Design Pattern Exercises with the Asteroids Game Example
[from a book chapter “Teaching Design Patterns” by Bernd Brügge and Timo Wolf]
Asteroids Game Description
The Asteroids player controls a space shuttle on a game board, representing the outer space. Two
different types of asteroids are also present on the game board: (a) small and fast asteroids,
which change their direction only when they hit the edge of the game board, and (b) big and slow
asteroids, which may change their direction any time. All asteroids can change their speed,
ranging from a minimum to a maximum speed. The player can fire rockets and change the
direction and speed of the space shuttle by using the keyboard. Striking a big asteroid with a
rocket replaces the big asteroid with three small asteroids. Striking a small asteroid removes the
asteroid from the game board. The player wins if he destroys all asteroids. He loses if his space
shuttle collides with any asteroid. Figure 1 shows a screenshot of the initial version of Asteroids.
Figure 1. Screenshot of the initial Asteroids game. Fig. 2 shows the initial class diagram obtained from the game description. We focus on the main
classes and concepts and omit unnecessary details. In the following we describe the classes of
Fig. 2:
Page 1 Figure 2. Initial class model of Asteroids. Class : Game
The Game class represents the main window of the asteroids application. It consists of a
GameBoard and a ToolBar class. It is the root component for all graphical user interface classes.
Class : ToolBar
The class ToolBar is a graphical component that presents a start and stop button to the Asteroids
player.
Class : SolidBody
The class SolidBody represents an abstraction of all objects that move over the GameBoard. The
SolidBody has a body attribute that represents an image used for painting. The flight direction is
stored in the direction attribute. Values from 0 and 360 are valid and represent the direction in
degree. Zero degree is the direction pointing to the top. The position attribute has a horizontal
and a vertical coordinate and holds the current position of a SolidBody. Each SolidBody has a
speed attribute. All attributes are protected to prevent modifications from foreign objects. Getter
operations are provided to access the attributes of the SolidBody. The operation updatePosition ()
computes and sets a new position, based on the speed and direction attribute values. If the new
position reaches the boundary of the GameBoard, the direction of the solid body gets changed
with the rule: the angle of incidence is the angle of reflection.
Class: Asteroid
The class Asteroid represents an asteroid that moves over the GameBoard. The Asteroid class
overwrites the updatePosition () operation and randomly increments or decrements the speed.
Class: BigAsteroid
Page 2 The class BigAsteroid specializes the Asteroids class by adding extra functionality that randomly
changes the direction.
Class: SmallAsteroid
This represents a small asteroid that moves with higher speed than do objects of the BigAsteroid
class.
Class: SpaceShuttle
The space shuttle is the solid body, controlled by the player. The player loses the game if the
space shuttle is destroyed. He wins if the space shuttle is the only solid body that is left on the
GameBoard. The class provides public operations to change the speed, the direction, and to fire
rockets.
Class: Rocket
Rocket objects get created by the fireRocket() operation of the SpaceShuttle class. Their
direction is the space shuttle’s direction and do not change. Rockets destroy any other solid
bodies. They get removed from the GameBoard when reaching the GameBoard boundaries or
when hitting other solid bodies.
Class: GameBoard
The GameBoard represents the outer space of the game asteroids. It consists of many SolidBody
objects. During a game, the solid bodies are moving on the GameBoard. The GameBoard paints
the images of the SolidBodies at their position. The positions of the solid bodies are always
within the surface of the GameBoard.
Class: Referee
The Referee class is the main controller of the game Asteroids. It is responsible for starting,
controlling, and stopping the game. Starting the game creates a separate control thread that
invokes the operation moveSolidBodies() in frequent intervals. The operation moveSolidBodies()
calls the updatePosition() operation of all solid-body objects and forces the GameBoard to
repaint, thus enabling the SolidBody movement. After moving all solid bodies, the operation detectCollision (. . .) : SolidBody is used to identify collisions. It computes if the two solid
bodies body1 and body2 intersect and decides which solid body gets destroyed. The destroyed
solid body is returned. If two asteroids intersect, none of them get destroyed. If an asteroid or the
space shuttle intersects with a rocket, the asteroid or space shuttle gets destroyed. If an asteroid
intersects with a space shuttle, the space shuttle gets destroyed. After detecting collisions, the
Referee tests if the player has won or lost the game. He wins if no asteroids are left on the
GameBoard. He loses if the space shuttle gets destroyed. In these cases, or if the stopGame ()
operation is invoked, the control thread and the game get stopped.
Class: SpaceShuttleController
The SpaceShuttleController listens to keyboard and mouse events. Depending on the events,
space shuttle operations are invoked, to change the direction and speed or to fire a new rocket.
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