OOP MCQ with Answer and Explanation in details for practice and interview.
Question 1:
Which of the following is NOT one of the four fundamental principles of Object-Oriented Programming?
A) Encapsulation
B) Inheritance
C) Polymorphism
D) Compilation
Answer: D) Compilation
Explanation: The four fundamental principles of OOP are:
Encapsulation: This is the bundling of data and methods that operate on that data within a single unit (class). It restricts direct access to some of an object's components.
Inheritance: This allows a new class to inherit properties and behaviors (methods) from an existing class. It promotes code reusability.
Polymorphism: This enables objects to be treated as instances of their parent class, with the ability to take on multiple forms (e.g., method overloading and overriding).
Compilation is a process of converting code written in a high-level programming language into machine code, which is not specific to OOP.
Question 2:
What does it mean when we say a class is an "abstract class"?
A) It can be instantiated.
B) It cannot be instantiated and may contain abstract methods.
C) It is a class that cannot have any properties.
D) It is a class with only static methods.
Answer: B) It cannot be instantiated and may contain abstract methods.
Explanation: An abstract class serves as a blueprint for other classes. It cannot be instantiated directly, meaning you cannot create an object of an abstract class. It can include abstract methods (methods without an implementation) that must be implemented by derived (child) classes. This allows for a common interface while enforcing certain behaviors in the subclasses.
Question 3:
What is the primary purpose of encapsulation in OOP?
A) To increase code complexity.
B) To allow for method overloading.
C) To hide the internal state and require all interaction to occur through an object's methods.
D) To facilitate multiple inheritance.
Answer: C) To hide the internal state and require all interaction to occur through an object's methods.
Explanation: Encapsulation is primarily aimed at protecting the internal state of an object. By exposing only necessary methods (the public interface) and keeping the rest private, it prevents external code from manipulating the object's state directly. This leads to increased security and maintainability of the code, as the internal representation of the object can change without affecting external code that uses it.
Question 4:
Which of the following best describes polymorphism?
A) The ability to create multiple objects from a single class.
B) The ability to define methods in a child class with the same name as methods in the parent class.
C) The ability to hide data.
D) The ability to combine different objects into a single entity.
Answer: B) The ability to define methods in a child class with the same name as methods in the parent class.
Explanation: Polymorphism allows methods to be defined in a way that they can perform different functions based on the object that is calling them. This is primarily achieved through:
Method Overloading: Same method name with different parameters within the same class.
Method Overriding: A child class providing a specific implementation of a method that is already defined in its parent class. This allows for dynamic method resolution during runtime.
Question 5:
What is the purpose of a constructor in a class?
A) To create a new instance of a class.
B) To define methods for the class.
C) To destroy instances of a class.
D) To manage access levels for class members.
Answer: A) To create a new instance of a class.
Explanation: A constructor is a special type of method that is automatically called when an object of a class is instantiated. Its primary purpose is to initialize the object's attributes and allocate resources. Constructors can also take parameters to allow for initialization with specific values when creating an object.
Question 6:
Which of the following statements about inheritance is true?
A) A subclass can access private members of its superclass.
B) A superclass can inherit from a subclass.
C) Multiple inheritance allows a class to inherit from multiple classes.
D) Inheritance does not promote code reusability.
Answer: C) Multiple inheritance allows a class to inherit from multiple classes.
Explanation: Inheritance allows one class (subclass) to inherit properties and behaviors from another class (superclass). While single inheritance is common (a subclass inherits from one superclass), multiple inheritance (where a subclass can inherit from multiple superclasses) is supported in some programming languages (like C++), but not in others (like Java). A subclass cannot directly access private members of its superclass, and a superclass cannot inherit from a subclass.
Question 7:
What is method overloading?
A) Defining multiple methods with the same name in the same class but with different parameters.
B) Overriding a method in a subclass.
C) Creating a method that can take any number of parameters.
D) Hiding a method from subclasses.
Answer: A) Defining multiple methods with the same name in the same class but with different parameters.
Explanation: Method overloading is a feature that allows a class to have more than one method with the same name, as long as their parameter lists differ in type or number. This increases the readability and usability of the code. Method overriding, on the other hand, occurs when a subclass provides a specific implementation for a method that is already defined in its parent class.
