Design Patterns Interview Questions

A design pattern is a reusable solution to a common software engineering problem. It provides a template or a blueprint for solving recurring software design problems in an efficient and effective way. Design patterns are not specific to any particular programming language, technology or architecture; rather, they are general solutions that can be applied in various contexts. Design patterns were first introduced by the Gang of Four (GoF) in their book Design Patterns: Elements of Reusable Object-Oriented Software. They identified 23 design patterns, which were classified into three categories: creational, structural, and behavioral.Using design patterns can improve the quality, reusability, and maintainability of software. By following established design patterns, developers can avoid reinventing the wheel and can focus on the unique aspects of their problem domain.

Design Patterns are classified into three categories: creational, structural, and behavioral. Creational patterns deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. Structural patterns deal with object composition, such as class and object composition, trying to identify simple ways to realize relationships between objects. Behavioral patterns deal with communication between objects, trying to identify patterns of communication between objects to simplify and decouple communication.

The Singleton Design Pattern is a creational design pattern that restricts the instantiation of a class to only one instance and provides a single point of access to this instance for all other objects.

The Factory Design Pattern is a creational design pattern that provides an interface for creating objects in a super class, but allows subclasses to alter the type of objects that will be created.

The Observer Design Pattern is a behavioral design pattern that allows objects to subscribe to be notified of changes to the subject they are observing.

The Decorator Design Pattern is a structural design pattern that allows behavior to be added to an individual object, dynamically, without affecting the behavior of other objects from the same class.

The Command Design Pattern is a behavioral design pattern that allows requests to be encapsulated as objects, thereby allowing clients to be parametrized with different requests, queue or log requests, and support undo-redo.

The Adapter Design Pattern is a structural design pattern that allows objects with incompatible interfaces to work together by wrapping one object within another.

The Facade Design Pattern is a structural design pattern that provides a simplified interface to a complex subsystem, hiding its implementation details from the client.

The Bridge Design Pattern is a structural design pattern that separates an object's interface from its implementation, allowing the two to vary independently.

The Composite Design Pattern is a structural design pattern that allows clients to treat individual objects and compositions of objects uniformly.

The Flyweight Design Pattern is a structural design pattern that allows objects to be shared, rather than instantiated for each instance, to improve performance when dealing with a large number of objects.

The Proxy Design Pattern is a structural design pattern that provides a surrogate or placeholder object, which references an underlying object and controls access to it.

The Chain of Responsibility Design Pattern is a behavioral design pattern that passes requests along a dynamic chain of receivers until one of them handles it.

The Mediator Design Pattern is a behavioral design pattern that defines a mediator object that controls communication between objects, thereby reducing coupling between objects.

The Template Method Design Pattern is a behavioral design pattern that defines the skeleton of an algorithm as an abstract class, allowing subclasses to provide concrete behavior.

The Iterator Design Pattern is a behavioral design pattern that provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

The State Design Pattern is a behavioral design pattern that allows an object to change its behavior when its internal state changes, providing a more object-oriented approach to handle state-based behavior.

The Visitor Design Pattern is a behavioral design pattern that separates algorithms from the objects on which they operate, allowing new operations to be added to objects without modifying their class.

The Memento Design Pattern is a behavioral design pattern that provides the ability to restore an object to its previous state (undo via rollback).

The Strategy Design Pattern is a behavioral design pattern that allows multiple algorithms or approaches to be used interchangeably within an object, providing a more flexible and reusable solution.

The Interpreter Design Pattern is a behavioral design pattern that provides a way to evaluate sentences in a language, defined by a grammar, by providing a representation for each grammar rule and interpreting the sentence.

The Command Design Pattern is a behavioral design pattern that allows objects to be treated as methods, allowing operations to be executed and undone, as well as being queued and executed at a later time.

The Observer Design Pattern is a behavioral design pattern that allows objects to be notified of changes in the state of another object, so that they can react to the change.

