Programming paradigms and object-oriented programming – A-level Computer Science exam questions with mark scheme

1Core3 marks

(a) State what is meant by encapsulation in object-oriented programming. [1]

(b) Explain two differences between a local variable and a global variable. [2]

(a) Combining/bundling the data (attributes) and the methods that act on that data into a single object/class; accept: hiding an object's internal state so it can only be accessed through its methods(1)
(b) A local variable is only accessible/has scope within the subroutine/block in which it is declared, whereas a global variable is accessible throughout the whole program(1)
(b) A local variable only exists (is allocated memory) while its subroutine is executing, whereas a global variable exists for the whole time the program runs; accept: a local variable with the same name as a global variable takes precedence inside that subroutine(1)
Answer: (a) Bundling data and the methods that operate on it into one class/object (with internal state hidden). (b) Scope: local = only within its subroutine, global = whole program; Lifetime: local exists only while subroutine runs, global for the whole run.
Examiner tip: For 'explain two differences' each mark needs both sides of the comparison – saying only what a local variable does is not a difference.

2Standard6 marks

The Python program below defines two classes.

class Vehicle:
    def __init__(self, reg, wheels):
        self.__reg = reg
        self._wheels = wheels

    def describe(self):
        return self.__reg + " has " + str(self._wheels) + " wheels"

class Lorry(Vehicle):
    def __init__(self, reg, wheels, maxLoad):
        super().__init__(reg, wheels)
        self.__maxLoad = maxLoad

    def describe(self):
        return super().describe() + ", max load " + str(self.__maxLoad) + " tonnes"

l = Lorry("KX70 ABC", 6, 26)
print(l.describe())

(a) Identify the name of the subclass. [1]

(b) Identify a method that has been overridden. [1]

(d) The attribute __reg is private. Explain why a programmer would make an attribute private and how other objects could still be allowed to read its value. [2]

(a) Lorry(1)
(b) describe; accept __init__ / the constructor (also redefined in Lorry)(1)
(c) KX70 ABC has 6 wheels(1)
(c) , max load 26 tonnes – i.e. full output KX70 ABC has 6 wheels, max load 26 tonnes on one line; ignore missing quotation marks(1)
(d) So that it cannot be accessed/changed directly by code outside the class, preventing accidental/invalid changes (encapsulation/information hiding); NE 'for security' on its own(1)
(d) A public getter/accessor method (e.g. getReg) could be provided that returns the value; accept: a public property/read-only property(1)
Answer: (a) Lorry (b) describe (c) KX70 ABC has 6 wheels, max load 26 tonnes (d) Stops other code changing it directly/invalidly; provide a public getter method.
Examiner tip: In (c) follow super().describe() into the parent class first – many students output only the Lorry part.

3Stretch6 marks

The class diagram shows part of a drawing application. Shape is an abstract class; its method area is abstract (shown in italics).

(a) State the type of relationship between Drawing and Shape shown by the hollow diamond, and explain what it means for the lifetime of the Shape objects. [2]

(b) Explain why area has been declared as an abstract method in Shape rather than being implemented there. [2]

(c) The attribute shapes is a list of Shape objects. Write the method totalArea for the Drawing class so that it returns the total area of all the shapes in the drawing. Your method must make use of polymorphism. You may use pseudo-code or a programming language you have studied. [2]

(a) Aggregation (association aggregation)(1)
(a) The Shape objects can exist independently of the Drawing / are not destroyed when the Drawing object is destroyed (unlike composition)(1)
(b) There is no general way to calculate the area of an unspecified shape, so Shape cannot provide a sensible implementation(1)
(b) It forces every (concrete) subclass to provide/override its own area method, so any Shape object can be asked for its area; accept: so that area() can be called polymorphically on any object in the list of shapes(1)
(c) Iterates over every object in shapes, accumulating a total (initialised to 0) which is returned(1)
(c) Calls area() on each object without checking its class (e.g. no IF type = Circle), so the correct overridden method is used for each object(1)
Answer: (a) Aggregation – shapes can exist after the drawing is deleted. (b) The area can't be calculated for a general shape; every subclass must override area. (c) e.g.
```
def totalArea(self):
    total = 0
    for s in self.shapes:
        total += s.area()
    return total
```
Examiner tip: Using IF statements to test which subclass an object is defeats polymorphism and loses the final mark in (c).

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