Question 8:
What is the purpose of the super keyword in a subclass?
A) To create a new instance of a class.
B) To call a method from the subclass.
C) To access methods and properties of the superclass.
D) To define abstract methods.
Answer: C) To access methods and properties of the superclass.
Explanation: The super keyword is used to refer to the superclass (parent class) of the current object. It allows a subclass to access methods and properties defined in the superclass, enabling the subclass to call overridden methods or constructors. This is useful for code reuse and maintaining the hierarchy of classes.
Question 9:
Which of the following is true about interfaces in OOP?
A) An interface can contain implementation of methods.
B) A class can implement multiple interfaces.
C) An interface cannot extend another interface.
D) Interfaces can have instance variables.
Answer: B) A class can implement multiple interfaces.
Explanation: An interface is a contract that defines a set of methods without implementing them. A class can implement multiple interfaces, allowing it to inherit abstract behaviors from multiple sources. Interfaces cannot have method implementations (unless they use default methods in some languages) or instance variables; they can only define method signatures.
Question 10:
What does it mean when a method is declared as abstract?
A) It can have a default implementation.
B) It must be defined in the same class.
C) It cannot be called directly and must be implemented by subclasses.
D) It is a static method.
Answer: C) It cannot be called directly and must be implemented by subclasses.
Explanation: An abstract method is a method declared without an implementation. This means it cannot be called directly and must be overridden in a derived class. Abstract methods are used in abstract classes to enforce that certain methods are implemented in any subclass, thereby defining a common interface while allowing for flexibility in implementation.
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Question 11:
What is encapsulation often associated with in OOP?
A) Data hiding.
B) Inheritance.
C) Method overriding.
D) Polymorphism.
Answer: A) Data hiding.
Explanation: Encapsulation is often associated with data hiding because it restricts direct access to an object's internal state. By using access modifiers (like private, protected, and public), encapsulation prevents unauthorized or accidental modification of data, ensuring that an object's internal representation can only be modified through well-defined methods.
Question 12:
Which of the following is a benefit of using polymorphism in OOP?
A) Increased code complexity.
B) Reduced memory usage.
C) Improved flexibility and maintainability of code.
D) Eliminating the need for inheritance.
Answer: C) Improved flexibility and maintainability of code.
Explanation: Polymorphism allows methods to be used in a generic way, letting one interface handle different data types. This flexibility enhances code maintainability, as changes to one part of the code (like adding a new subclass) don’t necessarily require changes to other parts of the code that use polymorphic behavior.
Question 13:
What is the main purpose of a destructor in a class?
A) To create new objects.
B) To initialize object properties.
C) To free up resources when an object is no longer needed.
D) To define abstract methods.
Answer: C) To free up resources when an object is no longer needed.
Explanation: A destructor is a special method invoked when an object is about to be destroyed. Its main purpose is to release resources (like memory or file handles) that the object may have acquired during its lifetime. This helps in preventing memory leaks and ensuring proper resource management.
Question 14:
What does the term "constructor overloading" refer to?
A) Defining multiple constructors with the same name in a class but with different parameters.
B) Calling a constructor from another constructor.
C) Creating a constructor that cannot be used.
D) Overriding a constructor in a subclass.
Answer: A) Defining multiple constructors with the same name in a class but with different parameters.
Explanation: Constructor overloading allows a class to have more than one constructor, each with different parameter lists. This enables objects to be instantiated in different ways based on the parameters passed, enhancing flexibility in object creation.
Question 15:
Which of the following is true regarding static members in a class?
A) They can only be accessed by instances of the class.
B) They belong to the class rather than any particular instance.
C) They cannot be modified.
D) They cannot be inherited by subclasses.
Answer: B) They belong to the class rather than any particular instance.
Explanation: Static members (variables or methods) belong to the class itself rather than to any specific instance of the class. This means that they can be accessed without creating an instance of the class, and they are shared among all instances. Static members can be modified, and they can be inherited by subclasses.
Question 16:
In the context of OOP, what is the "Liskov Substitution Principle"?
A) A subclass must be able to substitute its parent class without affecting the program's functionality.
B) A subclass can have additional properties not present in its parent class.
C) A method can have different implementations in different classes.