The Abstract Factory Design Pattern is a creational design pattern that provides a way to create families of related objects, without specifying their concrete classes.

The Builder Design Pattern is a creational design pattern that provides a way to create objects step by step, providing a more flexible and reusable solution for object creation.

The Prototype Design Pattern is a creational design pattern that provides a way to create objects by cloning an existing object, providing a more flexible and reusable solution for object creation.

The Decorator Design Pattern is a structural design pattern that allows objects to be dynamically extended with additional responsibilities, providing a more flexible and reusable solution.

The Flyweight Design Pattern is a structural design pattern that allows multiple objects to be shared, reducing the amount of memory used, by storing objects in a pool and reusing them, rather than creating new objects each time.

The Bridge Design Pattern is a structural design pattern that separates the abstraction from its implementation, allowing both to vary independently, providing a more flexible and reusable solution.

The Facade Design Pattern is a structural design pattern that provides a simplified interface to a complex system, hiding its complexity and allowing for easier use and maintenance.

The Composite Design Pattern is a structural design pattern that allows objects to be treated as a single object, as well as a collection of objects, providing a more flexible and reusable solution.

The Adapter Design Pattern is a structural design pattern that allows objects with incompatible interfaces to work together, by converting the interface of one object to match the interface of another object.

The Proxy Design Pattern is a structural design pattern that provides a placeholder object that controls access to another object, allowing for additional functionality, such as lazy loading, caching, or security, to be added.

The Chain of Responsibility Design Pattern is a behavioral design pattern that allows a series of objects to be chained together, so that a request can be passed through the chain until it is handled, providing a more flexible and reusable solution.

The Mediator Design Pattern is a behavioral design pattern that allows objects to communicate with each other through a mediator object, reducing the coupling between objects and providing a more flexible and reusable solution.

The Template Method Design Pattern is a behavioral design pattern that provides a template method that defines the steps of an algorithm, allowing subclasses to provide their own implementation for some or all of the steps, providing a more flexible and reusable solution.

The Interpreter Design Pattern is a behavioral design pattern that provides a way to interpret a language, by defining a grammar and an interpreter that implements the grammar, providing a more flexible and reusable solution.

The Iterator Design Pattern is a behavioral design pattern that provides a way to access elements of an object, such as a collection, one at a time, without exposing its underlying representation, providing a more flexible and reusable solution.

The State Design Pattern is a behavioral design pattern that allows an object to change its behavior, depending on its state, by encapsulating the behavior in separate state objects, providing a more flexible and reusable solution.

The Strategy Design Pattern is a behavioral design pattern that provides a way to define a family of algorithms, encapsulating each algorithm in a separate strategy object, and allowing the algorithms to be interchangeable, providing a more flexible and reusable solution.

The Visitor Design Pattern is a behavioral design pattern that allows an operation to be performed on elements of an object structure, by defining a visitor object that implements the operation, providing a more flexible and reusable solution.

The Memento Design Pattern is a behavioral design pattern that provides a way to save and restore the state of an object, without exposing its internal representation, by using a memento object, providing a more flexible and reusable solution.

The Observer Design Pattern is a behavioral design pattern that allows objects to be notified of changes to the state of another object, by defining a subject object that maintains a list of its dependents, and notifying them when it changes state, providing a more flexible and reusable solution.

The Command Design Pattern is a behavioral design pattern that provides a way to request an action, by defining a command object that encapsulates the request, and allowing the request to be queued, undone, or redone, providing a more flexible and reusable solution.

The Flyweight Design Pattern is a structural design pattern that provides a way to reduce the memory usage of large numbers of objects, by sharing common data among objects, rather than duplicating it, providing a more efficient and reusable solution.

The Bridge Design Pattern is a structural design pattern that provides a way to separate the abstraction and implementation of an object, allowing them to vary independently, by defining an abstraction that delegates to an implementation, providing a more flexible and reusable solution.

