Possible improvements on Tank War solution

[ny83427]

• some flow control or game rule logic implementation are mixed in object draw method, which is not separated clearly and seems to add burden for maintenance
• a better approach might be let game objects just care about render, but separate game rule assertion, it will make changes to both part much easier
• introduce an abstract game object would be useful
• there seems no need to introduce CopyOnWriteArrayList here, as this is not a concurrent scenario of multi-threading, no need to confuse students with such kind of heavy stuff at this time
• tank move/fire and other logic is not very well organized, needs refactoring and reorganization