Set 5

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 the advantages of using subroutines:
Give 3 points.

Answer 4

Any 3 points:
- 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 on the owning class.

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 the “programming to interfaces, not implementation” design principle:

Answer 13

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

Question 14

What is a constant?

Answer 14

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

Question 15

Give two advantages of using constants over local variables:

Answer 15

• 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 16

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

Answer 16

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

Question 17

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

Answer 17

• It can be accessed by any class
• +

Question 18

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

Answer 18

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

Question 19

What is the purpose of a hierarchy chart?

Answer 19

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

Question 20

Constructing a hierarchy chart aids with which type of abstraction?

Answer 20

Decomposition

Question 21

What is meant by a structured programming approach?

Answer 21

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

Question 22

What are user-defined data types?

Answer 22

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

Question 23

Give advantages of OOP:

Answer 23

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

Question 24

Give drawbacks of OOP:

Answer 24

• 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 25

What is meant by favouring composition over inheritance? Why is this design principle followed?

Answer 25

Using objects composed of other objects rather than a tightly coupled inheritance system. It allows for more flexibility if you need to modify your program later.

Question 26

Describe the “encapsulate what varies” design principle

Answer 26

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 27

Name the OOP design principles:

Answer 27

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

Question 28

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

Answer 28

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 29

What is the `while` condition when coding binary search?

Answer 29

`lower<= upper && !found`

Question 30

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

Answer 30

**X** is not in the tree

Question 31

What is an abstract data type?

Answer 31

A theoretical description of a way of organising a collection of data, with particular features and access restrictions, that is independent of any particular data structure.

Question 32

What is a data structure?

Answer 32

The concrete realisation of an abstract data type in code.

Question 33

Is a stack FIFO or LIFO?

Answer 33

LIFO (Last in first out)

Question 34

Give the five core operations of a stack:

Answer 34

- **Push** - adding to top of stack - **Pop** - removing from top of stack and returning - **Peek** - returns top item *without* removing it - **IsEmpty** - checking if stack is empty - **IsFull** - checking if stack is full (only relevant when stored in a static structure)

Question 35

Describe the implementation of a stack

Answer 35

- Using an array or list to store the items - Initialise a pointer variable that points to the current top item - The pointer is initialised as -1 - The pointer is incremented if an item is pushed, and vice versa - The pointer will be -1 if stack is empty.

Question 36

What is a recursive subroutine?

Answer 36

One that calls itself (or is defined in terms of itself)

Question 37

Give three advantages of recursion:

Answer 37

1. Can lead to very elegant, short code 2. People think recursively (sometimes) 3. It is sometimes the only way to solve a problem

Question 38

Give two disadvantages of recursion:

Answer 38

1. Can be slower than iterative solutions 2. Can use more memory than iterative solutions

Question 39

What is meant by a ‘base case’?

Answer 39

The **terminating situation** in a recursive procedure that does not use recursion to produce a result

Question 40

What are the steps for linear search on a list?

Answer 40

1. Start at beginning of list 2. Compare each item to one you want until - you find it or - you reach the end of the list *(use an indefinite `while` loop!)*

Question 41

What are the steps for binary search on a value **X** (in English)?

Answer 41

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 42

What is the `while` condition when coding binary search?

Answer 42

`lower<= upper && !found`