The Chain of Responsibility Design Pattern is a behavioral design pattern that provides a way to process a request, by defining a chain of objects, each of which may process the request, or pass it to the next object in the chain, providing a more flexible and reusable solution.

The Mediator Design Pattern is a behavioral design pattern that provides a way to reduce coupling between objects, by defining a mediator object that acts as an intermediary between objects, handling their communication, providing a more flexible and reusable solution.

The Facade Design Pattern is a structural design pattern that provides a simplified interface to a complex system, by defining a facade object that encapsulates the system, and provides a simpler and more convenient interface, providing a more flexible and reusable solution.

The Proxy Design Pattern is a structural design pattern that provides a way to control access to an object, by defining a proxy object that acts as an intermediary, forwarding requests to the target object, and controlling access to it, providing a more flexible and reusable solution.

The Template Method Design Pattern is a behavioral design pattern that provides a way to define the structure of an algorithm, by defining a template method that contains the steps of the algorithm, and allowing concrete subclasses to provide the implementation for some of the steps, providing a more flexible and reusable solution.

The Singleton Design Pattern could be used to manage the shopping cart by defining a singleton class that implements the shopping cart, and ensuring that there is only one instance of the class in the system, that can be shared by all customers, providing a more efficient and reusable solution.

The Observer Design Pattern could be used to notify the users by defining a subject class that maintains a list of its dependents (the users), and notifying them when a new message is received, providing a more flexible and reusable solution.

The Command Design Pattern could be used to allow the users to undo and redo the operations by defining a command class for each text operation, that encapsulates the request, and allowing the requests to be queued, undone, or redone, providing a more flexible and reusable solution.

The Flyweight Design Pattern could be used to manage the objects by defining a flyweight class for each type of object, that shares common data among objects, rather than duplicating it, reducing memory usage and providing a more efficient and reusable solution.

The Bridge Design Pattern could be used to display the data by defining an abstraction class that encapsulates the data, and delegates to an implementation class that provides access to the data sources, allowing the abstraction and implementation to vary independently, and providing a more flexible and reusable solution.

The Composite Design Pattern could be used to display the views by defining a component class that represents a view, and allowing views to be composed of other views, either primitive views or composite views, allowing the views to be treated uniformly and providing a more flexible and reusable solution.

The Decorator Design Pattern could be used to manage the books and journals by defining a component class that represents a book or journal, and allowing books and journals to be decorated with additional information, such as borrowing dates or author names, providing a more flexible and reusable solution.

The Factory Design Pattern could be used to manage the user profiles by defining a factory class that creates user profiles, and encapsulating the profile creation process, allowing profiles to be created in a more flexible and reusable way, and hiding the implementation details from the client.

The State Design Pattern could be used to manage the behavior of the characters by defining a state class for each behavior, that represents the behavior and its state, and allowing the characters to change their behavior and state dynamically, providing a more flexible and reusable solution.

The Template Method Design Pattern could be used to manage the travel packages by defining a template class that represents a travel package, and defining the common steps in the package process, such as booking a flight and reserving a hotel, and allowing the specific steps to be overridden by concrete subclasses, providing a more flexible and reusable solution.

The Observer Design Pattern could be used to manage the articles by defining a subject class that represents the articles, and allowing the articles to be observed by multiple observer classes, such as displaying the articles in a list or sending notifications about new articles, providing a more flexible and reusable solution.

The Singleton Design Pattern could be used to manage the shopping cart by defining a singleton class that represents the shopping cart, and allowing only one instance of the shopping cart to exist, ensuring that there's always a single source of truth for the shopping cart, providing a more flexible and reusable solution.

The Flyweight Design Pattern could be used to manage the video content by defining a flyweight class that represents the video content, and allowing the video content to be shared among multiple clients, reducing the memory usage and improving the performance, providing a more flexible and reusable solution.

The Command Design Pattern could be used to manage the treatment plans by defining a command class that represents a treatment plan, and allowing the treatment plans to be executed, queued, or undone, providing a more flexible and reusable solution.

