    g2d.fillRect(50, 0, 30, 30);
    g2d.drawRect(50, 50, 30, 30);

    g2d.draw(new Ellipse2D.Double(0, 100, 30, 30));
}

Well, I am new to JPanel class and I have slight confusion. g is an object of Graphics class. So, what does (Graphics2D) g mean as in this line g seems as if it is a method and not an object? Further, can anyone tell me why Graphics2D class cannot be instantiated?

Answer 1

g is a variable, not a method. It is declared in the method declaration because it is a parameter of the method (i.e., it needs to be passed whenever the function is called).

The (Graphics2D) cast allows you to treat g as a Graphics2D object. See here for more information on casts.

You cannot initiate Graphics2D because it is an abstract class, meaning that it has some methods that are specific to implementation.