I saw many Ludum Dare games, which are written in Java (using its standard library). I checked the code of one or two of them and the thing that I saw is in their Game class, they extend Canvas.
So my question is why would they extend Canvas, not JPanel?
Very interesting question. I developed a game with JPanel in Linux and at the end discovered that it was not functioning in Windows (getBufferStrategy null). Switching to canvas fixed the problem for me.
Without seeing the actual code, it's difficult to say. However, I would guess that they are using both Canvas and JPanel somewhere (or at least some form of JPanel, e.g. JFrame or Applet etc..) and simply extending the game class from Canvas to make it more portable, i.e. so it can be added to a JPanel (or whatever) later.
As mentioned in the API
Component:
A component is an object having a graphical representation that can be displayed on the screen and that can interact with the user.
Canvas
A Canvas component represents a blank rectangular area of the screen onto which the application can draw or from which the application can trap input events from the user.
JComponent
The base class for all Swing components except top-level containers.
JLabel
JPanel is a generic lightweight container.
if you do not implement a Swing Application then you forgo JComponents and stick with the basic Components.