The Mediator Design Pattern could be used to manage the orders by defining a mediator class that represents the orders, and allowing the orders to be managed through the mediator, reducing the coupling between the objects and improving the maintainability, providing a more flexible and reusable solution.

The Decorator Design Pattern could be used to manage the posts by defining a base class that represents the post, and allowing the posts to be decorated with additional functionality, such as adding a filter, adding a location, or adding a time stamp, providing a more flexible and reusable solution.

The Factory Method Design Pattern could be used to manage the data by defining a factory class that creates the data, and allowing the data to be created based on different criteria, such as the location, the time, or the type of data, providing a more flexible and reusable solution.

The Abstract Factory Design Pattern could be used to manage the properties by defining an abstract factory class that represents the properties, and allowing the properties to be created based on different criteria, such as the type of property, the location, or the price, providing a more flexible and reusable solution.

The Builder Design Pattern could be used to manage the transactions by defining a builder class that represents the transactions, and allowing the transactions to be created step by step, such as setting the type, the amount, the date, or the security, providing a more flexible and reusable solution.

The Prototype Design Pattern could be used to manage the reservations by defining a prototype class that represents the reservations, and allowing the reservations to be cloned, providing a more flexible and reusable solution, without the need to create a new instance every time a reservation is made.

The Singleton Design Pattern could be used to manage the shopping cart by defining a singleton class that represents the shopping cart, and allowing only one instance of the shopping cart to exist throughout the lifetime of the application, providing a more efficient solution for managing the shopping cart.

The Chain of Responsibility Design Pattern could be used to manage the tasks by defining a series of classes that represent the tasks, and allowing the tasks to be handled by each class in turn, providing a more flexible solution for managing the tasks.

The Command Design Pattern could be used to manage the messages by defining a command class that represents the messages, and allowing the messages to be executed as commands, providing a more flexible solution for managing the messages.

The Mediator Design Pattern could be used to manage the payments by defining a mediator class that represents the payments, and allowing the payments to be managed by the mediator, providing a more flexible solution for managing the payments.

The Memento Design Pattern could be used to manage the itineraries by defining a memento class that represents the itineraries, and allowing the itineraries to be stored and retrieved, providing a more flexible solution for managing the itineraries.

The Factory Method Design Pattern could be used to manage database connections by defining a factory class that represents database connections, and allowing database connections to be created as objects, providing a more flexible solution for managing database connections.

The Observer Design Pattern could be used to manage user notifications by defining an observer class that represents user notifications, and allowing user notifications to be notified to multiple subscribers, providing a more flexible solution for managing user notifications.

The State Design Pattern could be used to manage project tasks by defining a state class that represents project tasks, and allowing project tasks to be managed based on their current state, providing a more flexible solution for managing project tasks.

The Template Method Design Pattern could be used to manage property listings by defining a template class that represents property listings, and allowing property listings to be managed based on a common template, providing a more flexible solution for managing property listings.

The Visitor Design Pattern could be used to manage web pages by defining a visitor class that represents web pages, and allowing web pages to be managed by the visitor, providing a more flexible solution for managing web pages.

The Chain of Responsibility Design Pattern could be used to manage the checkout process by defining a chain of classes that represent different steps in the checkout process, and allowing the checkout process to be managed by the chain, providing a more flexible solution for managing the checkout process.

The Command Design Pattern could be used to manage video playback by defining a command class that represents video playback actions, and allowing video playback actions to be managed by the command, providing a more flexible solution for managing video playback.

The Decorator Design Pattern could be used to manage job postings by defining a decorator class that represents job postings, and allowing job postings to be managed by adding or removing decorators, providing a more flexible solution for managing job postings.

The Flyweight Design Pattern could be used to manage recipe ingredients by defining a flyweight class that represents recipe ingredients, and allowing recipe ingredients to be managed by sharing common ingredients, providing a more flexible solution for managing recipe ingredients.

