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S.J.S. tutorial 16: Advanced inheritance

SECTION(16, Tutorial 16) m5_question(This tutorial shows you a practical example of inheritance. The file TT(StarWars.sjs) is comprised of three classes: CLSS(XWing), CLSS(TieFighter) and CLSS(StarWars). The first two represent spacecraft from the two sides of the Star Wars films. The class CLSS(StarWars) is the driver class and contains code for executing a battle between the X-Wings and the Tie Fighters.) m5_study(16.1) m5_question( POPEN m4_begin_indent NU()CLASS CLSS(XWing) EOL BEGIN EOL EOL PD PRIVATE PROPERTY INT VARI(shields); EOL PD PRIVATE PROPERTY INT VARI(weapon); EOL PD PRIVATE PROPERTY BOOLEAN VARI(dead); EOL EOL PD CONSTRUCTOR CLSS(XWing)() EOL PD BEGIN EOL PD PD shields = NUMB(1000); EOL PD PD weapon = NUMB(10); EOL PD END EOL EOL PD METHOD INT FUNC(getWeapon)() EOL PD BEGIN EOL PD PD RETURN weapon; EOL PD END EOL PD METHOD BOOLEAN FUNC(isDead)() EOL PD BEGIN EOL PD PD RETURN dead; EOL PD END EOL PD METHOD VOID FUNC(hit)(INT VARI(damage)) EOL PD BEGIN EOL PD PD shields = shields - damage; EOL PD PD IF (LT(shields,0)) EOL PD PD THEN BEGIN EOL PD PD PD SYSTEM_OUT_PRINTLN(STRI("BOOM!!!")); EOL PD PD PD dead = TRUE; EOL PD PD END EOL PD END EOL END EOL EOL CLASS CLSS(TieFighter) EOL BEGIN EOL EOL PD PRIVATE PROPERTY INT VARI(shields); EOL PD PRIVATE PROPERTY INT VARI(weapon); EOL PD PRIVATE PROPERTY BOOLEAN VARI(dead); EOL EOL PD CONSTRUCTOR CLSS(TieFighter)() EOL PD BEGIN EOL PD PD shields = NUMB(500); EOL PD PD weapon = NUMB(20); EOL PD END EOL EOL PD METHOD INT FUNC(getWeapon)() EOL PD BEGIN EOL PD PD RETURN weapon; EOL PD END EOL PD METHOD BOOLEAN FUNC(isDead)() EOL PD BEGIN EOL PD PD RETURN dead; EOL PD END EOL PD METHOD VOID FUNC(hit)(INT VARI(damage)) EOL PD BEGIN EOL PD PD shields = shields - damage; EOL PD PD IF (LT(shields,0)) EOL PD PD THEN BEGIN EOL PD PD PD SYSTEM_OUT_PRINTLN(STRI("BOOM!!!")); EOL PD PD PD dead = TRUE; EOL PD PD END EOL PD END EOL END EOL EOL CLASS CLSS(StarWars) EOL BEGIN EOL EOL PD PRIVATE FUNCTION VOID FUNC(duel)(CLSS(XWing) VARI(x), CLSS(TieFighter) VARI(t)) EOL PD BEGIN EOL EOL PD PD FUR (;;) EOL PD PD BEGIN EOL PD PD PD x.hit(t.getWeapon()); EOL PD PD PD IF (x.isDead()) EOL PD PD PD THEN BEGIN EOL PD PD PD PD SYSTEM_OUT_PRINTLN(STRI("X-Wing is dead")); EOL PD PD PD PD BREAK; EOL PD PD PD END EOL PD PD PD t.hit(x.getWeapon()); EOL PD PD PD IF (t.isDead()) EOL PD PD PD THEN BEGIN EOL PD PD PD PD SYSTEM_OUT_PRINTLN(STRI("Tie Fighter is dead")); EOL PD PD PD PD BREAK; EOL PD PD PD END EOL PD PD END EOL EOL PD END EOL EOL PD PRIVATE FUNCTION VOID FUNC(battle)(CLSS(XWing)[] VARI(good), CLSS(TieFighter)[] VARI(evil)) EOL PD BEGIN EOL EOL PD PD VAR INT VARI(g)PD PD PD = NUMB(0); EOL PD PD VAR INT VARI(e)PD PD PD = NUMB(0); EOL PD PD VAR INT VARI(goodDeaths) = NUMB(0); EOL PD PD VAR INT VARI(evilDeaths) = NUMB(0); EOL EOL PD PD WHILE (LT(g,good.length) AND LT(e,evil.length)) EOL PD PD BEGIN EOL PD PD PD SYSTEM_OUT_PRINTLN(STRI("battling X-Wing HASH") + g + STRI(" versus Tie Fighter HASH") + e); EOL PD PD PD duel(good[g],evil[e]); EOL PD PD PD IF (good[g].isDead()) EOL PD PD PD THEN BEGIN EOL PD PD PD PD g = g + NUMB(1); EOL PD PD PD PD goodDeaths = goodDeaths + NUMB(1); EOL PD PD PD END EOL PD PD PD IF (evil[e].isDead()) EOL PD PD PD THEN BEGIN EOL PD PD PD PD e = e + NUMB(1); EOL PD PD PD PD evilDeaths = evilDeaths + NUMB(1); EOL PD PD PD END EOL PD PD END EOL EOL PD PD VAR INT VARI(finalGood) = good.length - goodDeaths; EOL PD PD VAR INT VARI(finalEvil) = evil.length - evilDeaths; EOL EOL PD PD SYSTEM_OUT_PRINTLN(); EOL PD PD SYSTEM_OUT_PRINTLN(STRI("Battle Report: X-Wings Tie Fighters")); EOL PD PD SYSTEM_OUT_PRINTLN(STRI("----------------------------------------------")); EOL PD PD SYSTEM_OUT_PRINTLN(); EOL PD PD SYSTEM_OUT_PRINTLN(STRI("Initial ships:") + good.length + STRI(" ") + evil.length); EOL PD PD SYSTEM_OUT_PRINTLN(); EOL PD PD SYSTEM_OUT_PRINTLN(STRI("Killed ships:") + goodDeaths + STRI(" ") + evilDeaths); EOL PD PD SYSTEM_OUT_PRINTLN(); EOL PD PD SYSTEM_OUT_PRINTLN(STRI("Final ships:")PD + finalGood + STRI(" ") + finalEvil); EOL PD PD SYSTEM_OUT_PRINTLN(); EOL PD PD IF (GT(finalGood,finalEvil)) EOL PD PD THEN BEGIN EOL PD PD PD SYSTEM_OUT_PRINTLN(STRI("The