This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I thought only a character needs to be updated, not the background or any non-moving objects.
I thought about this after I removed the code that fills up the background in this program. The result was the image being shadowed, or the image the buffer replaced was never erased and left a trail of the image wherever the user dragged the mouse cursor.