Question

I’ve just started coding video games and I’ve heard that doing all your drawing on a JPanel and attaching that panel onto a JFrame is better than simply drawing onto the JFrame. I was just wondering why is this better?

Answer 1

It is better for various reasons, including:

• In most Swing components, custom painting is achieved by overriding the paintComponent(Graphics) method. Top-level Swing containers (e.g. JFrame, JApplet, JWindow) have only paint(Graphics). As a result of the common method for painting, people who answer often forget about this difference between common and top-level components, and therefore suggest silly advice. 😉
• A JComponent can be added to a JFrame, or a JApplet, or a JDialog, or a constraint of a layout in a JPanel, or a JInternalFrame, or a.. Just when you think your GUI is complete, the client says “Hey, wouldn’t it be great to throw in a tool-bar & offer it as an applet as well?” The JComponent approach adapts easily to that.
• As explained (complete with code!) by Richante, it is easier to calculate the co-ordinates required for painting the custom component, and if it has a preferred size, to size the JFrame to be ‘exactly the right size’ to contain the GUI (using pack()). That is especially the case when other components are added as well.

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Now, two minor disagreements with the advice offered.

If the entire component is custom painted, it might be better to simply put a BufferedImage inside an ImageIcon/JLabel combo. Here is an example of painting to an image.
When it comes time to update it:

1. Call getGraphics() for a Graphics or createGraphics() for a Graphics2D
2. Draw the custom painting to that graphics instance
3. Dispose of the graphics instance
4. Call repaint() on the label.