Object-Oriented Design

1 Explain what is meant by encapsulation, and give one benefit of using it. 3

One mark for each of the following, to a maximum of 3.

• Data and the methods that work on it are combined in one class
• Access to the data from outside the class is restricted, for example by making attributes private
• Benefit: the data can only be changed through methods that check it is valid, so it stays consistent

Model answer. Encapsulation means that the data of an object and the methods that operate on it are combined in a single class, and access to the data from outside is restricted, usually by making the attributes private and providing methods to read or change them. The benefit is that the data can only be changed through methods that can check it is valid, so an object cannot be put into an inconsistent state by other parts of the program.

2 Explain the difference between a class and an object, using an example. 3

One mark for each of the following, to a maximum of 3.

• A class is a template or blueprint that defines attributes and methods
• An object is an instance of a class, created from it
• An example, such as Dog as a class and rex as an object, where each object has its own attribute values

Model answer. A class is a template that defines the attributes and methods that its objects will have. An object is an instance of a class, created from that template, and it has its own values for the attributes. For example, a Dog class defines a name and a bark method, and rex and bella are two separate Dog objects, each with their own name.

3 A Vehicle class has a private attribute registration, a protected attribute speed that starts at 0, a method accelerate(amount) that increases the speed, and a method get_speed(). A subclass Lorry has an extra attribute load, and overrides accelerate so that the speed can never go above 60. Write Python class definitions for Vehicle and Lorry. 6

One mark for each of the following, to a maximum of 6.

• Vehicle class with a constructor that sets registration as private and speed as 0
• accelerate adds amount to speed
• get_speed returns the speed
• Lorry inherits from Vehicle
• Lorry constructor calls super().__init__ and sets load
• Lorry accelerate overrides, restricting the speed to a maximum of 60

class Vehicle:
    def __init__(self, registration):
        self.__registration = registration
        self._speed = 0

    def accelerate(self, amount):
        self._speed += amount

    def get_speed(self):
        return self._speed

class Lorry(Vehicle):
    def __init__(self, registration, load):
        super().__init__(registration)
        self.load = load

    def accelerate(self, amount):
        self._speed = min(self._speed + amount, 60)

truck = Lorry("AB12 CDE", 8000)
truck.accelerate(45)
truck.accelerate(45)
print(truck.get_speed())

4 Explain what is meant by polymorphism, using the shape classes in your answer as an example. 3

One mark for each of the following, to a maximum of 3.

• The same method name can be called on objects of different classes
• Each class provides its own version of the method, which overrides the one in the superclass
• The version that runs depends on the class of the object, so area() gives a different calculation for a circle and a rectangle

Model answer. Polymorphism means that the same method can be called on objects of different classes and each responds in its own way. Each subclass provides its own version of the method, overriding the one in the superclass. For shapes, calling area() on a Circle object calculates pi times the radius squared and calling it on a Rectangle calculates width times height, and the program does not need to know which class it has because the object's class decides which version runs.

5 A game has a Character class. Some characters can fly, some can swim and some can do both. Discuss whether inheritance or composition is the better way to model how a character moves. 4

One mark for each of the following, to a maximum of 4.

• Inheritance gives a fixed is-a hierarchy, and a character that can do both cannot easily inherit from two separate parents
• Composition lets each ability be a separate class held as a part
• New abilities can be added or swapped without changing existing classes
• Reasoned conclusion: composition is better because the abilities vary and combine

Model answer. Composition is the better choice. With inheritance, flying and swimming would each be a subclass, and a character that can do both would need to inherit from two parents or have a third class made for the combination, and every new ability would multiply the hierarchy. With composition each ability, such as Flying or Swimming, is its own class and a character holds the ones it has as attributes, so any combination is possible. A new ability can be added, or one swapped at run time, without changing any existing class, so composition suits behaviour that varies and combines.