D) Classes should not depend on abstract classes.
Answer: A) A subclass must be able to substitute its parent class without affecting the program's functionality.
Explanation: The Liskov Substitution Principle (LSP) is one of the SOLID principles of object-oriented design. It states that objects of a superclass should be replaceable with objects of a subclass without affecting the correctness of the program. This principle ensures that a subclass extends the behavior of the parent class without altering the desirable properties of the program.
Question 17:
What is the difference between an interface and an abstract class?
A) An interface can have state, while an abstract class cannot.
B) An abstract class can have both abstract and concrete methods, while an interface can only have abstract methods.
C) An interface can inherit from a class, while an abstract class cannot.
D) There is no difference; they are the same.
Answer: B) An abstract class can have both abstract and concrete methods, while an interface can only have abstract methods.
Explanation: An abstract class can contain both abstract methods (without implementation) and concrete methods (with implementation), whereas an interface is a contract that only defines method signatures. However, some programming languages allow interfaces to have default methods with implementations. Interfaces do not maintain state (no instance variables), while abstract classes can.
Question 18:
What is the term for a class that cannot be instantiated and is meant to be subclassed?
A) Concrete class.
B) Interface.
C) Abstract class.
D) Static class.
Answer: C) Abstract class.
Explanation: An abstract class is a class that cannot be instantiated directly. It is designed to be subclassed, allowing derived classes to provide implementations for the abstract methods defined in the abstract class. This establishes a base for other classes while enforcing a contract for method implementations.
Question 19:
Which of the following is a characteristic of a concrete class?
A) It cannot be instantiated.
B) It may contain abstract methods.
C) It must implement all abstract methods from its parent class.
D) It cannot have methods.
Answer: C) It must implement all abstract methods from its parent class.
Explanation: A concrete class is a class that can be instantiated and must provide implementations for all abstract methods inherited from its parent abstract class. This ensures that the concrete class is complete and can be used to create objects.
Question 20:
In OOP, what is a "getter" method?
A) A method that sets the value of a private variable.
B) A method that retrieves the value of a private variable.
C) A method that performs calculations.
D) A method that deletes an object.
Answer: B) A method that retrieves the value of a private variable.
Explanation: A "getter" method, also known as an accessor method, is used to read or retrieve the value of a private instance variable. This adheres to the principle of encapsulation, as it allows controlled access to the internal state of an object without exposing it directly.
Question 21:
Which of the following best defines the term "composition" in OOP?
A) A class inheriting from multiple classes.
B) A class containing references to other classes.
C) A class that cannot be subclassed.
D) A class defining only abstract methods.
Answer: B) A class containing references to other classes.
Explanation: Composition is a design principle in OOP where a class contains references to one or more objects of other classes as part of its state. This allows for building complex types by combining simpler ones, promoting reusability and flexibility. It contrasts with inheritance, where a class derives from another class.
Question 22:
What is an access modifier in OOP?
A) A tool to increase code complexity.
B) A feature that controls the visibility of class members.
C) A method that enhances performance.
D) A type of data structure.
Answer: B) A feature that controls the visibility of class members.
Explanation: Access modifiers (like public, private, protected) are keywords used to set the accessibility of classes, methods, and variables in OOP. They determine which parts of a program can access certain members, thus enforcing encapsulation and protecting the internal state of an object.
Question 23:
Which of the following is an example of an access modifier in Java?
A) void
B) this
C) public
D) constructor
Answer: C) public
Explanation: In Java, public is an access modifier that allows a class, method, or variable to be accessible from any other class in the program. Other access modifiers include private (accessible only within the same class) and protected (accessible within the same package and by subclasses).
Question 24:
What is the term for a relationship where one class is a specialized version of another class?
A) Aggregation
B) Composition
C) Inheritance
D) Interface
Answer: C) Inheritance
Explanation: Inheritance defines a relationship between classes where one class (the subclass or derived class) inherits properties and methods from another class (the superclass or base class). This relationship allows for code reuse and the establishment of hierarchical classifications.
Question 25:
Which of the following correctly describes the concept of "overriding"?
A) Implementing multiple methods with the same name.
B) Providing a new implementation for an inherited method in a subclass.
