OOP Design

Question 1

What is inheritance in object-oriented programming?

Answer 1

The process of a class inheriting attributes and methods from a base class to a derived class

Inheritance establishes ‘is a’ relationships among classes.

Question 2

What is a Superclass?

Answer 2

Class whose properties (methods and variables) are inherited by another class

Also known as a parent class.

Question 3

What is a Subclass?

Answer 3

Inherits from the superclass using the extends keyword

It can use or override the parent’s methods.

Question 4

What does the super keyword refer to?

Answer 4

Refers to the immediate parent class object

Used to call the parent’s constructor or methods.

Question 5

What are the benefits of inheritance?

Answer 5

Code reusability, allows sharing common functionality among related classes

Inheritance promotes a hierarchical class structure.

Question 6

Define encapsulation.

Answer 6

Process of hiding ‘sensitive data’ from users via access modifiers

Uses public methods to alter or view data instead of direct manipulation.

Question 7

What is the access level of private fields?

Answer 7

Only accessible within the same class

This access level enhances security.

Question 8

What is the access level of protected fields?

Answer 8

Accessible within the same package and subclasses

This allows controlled access for derived classes.

Question 9

What is the access level of public fields?

Answer 9

Accessible from any other class

Public fields can be accessed directly from outside the class.

Question 10

What does the default access modifier mean?

Answer 10

Accessible only within the same package

Default access is less restrictive than private but more than protected.

Question 11

Why should fields be private?

Answer 11

Improves security and structure/modularity

Hiding fields prevents unauthorized access.

Question 12

What are the benefits of using getters and setters?

Answer 12

Improves security and structure/modularity

Allows controlled access to private fields.

Question 13

What are the benefits of encapsulation?

Answer 13

• Data hiding: prevents direct access to internal data
• Improved maintainability: changes to internal implementation won’t affect external code

As long as the interface remains the same.

Question 14

Define abstraction.

Answer 14

Hiding unnecessary information and only displaying necessary information

This simplifies the interface for the user.

Question 15

What is an abstract class?

Answer 15

Cannot be instantiated directly, can have abstract methods and regular methods

Abstract methods do not have an implementation.

Question 16

What are interfaces in programming?

Answer 16

Defines a contract that implementing classes must follow

Interfaces allow for a consistent method signature across different classes.

Question 17

What are the benefits of abstraction?

Answer 17

• Hides implementation details
• Focus on result not process

Users interact through abstract interfaces.

Question 18

Define polymorphism.

Answer 18

One interface, many implementations

This allows different classes to be treated as instances of the same class through a common interface.

Question 19

What is Compile-Time Polymorphism?

Answer 19

Achieved through method overloading, where multiple methods share the same name but have different signatures

Also known as static binding.

Question 20

What is Run-Time Polymorphism?

Answer 20

Achieved through method overriding, where a subclass provides its own implementation of a method defined in the parent class

Also known as dynamic binding.

Question 21

What are the overriding rules?

Answer 21

• Must have the same method signature
• The overridden method cannot have a more restrictive access modifier
• Can call the parent’s overridden method using super

These rules ensure proper method overriding.

Question 22

What is the instanceof operator used for?

Answer 22

Used to verify whether an object is an instance of a particular class or interface

This operator helps in type checking at runtime.

Question 23

What are the advantages of using encapsulation in a class?

Answer 23

Improves security and modularity, only allows a class to be interacted the way it is designed to be.

Question 24

Why should class variables typically be marked as private?

Answer 24

To prevent unauthorized access or changing

Question 25

How does encapsulation improve security in an application?

Answer 25

It prevents unauthorized access to variables, and only lets them be interacted in a way that it has been designed to.

Question 26

What's the difference between an abstract class and an interface? When would you use one over the other?

Answer 26

An abstract class can have both abstract and concrete methods, allowing partial implementation, whereas an interface only defines abstract methods. An abstract class should be used as a base class that needs to be inherited, interface should be used when you want to define a behavior that multiple unrelated classes can use.

Question 27

Can an abstract class have a constructor? If yes, when is it called?

Answer 27

It can have a constructor, even though you can't directly create an instance of that class. It is used when a concrete subclass is created

Question 28

Explain why abstraction is important in software design patterns.

Answer 28

Removes unnecessary information. For users it hides implementation details, where the user only needs to focus on results not the process.

Question 29

What's the difference between is-a and has-a relationships in inheritance?

Answer 29

Is-a refers to a sub class that inherits a superclass, for example dog is an animal. Has-a is a reference to another class.

Question 30

Can a child class override a final method in Java? Why or why not?

Answer 30

Final methods cannot be overridden.

Question 31

What is multiple inheritance and why doesn't Java support it directly? How does Java handle this limitation?

Answer 31

Multiple inheritance is a child class inheriting properties of many other classes. Java doesn't support it as it leads to complexity and ambiguity (the diamond problem). It works around it via interfaces.

Question 32

What is the difference between method overloading and method overriding?

Answer 32

Method overloading is multiple methods of the same name and but different signatures. Method overriding is the process of a subclass overriding its supper class method

Question 33

How does Java achieve runtime polymorphism?

Answer 33

Method overloading

Question 34

Can an interface extend another interface? If so, how?

Answer 34

Yes an interface can extend any number of interfaces.

Question 35

What are default methods in Java interfaces, and why were they introduced?

Answer 35

Default methods allow you to add new functionality to existing and ensure binary compatibility. Allowing for new code to work for older versions.

Question 36

Explain the Liskov Substitution Principle with a real-world analogy.

Answer 36

Objects of a superclass should be replaceable with objects of a subclass without breaking the functionality of a program. E.g. a paymentMethod superclass, and creditcardPayment or payPalPayment subclass. Wherever you code expects a PaymentMethod, it should be able to use either CreditCardPayment or PayPalPayment

Question 37

How does dependency injection relate to OOP principles?

Answer 37

Dependency Injection helps enforce OOP principles by, promoting loose coupling between classes, making code more maintainable, and encouraging the use of abstractions and interfaces.

Question 38

What is the difference between composition and inheritance? Which one would you prefer for re-usability?

Answer 38

Composition is where a class contains objects of other classes to reuse functionality. Inheritance is where class inherits behavior and properties. For re-usability composition is more suitable, as they can be swapped out or modified independently, and better flexibility, also avoids fragile hierarchies.