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Beginning of the class named Slick0110a

Consider the code fragment shown in Listing 3 . (This code was extracted from Listing 1 to make it easier to discuss.)

Listing 3 . Beginning of the class named Slick0110a.
public class Slick0110a implements Game{ public static void main(String[]args) throws SlickException{AppGameContainer app = new AppGameContainer(new Slick0110a());app.start();//this statement is required }//end main

Listing 3 shows the beginning of the class named Slick0110a along with the entire main method.

An object of the interface type Game

To begin with, note that the Slick0110a class implements the interface named Game . Therefore, an object of this class satisfies the requirement for the second type of object identified as item 2 in the above list . In other words, an object of this class is an object of the interface type Game .

An object of the AppGameContainer class

Now note the first statement in the main method in Listing 3 that instantiates a new object of the class named AppGameContainer and saves that object's reference in the local variable named app . This object satisfies the requirement for the first type of primary object identified as item 1 in the above list . In other words, an object of this class is an object of the type GameContainer because the class named AppGameContainer is a subclass of GameContainer .

Tying the two objects together

The class named AppGameContainer provides two overloaded constructors, each of which requires an incoming parameter that is a reference toan object instantiated from a class that implements the interface named Game . The code in the main method in Listing 3 uses the simpler of the two overloaded constructors to instantiate a new object of the class named AppGameContainer and to save its reference in the local variable named app .

The code in Listing 3 also instantiates a new object of the class named Slick0110a and passes that object's reference to the constructor for the class named AppGameContainer . This is legal because the class named Slick0110a implements the interface named Game .

At this point, the two objects described in the above list exist and occupy memory. From this point forward, the container object knows about the game object and has access to its members.

Start the game program running

Finally, the last statement in the main method calls the start method on the new container object to cause the program to be initialized and to cause the game loop to start running.

The Game interface

One of the rules in Java programming is that whenever a new class definition inherits an abstract method declaration, either the new class definition mustprovide a concrete definition for the abstract method or the class itself must be declared abstract.

All of the methods declared in an interface are implicitly abstract. Therefore, whenever a new class definition implements an interface that declaresmethods, it inherits one or more abstract method declarations and the above rule applies.

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Source:  OpenStax, Anatomy of a game engine. OpenStax CNX. Feb 07, 2013 Download for free at https://legacy.cnx.org/content/col11489/1.13
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