Set 6

Question 1

What is the advantage of favouring composition over inheritance?

Answer 1

It makes it easier to test each class using unit testing

Question 2

What is the similarity between composition and aggregation?

Answer 2

Both “has a” relationships (ownership)

Question 3

What is the difference between composition and aggregation?

Answer 3

• In composition, if the containing object is destroyed so are the objects it contains
• This is not the case with aggregation

(composition = strong ownership, aggregation = weak ownership)

Question 4

Give five advantages of using subroutines:

Answer 4

• Code re-use
• Allows for modularisation of the program
• It makes it easier to identify bugs
• It makes it easier to test individual tasks NE easier to test
• It makes it easier for other programmers to interpret and understand your code

Question 5

What is an object reference variable?

Answer 5

• A fixed length memory address that points to a memory location (in the heap)
• It is used to refer to an object

Question 6

What is the purpose of a class?

Answer 6

To define the method and property fields that capture the common behaviours and characteristics of objects

Question 7

How is composition represented in UML?

Answer 7

Black diamond

Question 8

How is aggregation represented in UML?

Answer 8

White diamond

Question 9

What is encapsulation?

Answer 9

• The process of combining properties and methods together into an object
• and being able to restrict access to an object’s state / behaviour

Question 10

What is a constructor?

Answer 10

A method that is called when an object is first created

Question 11

What is the purpose of inheritance?

Answer 11

To create a hierarchy of specialisation for classes

Question 12

What is (subtype) polymorphism? What is its purpose?

Answer 12

• When a method in a parent class also exists in a derived class
• The method has the same name and parameter list, but a different implementation
• It allows objects up and down the inheritance hierarchy chain to respond differently to the use of a common interface

Question 13

Give two advantages of “programming to interfaces, not implementation”:

Answer 13

• Allows you to test each part of your program separately
• Allows you to modify your code easily in the future

Question 14

Give two advantages of local variables over global variables:

Answer 14

• You can give local variables the same name in different subroutines because they are only recognised by the subroutine they are declared in
• Local variables are deleted as soon as the subroutine is over and release the memory space which it occupies

Question 15

What is a constant?

Answer 15

A data item that, once declared, retains the same value for the entire duration of the program run

Question 16

Give two advantages of using constants over local variables:

Answer 16

• Makes your code more readable, by clearly indicating that a value should not be changed throughout the program
• Enhances code safety by preventing accidental modifications to values that shouldn’t change

Question 17

What does the access modifier protected mean? Give its UML symbol.

Answer 17

• It can be accessed by any subclasses
• and from within the class itself
• #

Question 18

What does the access modifier public mean? Give its UML symbol.

Answer 18

• It can be accessed by any class
• +

Question 19

What does the access modifier private mean? Give its UML symbol.

Answer 19

• It can only be accessed from within the class
• -

Question 20

What is the purpose of a hierarchy chart?

Answer 20

• To represent the structure of the program
• By showing which subroutine is called from which subroutine

Question 21

Constructing a hierarchy chart aids with which type of abstraction?

Answer 21

Decomposition
(possibly others)

Question 22

What is meant by a structured programming approach?

Answer 22

• When decomposition of a problem occurs by using block structures (subroutines)
• Structured programming makes use of control structures - sequence / selection / iteration

Question 23

What are user-defined data types?

Answer 23

Data types that are derived from existing built-in types in order to create a customised data structure

Question 24

Give three advantages of OOP:

Answer 24

• Classes are highly modular, meaning they are easy to maintain
• Promotes code reuse
• Encapsulation provides a high level of protection for data

Question 25

Give two drawbacks of OOP:

Answer 25

• Can lead to very large and complex systems
• Objects can take up a relatively large amount of memory, which is an issue if there are hardware restrictions

Question 26

What is meant by favouring composition over inheritance?

Answer 26

Using objects composed of other objects rather than a tightly coupled inheritance system as it allows for more flexibility

Question 27

Describe the “encapsulate what varies” design principle

Answer 27

When designing a program in the OOP, any requirements ​which are likely to change in the future ​should be ​encapsulated in a class ​so that any changes can be​ easily made when required​.

Question 28

Name three OOP design principles:

Answer 28

• Encapsulate what varies
• Favour composition over inheritance
• Program to interfaces, not implementation

Question 29

Steps for binary search on a value X (in English/Pseudocode)

Answer 29

1. Look at item in centre of sorted list (or array) (sort first if needed)
2. If it isn’t X…
   2.1. If it is larger than X, you can ignore all of the items above
   2.2. If it is smaller than X, you can ignore all of the items below
3. Check middle of halved list and repeat, until you find X or the sublist cannot be halved any more.

Question 30

What is the while condition when coding binary search?

Answer 30

lower<= upper && !found

Question 31

What does it mean if you reach a leaf node before you find X in a binary tree search?

Answer 31

X is not in the tree

Question 32

Bubble sort (ascending order) pseudocode on array A

Answer 32

N ← length(A)
swapped ← True
while swapped and N > 1:
__swapped ← False
__for position = 0, 1, ... , N-2:
____if A[position] > A[position+1]:
______swap( A[position] , A[position+1])
______swapped ← True
__N ← N-1