C) Calling a superclass method from a subclass.
D) Creating a static method in a subclass.
Answer: B) Providing a new implementation for an inherited method in a subclass.
Explanation: Overriding occurs when a subclass provides a specific implementation of a method that is already defined in its superclass. This allows the subclass to modify or extend the behavior of the inherited method while maintaining the same method signature.
Question 26:
In OOP, what is a "mixin"?
A) A class that can be instantiated.
B) A type of abstract class.
C) A class that provides methods to be used by other classes without being a superclass.
D) A type of interface.
Answer: C) A class that provides methods to be used by other classes without being a superclass.
Explanation: A mixin is a class that provides methods that can be used by other classes without being a direct ancestor of those classes. Mixins allow for code reuse and the addition of functionality to classes in a flexible way, often seen in languages that support multiple inheritance.
Question 27:
Which of the following statements is true regarding the final keyword in OOP?
A) It can be applied to variables only.
B) It prevents a class from being subclassed.
C) It allows for method overriding.
D) It can only be used with abstract classes.
Answer: B) It prevents a class from being subclassed.
Explanation: In many programming languages, the final keyword is used to denote that a class cannot be subclassed (in the case of classes), a method cannot be overridden (in the case of methods), or a variable's value cannot be changed (in the case of variables). This is useful for creating immutable classes and ensuring certain behaviors are fixed.
Question 28:
What is a "singleton" pattern in OOP?
A) A design pattern that restricts a class to a single instance.
B) A method for handling multiple instances of a class.
C) A type of factory pattern.
D) A pattern that allows for multiple inheritance.
Answer: A) A design pattern that restricts a class to a single instance.
Explanation: The singleton pattern is a design pattern that ensures a class has only one instance and provides a global point of access to it. This is often used in cases where a single instance is required to coordinate actions across the system, such as in configuration settings or logging.
Question 29:
Which of the following describes the term "aggregation"?
A) A strong "has-a" relationship where a child cannot exist without the parent.
B) A weak "has-a" relationship where the child can exist independently of the parent.
C) A relationship where one class inherits from another.
D) A relationship that uses interfaces.
Answer: B) A weak "has-a" relationship where the child can exist independently of the parent.
Explanation: Aggregation represents a relationship where one class (the parent) contains references to another class (the child), but the child can exist independently of the parent. This means that if the parent is destroyed, the child can still exist on its own, which contrasts with composition, where the child cannot exist without the parent.
Question 30:
What is the purpose of the interface keyword in a programming language?
A) To define a concrete class.
B) To specify a set of methods that implementing classes must provide.
C) To create an abstract class.
D) To allow for multiple inheritance.
Answer: B) To specify a set of methods that implementing classes must provide.
Explanation: The interface keyword is used to define an interface, which is a contract that specifies a set of methods that implementing classes must provide. It does not contain any implementation details, allowing different classes to provide their own implementations of the methods defined in the interface.
Question 31:
What is a "protected" member in a class?
A) It can only be accessed within the same class.
B) It can be accessed within the same package and by subclasses.
C) It can be accessed from any class.
D) It can only be accessed from subclasses in the same package.
Answer: B) It can be accessed within the same package and by subclasses.
Explanation: A "protected" member in a class can be accessed by classes in the same package and by subclasses, even if those subclasses are in different packages. This access modifier allows for greater flexibility compared to private members while still providing some encapsulation.
Question 32:
Which of the following can be considered an advantage of using OOP?
A) Code is less modular.
B) Data and behavior are tightly coupled.
C) Improved code reusability and maintainability.
D) Increased complexity in data handling.
Answer: C) Improved code reusability and maintainability.
Explanation: One of the primary advantages of OOP is the improvement in code reusability and maintainability. By organizing code into classes and objects, developers can reuse existing code through inheritance and composition, leading to a more organized and manageable codebase.
Question 33:
What is the difference between "method overriding" and "method overloading"?
A) Overriding changes the method signature, while overloading does not.
B) Overriding is related to subclasses, while overloading occurs within the same class.
C) Overloading allows for different return types, while overriding does not.
D) There is no difference; they are the same.
Answer: B) Overriding is related to subclasses, while overloading occurs within the same class.