The Memento Design Pattern could be used to manage weather data by defining a memento class that represents weather data, and allowing weather data to be managed by creating and restoring mementos, providing a more flexible solution for managing weather data.

The Observer Design Pattern could be used to manage friend requests by defining an observer class that represents friend requests, and allowing friend requests to be managed by observing changes in the state of the friend requests and updating accordingly, providing a more flexible solution for managing friend requests.

The State Design Pattern could be used to manage chat rooms by defining a state class that represents the state of chat rooms, and allowing chat rooms to be managed by changing the state of chat rooms based on user actions, providing a more flexible solution for managing chat rooms.

The Template Method Design Pattern could be used to manage tasks by defining a template method that represents the steps involved in managing tasks, and allowing tasks to be managed by defining the steps involved in managing tasks and providing a consistent method for managing tasks, providing a more flexible solution for managing tasks.

The Visitor Design Pattern could be used to manage transactions by defining a visitor class that represents transactions, and allowing transactions to be managed by visiting transactions and performing operations on transactions, providing a more flexible solution for managing transactions.

The Interpreter Design Pattern could be used to manage voice commands by defining an interpreter class that represents voice commands, and allowing voice commands to be managed by interpreting voice commands and performing the corresponding action, providing a more flexible solution for managing voice commands.

The Command Design Pattern could be used to manage orders by defining a command class that represents orders, and allowing orders to be managed by encapsulating orders as objects and executing orders when requested, providing a more flexible solution for managing orders.

The Composite Design Pattern could be used to manage weather data by defining a composite class that represents weather data, and allowing weather data to be managed by treating individual weather data and collections of weather data uniformly, providing a more flexible solution for managing weather data.

The Factory Method Design Pattern could be used to manage movie rentals by defining a factory method that creates movie rental objects, and allowing movie rentals to be managed by creating movie rental objects dynamically, providing a more flexible solution for managing movie rentals.

The Decorator Design Pattern could be used to manage investments by defining a decorator class that represents investments, and allowing investments to be managed by dynamically adding and removing investment-related responsibilities, providing a more flexible solution for managing investments.

The Iterator Design Pattern could be used to manage playlists by defining an iterator class that represents playlists, and allowing playlists to be managed by iterating over playlists and accessing playlist elements, providing a more flexible solution for managing playlists.

The Observer Design Pattern could be used to manage users by defining an observer class that represents users, and allowing users to be managed by subscribing to updates from other users and being notified of any changes, providing a more flexible solution for managing users.

The Builder Design Pattern could be used to manage shopping carts by defining a builder class that represents shopping carts, and allowing shopping carts to be managed by building shopping carts incrementally, providing a more flexible solution for managing shopping carts.

The State Design Pattern could be used to manage tasks by defining a state class that represents the state of tasks, and allowing tasks to be managed by transitioning between different states, providing a more flexible solution for managing tasks.

The Proxy Design Pattern could be used to manage books by defining a proxy class that represents books, and allowing books to be managed by providing a substitute or place holder for a real object, providing a more flexible solution for managing books.

The Template Method Design Pattern could be used to manage courses by defining a template method that represents courses, and allowing courses to be managed by defining the skeleton of an algorithm and allowing concrete classes to provide the details, providing a more flexible solution for managing courses.

The Factory Method Design Pattern could be used to manage flight tickets by defining a factory method that creates flight tickets, and allowing flight tickets to be managed by defining a factory method that creates objects without specifying the exact class of object that will be created, providing a more flexible solution for managing flight tickets.

The Singleton Design Pattern could be used to manage trades by defining a singleton class that represents trades, and allowing trades to be managed by ensuring that only one instance of the class can exist in the system, providing a more flexible solution for managing trades.

The Adapter Design Pattern could be used to manage movie recommendations by defining an adapter class that represents movie recommendations, and allowing movie recommendations to be managed by converting the interface of a class into another interface that a client expects, providing a more flexible solution for managing movie recommendations.

