Of Garbage Man and Banjos


This year the first year students can create a game as part of their programming coursework. The game is "Alien Banjo Attack" and above you can see a screenshot of my demo version. At the top are various different types of evil banjo and your mission is to defend the earth using nothing more than your accordion which will shoot notes of music to destroy the incoming swarm.  If the banjos reach the bottom of the screen you lose the game. If you crash into a banjo you lose a life. Space Invaders with a banjo feel. What's not to love?

I want there to be loads of objects (notes, banjos etc) on the screen at the same time, which means a lot of instances of the various classes that represent the game objects. There has been some discussion about the best way to handle this, as banjos are destroyed and new ones appear.

I reckon the best way to do this is to make a banjo stateful. The banjo will contain a flag to represent whether or not it is involved in the game. If a banjo is destroyed this flag set to indicate that it is dead and the banjo plays no further part in the gameplay. When we need a new banjo we just have to find one which is marked as dead and "resurrect" it by moving the banjo to a new position and then changing the flag to bring it back to life. 

You might think that creating new instances of banjos to replace discarded ones would be another way to achieve this behaviour but I don't like that at all. If we start creating and destroying objects this will make work for the Garbage Collector who will have to come in and tidy up memory every now and then which might slow things down a bit. 

Another way to reuse objects would be to keep a list of "dead" banjos. Each time a banjo is killed we move it out of the "active" list into the list of dead ones. That way, if we need a new banjo we just have to look in the list. This is a bit more complicated than just searching for a banjo that can be resurrected, but it does have the advantage that the game never wastes time working through "dead" display objects, as these are no longer in the active list.  Many operating systems use this technique in what is called a "thread pool" where previously used threads are kept ready for use by processes that might need them.