Top Design Patterns Interview Questions

A design pattern is simply a generalized and reusable solution to a typical issue that occurs during software development and design. Design patterns are not specific to a particular programming language or technology. They provide abstract blueprints or templates for solving common design-related issues that occur repeatedly. They help software developers to create flexible, organized and maintainable code by following best practices that worked on time.

1. What is a Design Pattern?

Design patterns are re-usable solutions to common problems faced during software development.

• Represent best practices that help developers avoid reinventing the wheel. Patterns resolve problems such as duplicated code, duplicated logic, and flexibility of code, leading to solid, flexible, and maintainable software.
• Widely applied in object-oriented software development.

2. How Are Design Patterns Different from Algorithms?

While algorithms and design patterns both offer solutions for recurring problems, their difference lies in their purpose:

• Algorithms give a step-by-step solution to perform a specific task, often focusing on solving computational problems.
• Design Patterns, however, give general guidelines or blueprints on how to organize software to address repeated design problems. Patterns are more concerned with the architecture and object interactions, whereas algorithms are concerned with an exact computational step.

3. How Are Design Principles Different from Design Patterns?

Design Principles are guidelines followed during software design, such as SOLID, which focus on making software more scalable, extensible, and maintainable. These principles apply to the entire development process and programming practices.

Design Patterns, however, are predefined solutions to specific design problems. They are ready-to-use solutions that can be customized based on specific needs. For example, Factory, Singleton, and Strategy patterns are solutions for specific issues that arise during software development.

4. What are the Types of Design Patterns?

Three main types of Design Patterns are as follows

• Creational Patterns: Deal with object creation mechanisms (e.g., Singleton, Factory).
• Structural Patterns: Deal with object composition and inheritance (e.g., Adapter, Facade).
• Behavioral Patterns: Deal with object interactions and communication (e.g., Observer, Strategy).

5. What are the Advantages of Using Design Patterns?

There are many advantages of using Design Patterns

1. Proven Solutions: They offer tested solutions to common problems, saving time and effort.
2. Reusability: They are reusable and can be used in multiple projects.
3. Better Communication: They provide a common language for developers to discuss designs clearly.
4. Improved Architecture: They help in creating a well-organized and easy to understand system.
5. Clarity: Design patterns make the design process transparent and easier to follow.

6. What are the types of creational Patterns?

The following are five types of Creational Patterns:

1. Factory method
2. Abstract Factory
3. Builder
4. Prototype
5. Singleton

7. What are the types of Structural patterns?

The types of Structural patterns are as follow :

1. Adapter
2. Bridge
3. Filter
4. Composite
5. Decorator
6. Facade
7. Flyweight
8. Proxy

8. What are the types of Behavioral patterns?

The types of Behavioral Patterns are as follow:

1. Interpreter Pattern
2. Template Method Pattern
3. Chain of responsibility Pattern
4. Command Pattern
5. Iterator Pattern
6. Strategy Pattern
7. Visitor Pattern

9. What Are Some of the Design Patterns Used in Java’s JDK Library?

Some of the design patterns used in Java's JDK library include:

• Decorator Pattern: Used by wrapper classes like BufferedReader and BufferedWriter.
• Singleton Pattern: Used by Runtime and Calendar classes to ensure only one instance exists.
• Factory Pattern: Used in methods like Integer.valueOf() for converting strings to integers.
• Observer Pattern: Used in event-handling frameworks like AWT and Swing for UI event management.

10. What Are the SOLID Principles?

The SOLID Principles are five design principles developers use to write clean, maintainable, and scalable code:

1. Single Responsibility Principle (SRP): A class should have a single reason to change.
2. Open-Closed Principle (OCP): Software entities must be open for extension but closed for modification.
3. Liskov Substitution Principle (LSP): Objects of a superclass should be replaceable with objects of its subclasses without changing the correctness of the program.
4. Interface Segregation Principle (ISP): Clients shouldn't be made to depend on interfaces they don't use.
5. Dependency Inversion Principle (DIP): High-level modules must not be dependent on low-level modules. Both of them must be dependent upon abstractions.

11. What is Known as Gang of Four?

The four authors who published the book Design Patterns Elements of Reusable Object-Oriented Software are known as Gang of Four. The name of four authors are Erich Gamma, Ralph Johnson, Richard Helm, and John Vlissides.

12. What is the Singleton pattern, and when would you use it?

The Singleton Pattern ensures that a class has only one instances and provides a global point of access to that instance. It is used when we want to limit creation of an object to only one instance. It ensures controlled access to a resource.

For example :

public class Singleton {
    private static Singleton instance;
    private Singleton() {} // Private constructor

    public static Singleton getInstance()
    {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

When to Use:

Use the Singleton pattern when:

• You need to control access to a shared resource like a database connection or a configuration manager.
• You want to ensure that there is only one instance of a class preventing redundant copies.

