Summary
You have achieved a lot in this chapter. You have built a solid foundation for the Space Invaders ++ game and you have also coded a reusable system that can be used for almost any game that is divided up into different "screens".
We now have an input handling system in place that can detect keyboard presses and mouse clicks and route the responsibility to handle them to a specific panel that is part of a specific screen. Furthermore, the abstraction of the concept of a screen allows us to set up as many different game loops as we like. The GameScreen class will be the main class to handle the logic of this game but, once you see how over the next few chapters, you could easily code another screen to play a completely different game. Of course, the most likely thing you will do is get started with your own ideas.
In the next chapter, we will code the game objects and components which are the basis of our entity-component pattern implementation.