20 Essential Low-Level Design (LLD) MCQs for Software Developers: Test Your Knowledge!

1. Which of the following principles promotes code reusability by allowing a new class to be created from an existing class?

a) Encapsulation b) Abstraction c) Inheritance d) Polymorphism

The correct answer is:

c) Inheritance

Explanation: Inheritance is a fundamental principle of object-oriented programming that promotes code reusability by allowing a new class (called a subclass or derived class) to be created from an existing class (called a superclass or base class). The subclass inherits attributes and methods from its superclass, which promotes code reuse and avoids redundant code. Through inheritance, subclasses can extend or specialize the behavior of the superclass, adding new functionality or overriding existing methods as needed. This enables developers to build upon existing code and create hierarchical relationships between classes, leading to more modular and maintainable codebases.

2. What principle of object-oriented programming involves bundling data and methods into a single unit?

a) Inheritance b) Polymorphism c) Encapsulation d) Abstraction

The correct answer is:

c) Encapsulation

Explanation: Encapsulation is the principle of object-oriented programming that involves bundling data (attributes) and methods (functions) that operate on that data into a single unit, known as an object. Encapsulation hides the internal state of an object from the outside world and restricts access to it, providing a well-defined interface for interacting with the object. This helps to prevent unauthorized access to the object's data and ensures that it can only be modified through controlled methods, maintaining the integrity of the object's state. Encapsulation promotes modularity, code organization, and information hiding, making it easier to manage and maintain complex systems.

3. Which design principle involves simplifying complex systems by modeling only the relevant aspects and hiding unnecessary details?

a) Encapsulation b) Inheritance c) Abstraction d) Polymorphism

The correct answer is:

c) Abstraction

Explanation: Abstraction is the design principle that involves simplifying complex systems by modeling only the relevant aspects and hiding unnecessary details. It focuses on capturing the essential characteristics of an object or system while ignoring implementation specifics. Abstraction allows developers to create models that represent real-world entities or concepts in a simplified manner, making it easier to understand and work with complex systems. In object-oriented programming, abstraction is achieved through classes, interfaces, and abstract data types, which define a common interface for interacting with objects while hiding their internal implementation details. This promotes code modularity, reusability, and maintainability by separating concerns and providing a clear and concise interface for interacting with objects.

4. What is the primary purpose of using an abstract class in object-oriented design?

a) To create objects directly from it

b) To provide a default implementation for methods

c) To define a common interface for subclasses

d) To encapsulate data within it

The correct answer is:

c) To define a common interface for subclasses

Explanation: An abstract class in object-oriented design serves as a blueprint or template for other classes. The primary purpose of an abstract class is to define a common interface for its subclasses. This means that the abstract class may contain one or more abstract methods (methods without implementation), which the subclasses are required to implement. By defining a common interface, abstract classes ensure that all subclasses adhere to a certain contract or behavior, promoting consistency and standardization in the design. Abstract classes themselves cannot be instantiated directly, but they provide structure and guidelines for creating subclasses that share common characteristics and behaviors. Therefore, option (c) is the correct answer.

5. Which of the following is NOT a type of relationship in UML (Unified Modeling Language) diagrams?

a) Inheritance b) Aggregation c) Encapsulation d) Composition

The correct answer is:

c) Encapsulation

Explanation: In the context of UML (Unified Modeling Language) diagrams, encapsulation is not considered a type of relationship. Encapsulation is a fundamental principle of object-oriented programming that involves bundling data and methods into a single unit (an object) and restricting access to the internal state of the object. However, in UML diagrams, the commonly recognized types of relationships are:

Inheritance: Represents an "is-a" relationship between classes, where one class (subclass or derived class) inherits attributes and methods from another class (superclass or base class). Aggregation: Represents a "has-a" relationship between classes, where one class contains (or is composed of) other classes as part of its structure. Aggregation is often depicted as a diamond shape on the side of the containing class. Composition: Represents a stronger form of aggregation, where the contained objects have a strong ownership relationship with the containing object. Composition implies that the contained objects cannot exist independently of the containing object. Composition is often depicted as a filled diamond shape on the side of the containing class.

Therefore, option (c) Encapsulation is not a type of relationship in UML diagrams.

6. Which design principle involves breaking down a program into separate, self-contained modules that can be independently developed and maintained?

a) Inheritance b) Polymorphism c) Modularity d) Encapsulation

The correct answer is:

c) Modularity

Explanation: Modularity is the design principle that involves breaking down a program into separate, self-contained modules that can be independently developed and maintained. Each module encapsulates a specific functionality or aspect of the system, and modules communicate with each other through well-defined interfaces. This promotes code organization, reusability, and ease of maintenance by isolating changes within individual modules and reducing dependencies between them. Modularity allows developers to work on different parts of a system concurrently and facilitates testing, debugging, and scalability. Therefore, option (c) Modularity is the correct answer.