13. Explain the Factory Method Pattern and provide an Example of its use.

The Factory Method Pattern defines an interface for creating objects because it allows subclasses to alter the type of objects that will be created. This pattern helps in assigining the object creation to subclasses that enables flexibility and extensibility.

Use the Factory Method Pattern when:

• You need to create objects, but the exact type of the object should be determined by subclasses.
• You want to transfer the responsibility of object creation to subclasses without altering the code that uses the objects.

Example in python :

from abc import ABC, abstractmethod


class Creator(ABC):
    @abstractmethod
    def factory_method(self):
        pass

    def some_operation(self):
        product = self.factory_method()
        return f"Creator: {product.operation()}"


class ConcreteCreator1(Creator):
    def factory_method(self):
        return ConcreteProduct1()


class ConcreteCreator2(Creator):
    def factory_method(self):
        return ConcreteProduct2()


class Product(ABC):
    @abstractmethod
    def operation(self):
        pass


class ConcreteProduct1(Product):
    def operation(self):
        return "Product 1"


class ConcreteProduct2(Product):
    def operation(self):
        return "Product 2"

14. Describe the Adapter Pattern and provide an Example of where it can be applied.

The Adapter pattern allows the interface of an existing class to be used as another interface. It is often used to make existing classes work with others without modifying their source code.

Use the Adapter Pattern when:

• You need to integrate classes that have incompatible interfaces.
• You cannot modify the existing class but need it to work with new code or libraries.

Example:

interface ITarget {
    void Request();
}

class Adaptee {
    public void SpecificRequest()
    {
        Console.WriteLine("Adaptee's method called");
    }
}

class Adapter : ITarget {
    private Adaptee adaptee = new Adaptee();

    public void Request() { adaptee.SpecificRequest(); }
}

15. Provide a scenario where the Command pattern would be preferable to the Strategy pattern.

The Command pattern is preferable you want to encapsulate a request as an object with additional metadata such as the request's originator or ability to queue commands for later execution. It allows you to undo/redo operations, queue them for execution or log them.

Imagine a remote control for multiple devices (TV, lights, fan). Each button on the remote represents a command. The Command Pattern allows you to treat each button's action as a command object, which can be queued, executed, or undone. This is more complex than simply switching algorithms and involves additional metadata (such as the device's state or the command's origin).

The Strategy Pattern focuses on encapsulating interchangeable algorithms where there is no need for metadata about the request itself.

16. Explain the Single Responsibility Principle and its significance in Software Design.

The SRP defines that one class should have just a single reason to change i.e it should have only one responsibility or task. This principle ensures that one class is dedicated to a single concern and is not overloaded with several unrelated responsibilities.

Significance:

• Modularity: By implementing SRP the system is more modular and easier to maintain and extend.
• Maintainability: Changes in one area of the system are less likely to impact other areas that are unrelated, limiting the risk of bugs when changing the code.
• Readability: Code becomes more readable and easier to comprehend since each class has a defined, singular responsibility.

17. What is the Observer Pattern, and how does it allow objects to inform other objects about changes in State?

The Observer pattern establishes a one-to-many relationship between objects such that one object (the subject) keeps a list of its dependents (observers) and informs them of changes to the state. The observers are updated automatically whenever the subject's state is changed.

For example:

import java.util.ArrayList;
import java.util.List;

interface Observer {
    void update(String message);
}

class ConcreteObserver implements Observer {
    private String name;

    public ConcreteObserver(String name)
    {
        this.name = name;
    }

    public void update(String message)
    {
        System.out.println(
            name + " received message: " + message);
    }
}

class Subject {
    private List<Observer> observers = new ArrayList<>();

    public void attach(Observer observer)
    {
        observers.add(observer);
    }

    public void detach(Observer observer)
    {
        observers.remove(observer);
    }

    public void notifyObservers(String message)
    {
        for (Observer observer : observers) {
            observer.update(message);
        }
    }
}

Explanation: The Subject maintains a list of observers and notifies them of changes. Each observer gets updated with the new message.

18. Define the Open/Closed Principle and how design patterns enforce it.

The Open/Closed Principle (OCP) defines that software entities (classes, modules, functions) must be open for extension but closed for modification. This implies you can extend the functionality without changing existing code.

Design patterns such as Strategy and Decorator Patterns enable new behavior to be added by inheriting existing classes or modules instead of modifying them. This follows the OCP by avoiding modifications to the fundamental code while still allowing additional behavior.

19. How is the Bridge Pattern different from the Adapter Pattern?

• The Bridge Pattern is used to separate an abstraction from its implementation, allowing both to vary independently. It’s ideal when you want to decouple a class’s interface from its implementation so that you can change either without affecting the other.
• The Adapter Pattern, on the other hand converts one interface into another expected by the client allowing incompatible interfaces to work together. The Adapter is a structural pattern used to make existing classes compatible with others.

20. In what way does the Dependency Inversion Principle facilitate loose coupling and how does it relate to design patterns?