Explanation: Method overriding occurs when a subclass provides a specific implementation for a method already defined in its superclass. In contrast, method overloading refers to defining multiple methods with the same name in the same class but with different parameter lists. This distinction is essential for understanding how polymorphism works in OOP.
Question 34:
What is a "factory method"?
A) A method that creates a new instance of a class.
B) A method that retrieves an object from memory.
C) A method that destroys an instance of a class.
D) A method that overrides a superclass method.
Answer: A) A method that creates a new instance of a class.
Explanation: A factory method is a design pattern that provides an interface for creating instances of a class, with subclasses deciding which class to instantiate. This pattern promotes loose coupling and allows for flexibility in object creation, making it easier to manage and extend.
Question 35:
In OOP, what does "decomposition" refer to?
A) Breaking down complex problems into smaller, manageable parts.
B) The process of creating objects.
C) The combination of multiple objects into one.
D) The analysis of inheritance hierarchies.
Answer: A) Breaking down complex problems into smaller, manageable parts.
Explanation: Decomposition in OOP refers to the practice of breaking down complex problems into smaller, more manageable components or classes. This promotes better organization, easier debugging, and more maintainable code, as each component can be developed and tested independently.
Question 36:
Which of the following is an example of a design pattern in OOP?
A) Inheritance
B) Polymorphism
C) Observer
D) Encapsulation
Answer: C) Observer
Explanation: The Observer pattern is a behavioral design pattern that defines a one-to-many dependency between objects. When one object (the subject) changes its state, all dependent observers are notified and updated automatically. This pattern is widely used for implementing distributed event-handling systems.
Question 37:
What is a "callback" in OOP?
A) A method that calls itself.
B) A method passed as an argument to another method.
C) A method that cannot be overridden.
D) A method that initializes an object.
Answer: B) A method passed as an argument to another method.
Explanation: A callback is a function or method that is passed as an argument to another method, allowing it to be called at a later time, typically in response to an event or completion of a task. This is often used in asynchronous programming to handle events or responses.
Question 38:
In OOP, what does the term "composition over inheritance" mean?
A) Using inheritance as the primary way to achieve code reuse.
B) Preferring composition to create complex types instead of relying solely on class hierarchies.
C) Avoiding the use of classes altogether.
D) Using multiple inheritance exclusively.
Answer: B) Preferring composition to create complex types instead of relying solely on class hierarchies.
Explanation: "Composition over inheritance" is a design principle that suggests favoring composition (combining simple objects or classes) to build complex types rather than relying solely on class inheritance. This approach promotes flexibility and reduces tight coupling between classes, making systems easier to change and extend.
Question 39:
Which of the following concepts allows for different classes to be treated as instances of the same class through a common interface?
A) Abstraction
B) Inheritance
C) Polymorphism
D) Encapsulation
Answer: C) Polymorphism
Explanation: Polymorphism allows different classes to be treated as instances of the same class through a common interface. This can be achieved through method overriding or method overloading, enabling a unified way to call methods without knowing the specific types of the objects at compile time.
Question 40:
What is the purpose of the static keyword in OOP?
A) To indicate that a method cannot be overridden.
B) To indicate that a variable or method belongs to the class rather than any instance.
C) To indicate that a class cannot be instantiated.
D) To indicate that a method is abstract.
Answer: B) To indicate that a variable or method belongs to the class rather than any instance.
Explanation: The static keyword indicates that a variable or method belongs to the class itself, rather than to any particular instance of the class. Static members are shared among all instances of the class, making them useful for data that is common to all objects.
Question 41:
Which of the following is a potential drawback of using multiple inheritance?
A) Increased reusability of code.
B) The diamond problem, where ambiguity arises in method resolution.
C) Improved organization of code.
D) Simplification of the class hierarchy.
Answer: B) The diamond problem, where ambiguity arises in method resolution.
Explanation: Multiple inheritance can lead to the "diamond problem," which occurs when a class inherits from two classes that both inherit from a common ancestor. This can create ambiguity in method resolution, as it becomes unclear which inherited method should be called. Many languages, like Java, avoid multiple inheritance for this reason by allowing a class to implement multiple interfaces instead.
Question 42:
In OOP, what is an "object"?
A) A blueprint for creating classes.
