17 Design Pattern Interview Questions and Answers

The 17 design pattern interview questions and their answers discussed in this article are meant to help you prepare for your upcoming interview. These questions are tailored based on what most interviewers inquire from their clients on the subject of design. Design patterns form an integral part of the programming field cutting across the different languages available. Coding and design patterns complement each other in myriads of ways. Therefore, in your design patterns interview, your employer-to-be will ask these questions to determine whether you are good in design or not.

Question 1: Where would you prefer an abstract class over the interface?

It is one of the most challenging design pattern interview questions. This is because both the interface and the abstract class follow the ?writing code for interface than implementation? principle of design. Therefore, this issue can be answered with the help of these guidelines:

? It is possible to implement multiple interfaces but possible to extend only one class. Therefore, extending one class blocks your chances of extending other classes.
? The role of an interface is to represent adjectives or behaviors. Therefore, by using an abstract class to account for the behavior of a class, the class will cease to have the clonable and runnable qualities simultaneously. That is because, in Java, two classes cannot be extended at the same time.
? On-time critical application usually uses the abstract class since it is faster.
? An abstract class is preferred in the case where there is an actual behavior across the hierarchy of inheritance which can be coded.

Question 2: Describe what design patterns are in programming.

These are representations of best practices by program and software developers. They are best solutions to problems that are frequently experienced in the process of software development.

Question 3: What are the various design patterns?

Design patterns are usually classified into three distinct groups. These are:

? Creational patterns: they give us a way to create objects while concealing the creation logic instead of instantiating objects directly using new operators. The strategy is aimed at giving the program a flexibility attribute with regards to choosing which objects have to be created for a certain use case.
? Structural patterns: they have to do with the class object and composition. The concept of inheritance is utilized in the formation of interfaces. They also define ways of composing objects to get new functionalities.
? Behavioral patterns: they entail communication process between objects.

Question 4: Differentiate between the factory pattern and abstract factory pattern.

Among the design pattern interview questions, this one may not miss in your upcoming interview. Factory pattern is one of the widely used patterns in Java programming language based on its ability to provide the best way to create an object. Under this design pattern, an object is created without exposing the creation logic to the pattern. It also creates an object with a familiar interface.

Abstract factory pattern, on the other hand, is a superior facility which can be used to create other factories.

Question 5: Define a singleton pattern.

This is a model that plays a role in ensuring that a single instance of a class initiated.

Question 6: What is a service oriented architecture?

This is a logical encapsulation of self-contained business functionality.

Question 7: Differentiate between a layer and a tier?

A layer refers to the separation of the code and the design usually created for an application in varied files. A tier, on the other hand, is the physical location of the layer files.

Question 8: In what cases are the design patterns not recommended?

Design patterns are not applicable when the software being designed does not change with time and requirements.

They are not used in a scenario where the requirements of the source code of a given applications are unique and similar.

Question 9: What are the fundamental differences between a static and a singleton class?

A static class cannot be used as a top class or used to implement an interface. Singleton classes, on the other hand, can be utilized for that. While all the members of a static class are static, it is not a requirement in a Singleton class. The static class can only get initialized after being loaded to avoid lazy loading. Singleton classes can be lazily loaded. The static class is stored in a stack while the singleton is stored in a heap memory space.

Although it might seem easy, this is actually one of the trickiest design pattern interview questions because an interviewer can easily walk you?through a myriad of notions in just a few minutes.

Question 10: Differentiate between the strategy and state design patterns as used in Java.

Though their structures may appear same, they are meant for different purposes. The state model is used to manage and describe the condition of the object. The strategy pattern allows the user to outline and choose a single algorithm from a group of interchangeable algorithms. Therefore, strategy pattern is client driven.

Question 11: In Java programming language, it is prohibited to access non-static variables from static methods?

The non-static data cannot be accessed from a static context in Java because non-static variables are identified with a certain instance of objects. Static variables aren?t defined with any instance.

Question 12: What is the observer design pattern as used in Java?

These are patterns intended to communicate changes in the status of an object to their particular observers so that they can respond to the changes accordingly.

Question 13: Which creational design pattern can be used in step by step creation of a sophisticated object?

A builder pattern is your best bet in this scenario. It typically designs intricate patterns from simple ones. It abstracts steps of object creation which ensure that various implementations of the steps can yield differently-represented objects.

Question 14: Give the role of the director in the implementation of a builder pattern.

This one is one of the most basic design pattern interview questions. A director class is vital in the creation sequence of an object. It is the receptor of the concrete builder, the application of abstract interface for creating objects as a parameter. It then implements the necessary processes on it.

Question 15: What is a composite pattern?

This is a design pattern used when there is a need to treat a set of objects in the same way as an individual object. They are made up of objects built in a tree structure to symbolize part of or the whole hierarchy. It falls under structural patterns and uniquely creates a class containing a set of its objects. It, therefore, provides strategies through which a set of similar objects can be modified.

Question 16: Define the decorator pattern.

Decorator patterns are tools that enable the addition of new functionality to an object. Just like the composite pattern, it is also a structural model. The decorator pattern wraps the current class.

Question 17: Differentiate between the fa?ade and proxy pattern.

A fa?ade pattern conceals the complexities of the system. It gives the client an interface which they can access the system. Only a single class providing a simplified method to a client is involved. The system assigns a call to techniques of the current system classes.

There exist a plethora of design patterns in programming which play various roles to simplify the system development process. Hence, design pattern interview questions are very many. We have, however, presented you with the most common ones you are likely to meet in the interview room. Make sure to go through this article before going for the interview. You can also suggest this article for your friends to get the knowledge as well.

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