The drawing code

Listing 5 draws a black ball at the old coordinates (see Listing 4):

• That were saved in tempX and tempY.

That erases the old image:

• Because a black ball cannot be seen in front of a black background.

Then Listing 5 draws a new green ball:

• At the location specified by the x and y coordinates
  • Produced by the code in the switch statement in Listing 4.