— Scott McNealy, Sun Microsystems Chairman, President and ceo

instance of the Graphics2D class. 
Why do you care? Because there are things you can do with 
a Graphics2D reference that you can’t do with a Graphics 
reference. A Graphics2D object can do more than a Graphics 
object, and it really is a Graphics2D object lurking behind the 
Graphics reference.
Remember your polymorphism. The compiler decides which 
methods you can call based on the reference type, not the 
object type. If you have a Dog object referenced by an Animal 
reference variable:
Animal a = new Dog();
You can NOT say:
a.bark();
Even though you know it’s really a Dog back there. The 
compiler looks at ‘a’, sees that it’s of type Animal, and finds 
that there’s no remote control button for bark() in the Animal 
class. But you can still get the object back to the Dog it really is 
by saying:
Dog d = (Dog) a;
d.bark();
So the bottom line with the Graphics object is this:
If you need to use a method from the Graphics2D class, you 
can’t use the the paintComponent parameter (‘g’) straight 
from the method. But you can cast it with a new Graphics2D 
variable.
Graphics2D g2d = (Graphics2D) g;
Methods you can call on 
a 
Graphics2D
reference:
fill3DRect()
draw3DRect()
rotate()
scale()
shear()
transform()
setRenderingHints()
(these are not complete method lists, 
check the API for more)
To cast the 
Graphics2D
object to 
a 
Graphics2D
reference:
Graphics2D g2d = (Graphics2D) g;

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getting 
gui
you are here
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public void paintComponent(Graphics g) {