Multiple choice practice

OOP Concepts MCQ questions

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OOP Concepts tutorial Interview questions

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From lessons: 104
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OOP Concepts practice testTrue or false Question 1 of 10

Depending on interfaces makes it easy to use fake implementations in unit tests.

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All questions with answers

Which OOP principle hides internal state behind methods?
- AInheritance
- BEncapsulation
- CPolymorphism
- DOverloading
Show answer
Answer: B. Encapsulation
Encapsulation keeps data private and exposes controlled operations.
An object is…
- AA blueprint
- BAn instance of a class
- CA method
- DA namespace
Show answer
Answer: B. An instance of a class
The class is the blueprint; each object is a concrete instance.
Which relationship does inheritance represent?
- Ahas-a
- Bis-a
- Cuses-a
- Dpart-of
Show answer
Answer: B. is-a
A Dog is an Animal. Composition is has-a.
Which feature lets one interface have many implementations called the same way?
- AEncapsulation
- BPolymorphism
- CAbstraction
- DStatic binding
Show answer
Answer: B. Polymorphism
Polymorphism lets code call shape.Area() without knowing the concrete shape.
Method overloading is resolved…
- AAt runtime
- BAt compile time
- CBy the garbage collector
- DBy the database
Show answer
Answer: B. At compile time
The compiler picks the overload from the argument types.
Which can you NOT instantiate directly?
- AA sealed class
- BAn abstract class
- CA static method
- DA struct
Show answer
Answer: B. An abstract class
Abstract classes are meant to be inherited.
How many classes can a C# class inherit from?
- AOne
- BTwo
- CUnlimited
- DNone
Show answer
Answer: A. One
C# supports single class inheritance but multiple interfaces.
Which access modifier allows access from subclasses only (plus the class itself)?
- Aprivate
- Bpublic
- Cprotected
- Dinternal
Show answer
Answer: C. protected
protected members are visible to the class and derived classes.
What does the S in SOLID stand for?
- ASubstitution
- BSingle Responsibility
- CSeparation of Concerns
- DSimplicity
Show answer
Answer: B. Single Responsibility
A class should have one reason to change.
Which principle says subtypes must be usable in place of their base type?
- AOpen/Closed
- BLiskov Substitution
- CInterface Segregation
- DDependency Inversion
Show answer
Answer: B. Liskov Substitution
Derived classes must honour the base class’s contract.
Which pattern ensures a class has only one instance?
- AFactory
- BSingleton
- CObserver
- DStrategy
Show answer
Answer: B. Singleton
In modern apps a DI container’s singleton lifetime is usually preferred.
Which pattern swaps an algorithm at runtime through a common interface?
- AStrategy
- BSingleton
- CRepository
- DAdapter
Show answer
Answer: A. Strategy
For example different IShippingCalculator implementations.
Which pattern notifies subscribers when state changes?
- AObserver
- BFactory
- CDecorator
- DBuilder
Show answer
Answer: A. Observer
Events and pub/sub systems are observers.
What is wrong with this design?
C# Illustrative
```
1public class ReportService
2{
3    private readonly SmtpEmailSender _email = new SmtpEmailSender("smtp.example.com");
4}
```
- ANothing
- BReportService creates its own dependency, so it cannot be tested with a fake
- CConstructors cannot create objects
- DIt violates encapsulation by being public
Show answer
Answer: B. ReportService creates its own dependency, so it cannot be tested with a fake
Inject an IEmailSender instead of calling new inside the class.
In composition, the part’s lifetime…
- AIs independent of the whole
- BDepends on the whole
- CIs static
- DIs longer than the whole
Show answer
Answer: B. Depends on the whole
An OrderLine does not exist without its Order.
Which keyword lets a C# subclass replace a virtual method?
- Anew
- Boverride
- Cvirtual
- Dbase
Show answer
Answer: B. override
override replaces the implementation polymorphically; new only hides it.
An interface in C# can contain…
- AInstance fields
- BConstructors
- CMethod signatures (and default implementations)
- DPrivate state only
Show answer
Answer: C. Method signatures (and default implementations)
Interfaces describe members; C# 8+ allows default implementations but not instance fields.
What does a constructor return?
- Avoid
- BThe class type implicitly — it initialises the new object
- CA boolean
- DNothing can call a constructor
Show answer
Answer: B. The class type implicitly — it initialises the new object
Constructors have no declared return type; new returns the created object.
Which UML symbol marks a private member?
- A+
- B-
- C#
- D~
Show answer
Answer: B. -
+ public, - private, # protected, ~ package/internal.
“Favour composition over inheritance” mainly improves…
- AExecution speed
- BFlexibility and loose coupling
- CMemory usage
- DCompile time
Show answer
Answer: B. Flexibility and loose coupling
Behaviour can be combined and swapped without deep hierarchies.
What is the relationship between a class and an object?
- AThey are the same thing
- BAn object is a blueprint; a class is an instance
- CA class is a blueprint; an object is an instance of it
- DA class can only create one object
Show answer
Answer: C. A class is a blueprint; an object is an instance of it
Classes describe structure and behaviour; objects are concrete instances created from them.
From the lesson: Classes and Objects

What is logged?

JavaScript Illustrative

1class Counter { constructor() { this.n = 0; } inc() { this.n++; } }
2const a = new Counter();
3const b = new Counter();
4a.inc(); a.inc(); b.inc();
5console.log(a.n, b.n);

A3 3
B2 1
C1 2
D0 0

Show answer

Answer: B. 2 1

Each object has its own n. a was incremented twice, b once.

From the lesson: Classes and Objects

What is the main goal of encapsulation?
- AMake code run faster
- BHide internal state and control how it changes
- CAllow a class to inherit from many classes
- DCreate objects without classes
Show answer
Answer: B. Hide internal state and control how it changes
Encapsulation protects an object’s internal data and only allows changes through controlled methods.
From the lesson: Encapsulation
In JavaScript, a field declared as #secret can be read from outside the class with obj.#secret.
- ATrue
- BFalse
Show answer
Answer: B. False
Private fields are only accessible inside the class body; outside access is a syntax error.
From the lesson: Encapsulation
What is logged?
JavaScript Illustrative
```
1class Wallet {
2  #money = 100;
3  get money() { return this.#money; }
4}
5console.log(Object.keys(new Wallet()));
```
- A[ 'money' ]
- B[]
- C[ '#money' ]
- Dundefined
Show answer
Answer: B. []
Private fields are not properties, and the getter lives on the prototype, so the object has no own keys.
From the lesson: Encapsulation