• The Dependency Inversion Principle (DIP) encourages loose coupling by keeping high-level modules dependent on abstractions and not concrete implementations. This facilitates change in implementations without influencing the high-level modules that are dependent upon them.
• Many design patterns, like Factory Method and Dependency Injection, help implement DIP by ensuring that classes depend on abstractions and not on concrete implementations. This results in more flexible, testable, and maintainable code

21. Give a real-world example of using the Singleton Pattern in a common library or framework.

A common real-world example of using the Singleton Pattern is the Runtime class in Java. The Runtime class is a Singleton that provides access to the runtime environment of the Java application. It ensures that only one instance of the runtime is used throughout the application.

Example:

Runtime runtime = Runtime.getRuntime();

This guarantees that the runtime environment is being accessed in a controlled fashion without instantiating multiple objects.

22. Give an example where the Strategy pattern is applied to switch between various Algorithms.

The Strategy Pattern enables a class to alter its behavior (or algorithm) at runtime. An example would be sorting algorithms. You may employ various sorting strategies based on data size or type.

Example:

interface SortingStrategy {
    void sort(int[] array);
}

class QuickSort implements SortingStrategy {
    public void sort(int[] array) {
        // QuickSort algorithm
    }
}

class MergeSort implements SortingStrategy {
    public void sort(int[] array) {
        // MergeSort algorithm
    }
}

class SortContext {
    private SortingStrategy strategy;

    public SortContext(SortingStrategy strategy) {
        this.strategy = strategy;
    }

    public void setSortingStrategy(SortingStrategy strategy) {
        this.strategy = strategy;
    }

    public void sortArray(int[] array) {
        strategy.sort(array);
    }
}

Explanation: We can select a sort algorithm at runtime (QuickSort or MergeSort) with the Strategy pattern.

23. When should you avoid using design patterns and what are the potential drawbacks?

You should avoid using design patterns when they add unnecessary complexity or when the problem can be solved in a simpler manner. Too many patterns can cause code bloat, where the code is hard to read and maintain because of too many abstractions or too many layers.

Drawbacks of using patterns inappropriately:

• Increased complexity: Introducing patterns for problems that don’t need them.
• Code bloat: Too many classes and interfaces for simple problems.
• Over-engineering: Solving problems that are better solved by simpler approaches

24. How do you prevent design patterns from causing over-engineering and unnecessary complexity?

Avoid using design patterns to cause over-engineering by using them selectively. Select patterns that solve the particular problems you're attempting to solve and don't use too much in abstraction or indirection. Write understandable code and make things as simple as possible while still solving the problem you're working on.

25. Give an example of how the Decorator Pattern can be applied to extend the functionality of a pre-existing class in a Codebase.

The Decorator Pattern can be employed to extend an object dynamically without modifying its structure. A typical example is in text processing software, where you would like to include facilities such as spell-checking, formatting, or encryption in a text editor.

Example:

interface Text {
    String getText();
}

class PlainText implements Text {
    private String text;

    public PlainText(String text) {
        this.text = text;
    }

    public String getText() {
        return text;
    }
}

class TextDecorator implements Text {
    protected Text wrappedText;

    public TextDecorator(Text wrappedText) {
        this.wrappedText = wrappedText;
    }

    public String getText() {
        return wrappedText.getText();
    }
}

class BoldTextDecorator extends TextDecorator {
    public BoldTextDecorator(Text wrappedText) {
        super(wrappedText);
    }

    @Override
    public String getText() {
        return "<b>" + wrappedText.getText() + "</b>";
    }
}

Explanation: The Decorator Pattern is used for adding extra capabilities such as bold formatting without changing the original PlainText class.

26. What Is Inversion of Control?

Inversion of Control (IoC) is a design principle in which the management of object creation and control is moved from the program to a container or framework. It is often applied in dependency injection frameworks, where objects are injected into classes instead of being instantiated within them. This decouples things and enhances flexibility.

27. How can the Command Pattern be used in a User Interface (UI) Framework?

The Command Pattern may be applied in a UI framework to encapsulate user actions (such as button clicks) into command objects. These commands can be executed, undone, or stored in a history for features like undo/redo.

Example: Every user action (e.g., button click) is wrapped into a command object, which is then run by the UI framework. To implement undo/redo, prior commands are saved and replayed appropriately.

28. What is the purpose of a UML diagram in displaying design patterns?

Design patterns, classes, their relationships, and interactions are visualized using UML (Unified Modeling Language) diagrams. They facilitate clear communication of design ideas, making it simple to comprehend the structure of the pattern and how it would be placed within a system.

29. How can Design Patterns assist in refactoring existing code?

Design patterns offer proven solutions to recurring design issues. While refactoring code, you can spot areas where these patterns can be applied in order to enhance code structure, maintainability, and flexibility. which eliminates code duplication and makes the system easier to scale.

30. What Is the Role of Design Patterns in Software Development?

Design patterns offer proven solutions to common issues in software development. They encourage best practices, improve code maintainability, and increase software scalability, and this makes the codebase easier and more flexible to maintain over the long term.