rebel alliance is victorious!")); EOL PD PD END EOL PD PD ELSE BEGIN EOL PD PD PD SYSTEM_OUT_PRINTLN(STRI("The dark side has conquered!")); EOL PD PD END EOL PD PD SYSTEM_OUT_PRINTLN(); EOL PD END EOL EOL PD BEGIN_MAIN EOL EOL PD PD COMM(// defines the goodies array) EOL PD PD VAR CLSS(XWing)[] VARI(goodies) = NEW CLSS(XWing)[3]; EOL EOL PD PD COMM(// initialises the elements of the goodies array) EOL PD PD FUR (VAR INT VARI(i)=NUMB(0); LT(i,goodies.length); i = i + NUMB(1)) EOL PD PD BEGIN EOL PD PD PD goodies[i] = NEW CLSS(XWing)(); EOL PD PD END EOL EOL PD PD COMM(// defines the baddies array) EOL PD PD VAR CLSS(TieFighter)[] VARI(baddies) = NEW CLSS(TieFighter)[3]; EOL EOL PD PD COMM(// initialises the elements of the baddies array) EOL PD PD FUR (VAR INT VARI(i)=NUMB(0); LT(i,baddies.length); i=i+NUMB(1)) EOL PD PD BEGIN EOL PD PD PD baddies[i] = NEW CLSS(TieFighter)(); EOL PD PD END EOL EOL PD PD battle(goodies,baddies); EOL EOL PD END_MAIN EOL END EOL m4_end_indent ) m5_question(BO(Question 16.2:) Compile and run this file to see the battle between the X-Wings and the Tie Fighters unfold.) m5_question(BO(Question 16.3:) If you look at the Java code for the CLSS(XWing) and CLSS(TieFighter) classes you will notice that they are almost identical: They have the same methods and properties, the only difference is that the CLSS(XWing) objects are initialised with a different value for their shields and weapon properties to the CLSS(TieFighter) objects.) m5_question(The next few questions will guide you through the process of using inheritance to eliminate this unnecessary duplication of code. A new class called CLSS(SpaceShip) will be created and all of the code that is common to CLSS(XWing) and CLSS(TieFighter) will be moved into this class. The CLSS(XWing) and CLSS(TieFighter) classes will then be modified so that they both inherit from CLSS(SpaceShip).) m5_question(BO(Question 16.4:) The first step in this process is to create the outer shell of the CLSS(SpaceShip) class, which you should now type in: m4_begin_indent NU()CLASS CLSS(SpaceShip) EOL BEGIN EOL END EOL m4_end_indent ) m5_question(BO(Question 16.5:) Move the properties TT(shields), TT(weapon) and TT(dead) out of the CLSS(XWing) and CLSS(TieFighter) classes and into the CLSS(SpaceShip) class. You must change the privacy status of the properties from EM(private) to EM(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 subclasses of the class in which the method or property was defined.) m5_question(BO(Question 16.6:) Move the three methods TT(getWeapon), TT(isDead) and TT(hit) out of the CLSS(XWing) and CLSS(TieFighter) classses and into the CLSS(SpaceShip) class. At this point, the CLSS(XWing) and CLSS(TieFighter) classes should contain nothing but a constructor.) m5_question(BO(Question 16.7:) Finally, add the EM(extends) keyword to the first line of the CLSS(XWing) and CLSS(TieFighter) classes: m4_begin_indent NU()CLASS CLSS(XWing) EXTENDS CLSS(SpaceShip) EOL m4_end_indent NU()and m4_begin_indent NU()CLASS CLSS(TieFighter) EXTENDS CLSS(SpaceShip) EOL m4_end_indent ) m5_question(BO(Question 16.8:) Compile and run your program again, making sure that it produces the same results now that it is using inheritance.) m5_question(BO(Question 16.9:) The CLSS(SpaceShip) class is a superclass of both CLSS(XWing) and CLSS(TieFighter) containing everything that X-Wings and Tie Fighters contain in common. Because the role of the CLSS(SpaceShip) class is simply to hold these commonalities, we might choose to label the class with the EM(abstract) keyword: m4_begin_indent NU()ABSTRACT CLASS CLSS(SpaceShip) EOL m4_end_indent NU()This prevents us from creating instances of the CLSS(SpaceShip) class. Without the abstract modifier, we could happily create a NEW TT(CLSS(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 CLSS(SpaceShip) constructor. Change the class CLSS(SpaceShip) to be abstract and observe how the compiler will not accept any lines of the form: m4_begin_indent NU()VAR CLSS(SpaceShip) VARI(s) = NEW CLSS(SpaceShip)(); COMM(// compiler error) EOL m4_end_indent NU()Remove the ABSTRACT keyword and notice how the compiler will then allow this line to compile. )

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