Object-Oriented Programming

Question 1

What is a class?

Answer 1

The definition of an object. Classes define the attributes and methods that an object of the class will have.

Question 2

What is an object?

Answer 2

A specific instance of a class

Question 3

What is instantiation?

Answer 3

The process of creating an instance of an object, belonging to a class, by invoking a constructor method

Question 4

What is encapsulation? (in terms of OOP)

Answer 4

Combing data with the procedures and functions that manipulate it to form a new data type (class)

Question 5

What is information hiding in oop?

Answer 5

Protects an object’s implementation and internal state from being accessed by other objects

Question 6

What are the uses of information hiding?

Answer 6

Allows programmers to implement restrictions on how values are accessed and set. This is done through the use of access modifiers.

Question 7

What are the access modifiers and their descriptions?

Answer 7

• Public: accessed by objects of any other class (+)
• Private: accessed only by objects of that class (-)
• Protected: accessed by other objects of the class or subclass (#)

Question 8

What is a getter method?

Answer 8

A method that returns the requested attribute value

Question 9

What is a setter method?

Answer 9

A method that can modify the attribute’s value

Question 10

Why do we use getters and setters?

Answer 10

They allow developers to implement additional checks before accepting or providing data.

Question 11

What is polymorphism?

Answer 11

When objects of different classes respond differently to the use of a common interface

Question 12

What is polymorphism in OOP?

Answer 12

Polymorphism occurs when an inherited method (in a subclass) is redefined or overriden so that its implementation is different from its parent

Question 13

What is the interface?

Answer 13

The collection of a classes public methods, attributes and its identifier

Question 14

What is overriding?

Answer 14

• When a method is defined in a derived class with the same identifier as a method from the base class but is given a different implementation/ performs a different function
• Redefined method will be used instead of the base’s method when called

Question 15

What are methods that can be overridden by a derived class called?

Answer 15

Virtual methods

Question 16

What does an abstract method define?

Answer 16

The interface:
- identifier
- return type
- list of parameters

Question 17

What are the differences between a virtual and abstract class?

Answer 17

• Virtual methods can be overridden by a child class but abstract must be overridden by a child class
• Virtual have an implementation in base class but abstract do not
• Virtual can be declared in any class but abstract can only be defined in abstract classes

Question 18

What is object association?

Answer 18

Occurs when an object contains or has a reference to another object within it. Often described as a “has a” relationship.

Question 19

What are the two forms of association?

Answer 19

• Composition - “strong”
• Aggregation - “weak”

Question 20

What is composition?

Answer 20

Composition is a restricted form of association, where one object contains another. If the container object is deleted, the contained objects are also deleted.

Question 21

How is composition represented in a UML diagram?

Answer 21

A line between the two classes with a black filled diamond at the class which is the container (e.g. car to a wheel)

Question 22

What is aggregation?

Answer 22

Type of relationship where one class contains another class.However, the aggregated objects exist independently of the aggregating object and will not be deleted from the program if the aggregating object is deleted.

Question 23

How is aggregation represented in a UML diagram?

Answer 23

A line between the two classes and a white unfilled diamond at the aggregating object (e.g. car for a driver).

Question 24

What is inheritance?

Answer 24

Describes the relationship between classes where a derived “child” class gains the attributes and methods from a “parent” class

Question 25

How do you represent inheritance in a UML class diagram?

Answer 25

A filled arrow going from the derived class to the base class

Question 26

What are the 3 OOP design principles?

Answer 26

1. Encapsulate what varies 2. Favour composition over inheritance 3. Program to the interface, not implementation

Question 27

What is inheritance in object-oriented programming?

Answer 27

A relationship between classes where a derived class gains the attributes and methods of the base class that it inherits from

Question 28

What does the design principle “encapsulate what varies” mean?

Answer 28

- Encapsulate parts of our program that frequently changed or updated into classes - Ensures modules are loosely-coupled - Modifications can be tested within the class itself and has minimal impact on the rest of the program

Question 29

What does the design principle “favour composition over inheritance” mean?

Answer 29

- Composition isn more flexible and allows for complex functionalities by combining component objects - If class A requires functionality of class B, it is best to give class A an instance of class B rather than inheriting from it

Question 30

What does the design principle “program to the interface, not implementation” mean?

Answer 30

- To write loosely-coupled code our objects should be designed to only interact with other objects via their interface - This allows for implementation to be changed without affecting the rest of the program

Question 31

What are the access modifiers and how can they be represented in UML diagrams?

Answer 31

Public: + Private: - Protected: #

Question 32

What does the public access modifier mean?

Answer 32

Attribute or method can be accessed by objects of any other class within the program

Question 33

What does the private modifier mean?

Answer 33

Attribute or method can only be accessed by objects of the same class

Question 34

What does the protected modifier mean?

Answer 34

Attribute or method can be accessed by objects of the same class **or other classes that inherit from it**

Question 35

What are static methods?

Answer 35

Class methods that can be called without the need to **instantiate an object** of that class // A method that doesn't rely on the **internal state** of a specific **object** of the class.

Question 36

What are the uses of static variables?

Answer 36

- Only stored once per class so less memory used - Autoincrementing between objects (e.g. for an ID)