7. Which design pattern is used to restrict instantiation of a class to one object?

a) Factory Method b) Singleton c) Observer d) Builder

The correct answer is:

b) Singleton

Explanation: The Singleton design pattern is used to ensure that a class has only one instance and provides a global point of access to that instance. It restricts the instantiation of a class to one object by controlling the creation process, typically through static methods or static variables. Singleton is commonly used when a class needs to maintain a single global state or when only one instance of a class is required throughout the system. Therefore, option (b) Singleton is the correct answer.

const Singleton = (() => { let instance; // Private variable to hold the single instance

function createInstance() { // Private method to create the singleton instance const object = new Object("This is a singleton instance"); return object; }

return { getInstance: () => { // Public method to get the singleton instance if (!instance) { instance = createInstance(); } return instance; } }; })();

// Usage example const singleton1 = Singleton.getInstance(); const singleton2 = Singleton.getInstance();

console.log(singleton1 === singleton2); // Output: true, both instances refer to the same object console.log(singleton1.value); // Output: This is a singleton instance console.log(singleton2.value); // Output: This is a singleton instance

8. In UML class diagrams, what symbol is used to represent inheritance?

a) Arrow pointing to the base class

b) Solid line with a hollow arrowhead pointing to the base class

c) Dotted line with a solid arrowhead pointing to the base class

d) Solid line with a solid arrowhead pointing to the base class

The correct answer is:

b) Solid line with a hollow arrowhead pointing to the base class

Explanation: In UML (Unified Modeling Language) class diagrams, inheritance is represented by a solid line with a hollow arrowhead pointing from the subclass (or derived class) to the superclass (or base class). This arrow indicates the direction of the inheritance relationship, with the subclass inheriting attributes, operations, and relationships from the superclass. The solid line represents the connection between the two classes, while the hollow arrowhead indicates the direction of the inheritance relationship. Therefore, option (b) is the correct representation of inheritance in UML class diagrams.

9. What design principle allows objects of different types to be treated as objects of a common superclass?

a) Inheritance b) Encapsulation c) Polymorphism d) Abstraction

The correct answer is:

c) Polymorphism

Explanation: Polymorphism is the design principle that allows objects of different types to be treated as objects of a common superclass. It enables objects to be processed uniformly based on their common interface or superclass type, even though they may belong to different subclasses. Polymorphism allows for flexibility and extensibility in object-oriented programming by enabling code to be written in a generic manner, independent of specific implementations. This promotes code reuse, modularity, and maintainability. Therefore, option (c) Polymorphism is the correct answer.

10. What design pattern is used to decouple classes by having one class subscribe to changes in another class?

a) Observer b) Strategy c) Adapter d) Facade

The correct answer is:

a) Observer

Explanation: The Observer design pattern is used to decouple classes by having one class subscribe to changes in another class. It defines a one-to-many dependency between objects, where the subject (or publisher) maintains a list of observers (or subscribers) and notifies them of any state changes. This allows observers to react to changes in the subject without being tightly coupled to it. The Observer pattern promotes loose coupling and separation of concerns, making it easier to maintain and extend the system. Therefore, option (a) Observer is the correct answer.

11. What is the primary benefit of using interfaces in object-oriented design?

a) To provide a blueprint for creating objects

b) To define a common set of methods that classes must implement

c) To encapsulate data within a class

d) To establish a parent-child relationship between classes

The correct answer is:

b) To define a common set of methods that classes must implement

Explanation: The primary benefit of using interfaces in object-oriented design is to define a common set of methods that classes must implement. Interfaces specify a contract or behavior that classes must adhere to, without specifying how the methods are implemented. By defining a common interface, interfaces enable polymorphism, allowing objects of different classes to be treated uniformly based on their shared behavior. This promotes code reusability, modularity, and flexibility in design. Therefore, option (b) is the correct answer.

12. Which design pattern is used to create objects based on a template or prototype instance?

a) Prototype b) Factory Method c) Abstract Factory d) Singleton

The correct answer is:

a) Prototype

Explanation: The Prototype design pattern is used to create objects based on a template or prototype instance. Instead of creating new instances through constructors or factory methods, the Prototype pattern involves cloning an existing instance (the prototype) and customizing it as needed. This allows for the creation of new objects with minimal overhead and without having to specify their exact class. The Prototype pattern promotes flexibility and reusability by allowing clients to produce new objects from existing prototypes, which can be modified or extended dynamically. Therefore, option (a) Prototype is the correct answer.

