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This tutorial shows you a practical example of inheritance. The file StarWars.jtw is comprised of three classes: XWing, TieFighter and StarWars. The first two represent spacecraft from the two sides of the Star Wars films. The class StarWars is the driver class and contains code for executing a battle between the X-Wings and the Tie Fighters.
Question 16.1: Study, compile and run the following code:
class XWing begin private property int shields; private property int weapon; private property boolean dead; constructor XWing() begin shields = 1000; weapon = 10; end method int getWeapon() begin return weapon; end method boolean isDead() begin return dead; end method void hit(int damage) begin shields = shields - damage; if (shields<0) then begin System.out.println("BOOM!!!"); dead = true; end end end class TieFighter begin private property int shields; private property int weapon; private property boolean dead; constructor TieFighter() begin shields = 500; weapon = 20; end method int getWeapon() begin return weapon; end method boolean isDead() begin return dead; end method void hit(int damage) begin shields = shields - damage; if (shields<0) then begin System.out.println("BOOM!!!"); dead = true; end end end class StarWars begin private function void duel(XWing x, TieFighter t) begin for (;;) begin x.hit(t.getWeapon()); if (x.isDead()) then begin System.out.println("X-Wing is dead"); break; end t.hit(x.getWeapon()); if (t.isDead()) then begin System.out.println("Tie Fighter is dead"); break; end end end private function void battle(XWing[] good, TieFighter[] evil) begin var int g = 0; var int e = 0; var int goodDeaths = 0; var int evilDeaths = 0; while (g<good.length and e<evil.length) begin System.out.println("battling X-Wing #" + g + " versus Tie Fighter #" + e); duel(good[g],evil[e]); if (good[g].isDead()) then begin g = g + 1; goodDeaths = goodDeaths + 1; end if (evil[e].isDead()) then begin e = e + 1; evilDeaths = evilDeaths + 1; end end var int finalGood = good.length - goodDeaths; var int finalEvil = evil.length - evilDeaths; System.out.println(); System.out.println("Battle Report: X-Wings Tie Fighters"); System.out.println("----------------------------------------------"); System.out.println(); System.out.println("Initial ships:" + good.length + " " + evil.length); System.out.println(); System.out.println("Killed ships:" + goodDeaths + " " + evilDeaths); System.out.println(); System.out.println("Final ships:" + finalGood + " " + finalEvil); System.out.println(); if (finalGood>finalEvil) then begin System.out.println("The rebel alliance is victorious!"); end else begin System.out.println("The dark side has conquered!"); end System.out.println(); end beginMain // defines the goodies array var XWing[] goodies = new XWing[3]; // initializes the elements of the goodies array for (var int i=0; i<goodies.length; i = i + 1) begin goodies[i] = new XWing(); end // defines the baddies array var TieFighter[] baddies = new TieFighter[3]; // initializes the elements of the baddies array for (var int i=0; i<baddies.length; i=i+1) begin baddies[i] = new TieFighter(); end battle(goodies,baddies); endMain end
Question 16.2: Compile and run this file to see the battle between the X-Wings and the Tie Fighters unfold.
Question 16.3: If you look at the Java code for the XWing and TieFighter classes you will notice that they are almost identical: They have the same methods and properties, the only difference is that the XWing objects are initialized with a different value for their shields and weapon properties to the TieFighter objects.
The next few questions will guide you through the process of using inheritance to eliminate this unnecessary duplication of code. A new class called SpaceShip will be created and all of the code that is common to XWing and TieFighter will be moved into this class. The XWing and TieFighter classes will then be modified so that they both inherit from SpaceShip.
Question 16.4: The first step in this process is to create the outer shell of the SpaceShip class, which you should now type in:
class SpaceShip begin end
Question 16.5: Move the properties shields, weapon and dead out of the XWing and TieFighter classes and into the SpaceShip class. You must change the privacy status of the properties from private to protected. The protected modifier was invented as an intermediate level of privacy between public and private. Like private, it allows visibility to the same class in which the method or property was defined, but unlike private it also allows visibility to sub-classes of the class in which the method or property was defined.
Question 16.6: Move the three methods getWeapon, isDead and hit out of the XWing and TieFighter classses and into the SpaceShip class. At this point, the XWing and TieFighter classes should contain nothing but a constructor.
Question 16.7: Finally, add the extends keyword to the first line of the XWing and TieFighter classes:
class XWing extends SpaceShip
and
class TieFighter extends SpaceShip
Question 16.8: Compile and run your program again, making sure that it produces the same results now that it is using inheritance.
Question 16.9: The SpaceShip class is a superclass of both XWing and TieFighter containing everything that X-Wings and Tie Fighters contain in common. Because the role of the SpaceShip class is simply to hold these commonalities, we might choose to label the class with the abstract keyword:
abstract class SpaceShip
This prevents us from creating instances of the SpaceShip class. Without the abstract modifier, we could happily create a new SpaceShip(), which would be an object that is not an X-Wing, nor a Tie Fighter, but just a vague "space ship". If we consider this to be a logical mistake then we can use abstract to prevent such calls to the SpaceShip constructor. Change the class SpaceShip to be abstract and observe how the compiler will not accept any lines of the form:
var SpaceShip s = new SpaceShip(); // compiler error
Remove the abstract keyword and notice how the compiler will then allow this line to compile.
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