B) A variable that holds a reference to a method.
C) An instance of a class that encapsulates state and behavior.
D) A type of design pattern.
Answer: C) An instance of a class that encapsulates state and behavior.
Explanation: An object is an instance of a class that encapsulates both state (attributes) and behavior (methods). Objects are the fundamental building blocks of OOP, representing real-world entities or concepts in a structured way.
Question 43:
What is the purpose of the finalize method in some OOP languages?
A) To prevent a class from being subclassed.
B) To create a static method.
C) To perform cleanup before an object is destroyed.
D) To define an abstract method.
Answer: C) To perform cleanup before an object is destroyed.
Explanation: The finalize method is a special method that can be overridden in some OOP languages (like Java) to perform cleanup operations just before an object is garbage collected. It allows for the release of resources (like file handles or network connections) that the object may be holding.
Question 44:
Which of the following can be considered an example of encapsulation?
A) Making all class members public.
B) Using private variables with public getter and setter methods.
C) Allowing direct access to an object's internal state.
D) Not using classes at all.
Answer: B) Using private variables with public getter and setter methods.
Explanation: Encapsulation is achieved by restricting access to an object's internal state (usually through private variables) and providing public methods (getters and setters) to access and modify that state. This helps protect the integrity of the object's data and enforces control over how it is accessed and changed.
Question 45:
What does the term "abstraction" refer to in OOP?
A) Hiding implementation details and showing only the necessary features of an object.
B) Creating a new instance of a class.
C) Defining a method that can be overridden.
D) The process of aggregating multiple objects.
Answer: A) Hiding implementation details and showing only the necessary features of an object.
Explanation: Abstraction is a core principle of OOP that focuses on exposing only the relevant features of an object while hiding unnecessary implementation details. This allows developers to interact with objects at a high level without needing to understand their internal workings.
Question 46:
In the context of OOP, what is "inheritance"?
A) The ability of a class to have multiple interfaces.
B) The mechanism by which one class can inherit properties and methods from another class.
C) The process of creating objects.
D) The technique for hiding data.
Answer: B) The mechanism by which one class can inherit properties and methods from another class.
Explanation: Inheritance is a fundamental OOP principle that allows one class (the subclass or derived class) to inherit properties (attributes) and methods (behaviors) from another class (the superclass or base class). This promotes code reuse and establishes a hierarchical relationship between classes.
Question 47:
What is the purpose of a "virtual" method in OOP?
A) To prevent a method from being overridden.
B) To allow a method to be overridden in derived classes.
C) To define an interface.
D) To create a static method.
Answer: B) To allow a method to be overridden in derived classes.
Explanation: A virtual method is a method declared in a base class that can be overridden in derived classes. This allows for dynamic method resolution, where the method that gets executed is determined at runtime based on the actual object type, not the type of reference.
Question 48:
What does the "single responsibility principle" state?
A) A class should have multiple responsibilities.
B) A class should have one reason to change.
C) A class can have only one instance.
D) A class should only be abstract.
Answer: B) A class should have one reason to change.
Explanation: The single responsibility principle (SRP) is one of the SOLID principles of object-oriented design. It states that a class should have only one reason to change, meaning it should have one responsibility or purpose. This leads to more maintainable and understandable code.
Question 49:
What is the purpose of a "constructor chaining"?
A) To create multiple constructors in a class.
B) To call one constructor from another constructor within the same class.
C) To override a constructor in a subclass.
D) To call a superclass constructor.
Answer: B) To call one constructor from another constructor within the same class.
Explanation: Constructor chaining is the practice of calling one constructor from another constructor within the same class. This allows for reusing constructor code and providing different ways to initialize an object. It can be done using the this() keyword in languages like Java.
Question 50:
What does the "open-closed principle" state?
A) Software entities should be open for modification.
B) Software entities should be closed for modification but open for extension.
C) Classes should be public.
D) Interfaces should not be implemented.
Answer: B) Software entities should be closed for modification but open for extension.
Explanation: The open-closed principle (OCP) is one of the SOLID principles of object-oriented design. It states that software entities (like classes, modules, or functions) should be closed for modification but open for extension. This means that existing code should not be changed when new functionality is added; instead, new code should be written to extend existing behavior.