The Decorator Design Pattern could be used to manage weather forecasts by defining a decorator class that represents weather forecasts, and allowing weather forecasts to be managed by dynamically adding or modifying the behavior of an object, providing a more flexible solution for managing weather forecasts.

The Command Design Pattern could be used to manage events by defining a command class that represents events, and allowing events to be managed by encapsulating a request as an object, thereby allowing clients to parametrize clients with different requests, queue or log requests, and support undo-able operations, providing a more flexible solution for managing events.

The Observer Design Pattern could be used to manage user posts by defining an observer class that represents user posts, and allowing user posts to be managed by allowing objects to be notified of changes in the state of another object, providing a more flexible solution for managing user posts.

The Abstract Factory Design Pattern could be used to manage articles by defining an abstract factory class that represents articles, and allowing articles to be managed by providing an interface for creating families of related or dependent objects without specifying their concrete classes, providing a more flexible solution for managing articles.

The Bridge Design Pattern could be used to manage orders by defining a bridge class that represents orders, and allowing orders to be managed by separating the abstraction from its implementation, allowing the two to vary independently, providing a more flexible solution for managing orders.

The Facade Design Pattern could be used to manage deliveries by defining a facade class that represents deliveries, and allowing deliveries to be managed by providing a simple interface to a complex system of classes, hiding the complexity of the system and providing a more flexible solution for managing deliveries.

The Mediator Design Pattern could be used to manage patient data by defining a mediator class that represents patient data, and allowing patient data to be managed by allowing communication between objects to be handled by a mediator object, removing the need for objects to communicate directly with each other, and providing a more flexible solution for managing patient data.

The Observer Design Pattern could be used to manage user posts by defining an observer class that represents user posts, and allowing user posts to be managed by allowing objects to be notified of changes in the state of another object, providing a more flexible solution for managing user posts.

The Abstract Factory Design Pattern could be used to manage articles by defining an abstract factory class that represents articles, and allowing articles to be managed by providing an interface for creating families of related or dependent objects without specifying their concrete classes, providing a more flexible solution for managing articles.

The Bridge Design Pattern could be used to manage orders by defining a bridge class that represents orders, and allowing orders to be managed by separating the abstraction from its implementation, allowing the two to vary independently, providing a more flexible solution for managing orders.

The Facade Design Pattern could be used to manage deliveries by defining a facade class that represents deliveries, and allowing deliveries to be managed by providing a simple interface to a complex system of classes, hiding the complexity of the system and providing a more flexible solution for managing deliveries.

The Mediator Design Pattern could be used to manage patient data by defining a mediator class that represents patient data, and allowing patient data to be managed by allowing communication between objects to be handled by a mediator object, removing the need for objects to communicate directly with each other, and providing a more flexible solution for managing patient data.

The Factory Method Design Pattern could be used to manage truck deliveries by defining a factory method that represents truck deliveries, and allowing truck deliveries to be managed by allowing subclasses to override the factory method to change the class of objects that will be created, providing a more flexible solution for managing truck deliveries.

The Prototype Design Pattern could be used to manage character abilities by defining a prototype class that represents character abilities, and allowing character abilities to be managed by creating new objects by cloning existing objects, providing a more flexible solution for managing character abilities.

The Singleton Design Pattern could be used to manage flight bookings by defining a singleton class that represents flight bookings, and allowing flight bookings to be managed by ensuring that a class has only one instance, while providing a global point of access to this instance, providing a more flexible solution for managing flight bookings.

The Command Design Pattern could be used to manage transactions by defining a command class that represents transactions, and allowing transactions to be managed by encapsulating a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undo-able operations, providing a more flexible solution for managing transactions.

The Adapter Design Pattern could be used to manage weather reports by defining an adapter class that represents weather reports, and allowing weather reports to be managed by converting the interface of a class into another interface that clients expect, allowing classes that could not otherwise be used together to work together, providing a more flexible solution for managing weather reports.