13. Which of the following is a behavioral design pattern?

a) Singleton b) Observer c) Prototype d) Adapter

The correct answer is:

b) Observer

Explanation: The Observer design pattern is a behavioral design pattern. Behavioral patterns are concerned with the communication between objects and the assignment of responsibilities between them. The Observer pattern defines a one-to-many dependency between objects, where one object (the subject) notifies its dependents (observers) of any state changes, allowing them to react accordingly. This pattern is commonly used to implement distributed event-handling systems or to decouple classes by having one class subscribe to changes in another class. Therefore, option (b) Observer is the correct answer.

14. What design pattern is used to represent part-whole hierarchies where a change to one object affects its dependent objects?

a) Observer b) Composite c) Strategy d) Decorator

The correct answer is:

b) Composite

Explanation: The Composite design pattern is used to represent part-whole hierarchies where a change to one object affects its dependent objects. It allows clients to treat individual objects and compositions of objects uniformly through a common interface. The Composite pattern is composed of components that can be either individual objects or composite objects (composed of other components). Changes to composite objects propagate to their children recursively, ensuring that the entire hierarchy is updated consistently. Therefore, option (b) Composite is the correct answer.

15. What design pattern is used to define a family of algorithms and make them interchangeable?

a) Singleton b) Strategy c) Observer d) Decorator

The correct answer is:

b) Strategy

Explanation: The Strategy design pattern is used to define a family of algorithms and make them interchangeable. It defines a set of algorithms, encapsulates each one as an object, and makes them interchangeable within a context object. This allows the algorithm to vary independently from the clients that use it. The Strategy pattern promotes flexibility and modularity by allowing algorithms to be selected or replaced at runtime without altering the client code. Therefore, option (b) Strategy is the correct answer.

16. Which of the following statements about the Builder design pattern is true?

a) It is used to restrict instantiation of a class to one object

b) It is used to define a family of algorithms and make them interchangeable

c) It is used to separate the construction of a complex object from its representation

d) It is used to decouple classes by having one class subscribe to changes in another class

The correct answer is:

c) It is used to separate the construction of a complex object from its representation

Explanation: The Builder design pattern is used to separate the construction of a complex object from its representation. It allows the same construction process to create different representations of the object. This pattern is particularly useful when the construction process is complex or when the object construction needs to be isolated from the final object representation. Therefore, option (c) is the correct statement about the Builder design pattern.

17. What design pattern is used to convert the interface of a class into another interface clients expect?

a) Adapter b) Facade c) Bridge d) Decorator

The correct answer is:

a) Adapter

Explanation: The Adapter design pattern is used to convert the interface of a class into another interface that clients expect. It allows classes with incompatible interfaces to work together by providing a wrapper (the adapter) that translates the interface of one class into an interface that the client expects. This pattern is useful when integrating existing or third-party code with different interfaces into a system without modifying their source code. Therefore, option (a) Adapter is the correct answer.

18. Which design pattern is used to provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation?

a) Observer b) Iterator c) State d) Visitor

The correct answer is:

b) Iterator

Explanation: The Iterator design pattern is used to provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. It allows clients to traverse the elements of a collection (aggregate) without needing to know the internal structure of the collection. The Iterator pattern decouples the traversal algorithm from the collection, promoting flexibility and reusability. This pattern is commonly used in scenarios where different traversal algorithms may be needed or when the underlying representation of the collection may change. Therefore, option (b) Iterator is the correct answer.

19. What design pattern is used to encapsulate a request as an object, thereby allowing for parameterization of clients with queues, requests, and operations?

a) Mediator b) Command c) Proxy d) Memento

The correct answer is:

b) Command

Explanation: The Command design pattern is used to encapsulate a request as an object, thereby allowing for parameterization of clients with queues, requests, and operations. It decouples the sender of a request from the object that performs the action, allowing requests to be handled, queued, logged, and undone in a flexible and extensible manner. Commands are represented as objects, which contain all the information needed to perform an action or request, including the method to call, the arguments, and the receiver object. This pattern is commonly used in scenarios where clients need to issue requests without knowing the details of how the request is processed or by whom. Therefore, option (b) Command is the correct answer.

20. Which design pattern is used to define a dependency between objects so that when one object changes state, all its dependents are notified and updated automatically?

a) Observer b) Memento c) Proxy d) Chain of Responsibility

The correct answer is:

a) Observer

Explanation: The Observer design pattern is used to define a dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. In this pattern, the subject maintains a list of dependents (observers) and notifies them of any changes in its state. This allows the observers to react to the changes accordingly, ensuring consistency between dependent objects. The Observer pattern is commonly used in scenarios where objects need to be kept synchronized or where a change in one object should trigger updates in other related objects. Therefore, option (a) Observer is the correct answer.