Object-Oriented Programming
Home Lesson 6 — A Full Paper 4 Question

Q3 is the single most repeated OOP error in Paper 4 — a get method with a parameter appears in the examiner reports of all ten series since 2021. If you lost a mark on it, read Lesson 2 again before you start the clock.

In the exam you would move on, and parts are marked independently — so move on here too. If you open a hint, write "hint used" beside that part in the Evidence Document. Honesty is what makes the diagnosis work.

• (a) Three P attributes, two F attributes. Only the P ones are in the constructor header.
• (b) Neither method takes a parameter beyond self.
• (c) Two paths, two returns. Change the attributes before the return — nothing after a return runs.
• (d) Four parameters. The next line passes three of them to super().__init__.
• (e) Cost = super().Hire(Hours) on a line of its own — then test Cost.
• (f) Lesson 4's Club, with different names and ten elements.
• (g) Lesson 3's linear search, inside the class. Return what the bike's Hire() returns; return -1 only after the loop.
• (h) Loop to NumberOfBikes, not to 10. Ask each bike with GetAvailable().
• (i) Station.AddBike(EBike("E201", "Volt 500", 9, 85)) creates the object and passes it in one line.

B102 hired for 3 hours, cost 15
E201 hired for 2 hours, cost 18
E202 cannot be hired
B102 cannot be hired
B777 cannot be hired
Bikes available: 3

With the extension, a seventh line: E201 battery: 65. If yours differs, the difference tells you where to look:

Accepted alternatives

• MP12 and MP15: Bike.__init__(self, BikeID, Model, HireRate) and Bike.Hire(self, Hours).
• MP14: testing for the battery being 20 or more first, with the -1 in the else.
• MP24–MP25: a while loop with a found flag, provided the value returned by Hire() is the value returned by HireBike().
• MP28: creating each bike in a variable first, or a loop over a list of the test data.

Not credited, however good the code looks

• HoursHired, Available or a fixed value taken as a constructor parameter — MP2 lost, as well as the point for that attribute.
• print(Hours * self.__HireRate) in place of return — MP9 lost, and everything that relies on the value fails with it.
• Changing Available after the return line — MP8 lost. That line never runs.
• super().Hire(Hours) called inside an if condition and again on the next line — MP15 lost. The bike is hired twice: the second call finds it unavailable and returns -1.
• self.__Available or self.__HireRate anywhere in EBike — MP15 lost. They are private to Bike.
• class HireStation(Bike): — MP18 lost. Bike is in the data type column: a station has bikes.
• HireBike() that outputs a message instead of returning — MP24 lost. The design says returns, and part (i) depends on it.
• Calling HireBike() without storing what it returns, then calling it again to print the cost — MP29 lost, and the second call reports the bike as unavailable.
• Code pasted as an image — no code marks at all.

# 1(a)
class Bike:
    def __init__(self, BikeID, Model, HireRate):
        self.__BikeID = BikeID       # STRING
        self.__Model = Model         # STRING
        self.__HireRate = HireRate   # INTEGER
        self.__HoursHired = 0        # INTEGER
        self.__Available = True      # BOOLEAN

    # 1(b)
    def GetBikeID(self):
        return self.__BikeID

    def GetAvailable(self):
        return self.__Available

    # 1(c)
    def Hire(self, Hours):
        if self.__Available == True:
            self.__HoursHired = self.__HoursHired + Hours
            self.__Available = False
            return Hours * self.__HireRate
        else:
            return -1


# 1(d) an EBike IS A Bike
class EBike(Bike):
    def __init__(self, BikeID, Model, HireRate, BatteryLevel):
        super().__init__(BikeID, Model, HireRate)
        self.__BatteryLevel = BatteryLevel     # INTEGER

    # 1(e) overrides Bike.Hire
    def Hire(self, Hours):
        if self.__BatteryLevel < 20:
            return -1
        Cost = super().Hire(Hours)             # store what the parent's method returns
        if Cost != -1:
            self.__BatteryLevel = self.__BatteryLevel - 10 * Hours
        return Cost


# 1(f) a HireStation HAS bikes — no brackets
class HireStation:
    def __init__(self, StationName):
        self.__StationName = StationName       # STRING
        self.__Bikes = [None] * 10             # ARRAY[0:9] OF Bike
        self.__NumberOfBikes = 0               # INTEGER

    def AddBike(self, NewBike):
        if self.__NumberOfBikes < 10:
            self.__Bikes[self.__NumberOfBikes] = NewBike
            self.__NumberOfBikes = self.__NumberOfBikes + 1
            return True
        else:
            return False

    # 1(g)
    def HireBike(self, BikeID, Hours):
        for Index in range(0, self.__NumberOfBikes):
            if self.__Bikes[Index].GetBikeID() == BikeID:
                return self.__Bikes[Index].Hire(Hours)
        return -1

    # 1(h)
    def CountAvailable(self):
        Count = 0
        for Index in range(0, self.__NumberOfBikes):
            if self.__Bikes[Index].GetAvailable() == True:
                Count = Count + 1
        return Count


# 1(i) main program
Station = HireStation("Harbour Road")
Added = Station.AddBike(Bike("B101", "City Cruiser", 4))
Added = Station.AddBike(Bike("B102", "Trail Runner", 5))
Added = Station.AddBike(EBike("E201", "Volt 500", 9, 85))
Added = Station.AddBike(EBike("E202", "Volt 500", 9, 15))
Added = Station.AddBike(Bike("B103", "City Cruiser", 4))

Requests = [["B102", 3], ["E201", 2], ["E202", 1], ["B102", 2], ["B777", 1]]
for Request in Requests:
    Cost = Station.HireBike(Request[0], Request[1])
    if Cost == -1:
        print(Request[0] + " cannot be hired")
    else:
        print(Request[0] + " hired for " + str(Request[1]) + " hours, cost " + str(Cost))

print("Bikes available: " + str(Station.CountAvailable()))

• Polymorphism is doing the work in (g). HireBike calls Hire() on whatever is in the array. For E201 and E202 that is EBike's version, with the battery check; for the others it is Bike's. The station never asks which class a bike is.
• The value returned travels three levels. Bike.Hire returns the cost to EBike.Hire, which stores it and returns it to HireStation.HireBike, which returns it to the main program, which stores it and tests it. Break any link and the output fails — that chain is exactly what examiners mean by "using what a call returns".
• (i) uses Added = … even though nothing is tested here. Keeping what a method returns is a habit; in a question that asked for a club-full message, it would be the mark.