Design Patterns Interview Questions: Mastering the Art of Reusable Solutions

Please read these Design Pattern Interview Questions carefully. They are meant to help you get ready for the kinds of questions you might be asked during your Design Pattern interview. From what I’ve seen, good interviewers don’t usually plan to ask a specific question. Instead, they start with a basic question about the subject and then move on based on what you say or how you answer it. For example, “What are design patterns?”

Design patterns represent the best practices used by experienced object-oriented software developers. Design patterns are solutions to general problems that software developers faced during software development. Many software developers worked their way to these solutions by making mistakes over a long period of time. What is Gang of Four (GOF)?.

Four authors, Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, wrote a book in 1994 called Design Patterns – Elements of Reusable Object-Oriented Software. This book was the first to use the term “Design Pattern” in software development. These authors are collectively known as Gang of Four (GOF). Name types of Design Patterns?.

These design patterns are specifically concerned with the presentation tier. These patterns are identified by Sun Java Center. What is Factory pattern?.

Factory pattern is one of most used design pattern in Java. This kind of design pattern is called a creational pattern, and it gives you one of the best ways to make an object.

In the Factory pattern, we create an object without letting the client see how it was made, and we use a common interface to refer to the newly created object. What is Abstract Factory pattern?.

Abstract Factory patterns work around a super-factory which creates other factories. This factory is also called as factory of factories. This kind of design pattern is called a creational pattern, and it gives you one of the best ways to make an object.

In the Abstract Factory pattern, an interface is in charge of building a factory of related objects without saying what classes they are. Each generated factory can give the objects as per the Factory pattern. What is Singleton pattern?.

Singleton pattern is one of the simplest design patterns in Java. This kind of design pattern is called a creational pattern, and it gives you one of the best ways to make an object.

There is only one class in this pattern that is responsible for creating an object and making sure that only one object is made. This class gives you a way to get to its only object, which you can do directly without having to create the class object first. How can you create Singleton class in java?.

It is two step process. First, make the constructor private so that new operator cannot be used to instantiate the class. Give back an object of the object if it’s not null; if not, make the object and send it back through a method. What are the difference between a static class and a singleton class?.

Throw exception within the body of clone() method. Name some of the design patterns which are used in JDK library.

Factory pattern encapsulates the implementation details and underlying implementation can be changed without any impact on caller api. What is Builder pattern?.

Builder pattern builds a complex object using simple objects and using a step by step approach. This builder is independent of other objects. What is Prototype pattern?.

Prototype pattern refers to creating duplicate object while keeping performance in mind. This pattern involves implementing a prototype interface which tells to create a clone of the current object. When Prototype pattern is to be used?.

This pattern is used when creation of object directly is costly. For example, an object is to be created after a costly database operation. We can store the object in a cache, return a copy of it on the next request, and only update the database when we need to, which cuts down on database calls. What is Adapter pattern?.

Adapter pattern works as a bridge between two incompatible interfaces. This pattern involves a single class which is responsible to join functionalities of independent or incompatible interfaces. Give an example of Adapter pattern.

In real life, a card reader that connects a memory card to a laptop could be an example. You put the memory card into a card reader and then the reader into your laptop. This lets your laptop read the memory card. What is Bridge pattern?.

Bridge is used to separate an abstraction from its implementation so that the two can change on their own. This design pattern is a structural pattern because it separates the implementation class from the abstract class by building a bridge between them.

An interface in this pattern acts as a link between concrete classes and interface implementer classes, letting concrete classes do their own thing. Both types of classes can be altered structurally without affecting each other. What is Filter pattern?.

The filter pattern, also known as the criteria pattern, is a type of design pattern that lets developers sort a group of objects by different criteria and connect them in a way that doesn’t depend on the objects themselves. A structural pattern is what this type of design pattern is because it combines several criteria into a single one. What is Composite pattern?.

We use the composite pattern when we need to treat a group of objects the same way we would treat a single object. Composite pattern composes objects in term of a tree structure to represent part as well as whole hierarchy. This design pattern is a structural pattern because it arranges a group of objects in a tree structure.

This pattern creates a class that contains group of its own objects. This class provides ways to modify its group of same objects. What is Decorator pattern?.

Decorator pattern allows a user to add new functionality to an existing object without altering its structure. This design pattern is a structural pattern because it acts as a cover for an existing class of code.

This pattern makes a decorator class that covers the original class and adds extra features while keeping the class methods signature the same. What is Facade pattern?.

With the facade pattern, the complicated parts of the system are hidden, and the client is given a way to access the system. This type of design pattern is a structural pattern because it hides the complexity of a system by adding an interface to it.

This pattern uses a single class that gives clients simple methods they need and sends calls to methods of other system classes. What is Flyweight pattern?.

The flyweight pattern is mostly used to cut down on the number of objects that are created, which saves memory and speeds things up. It’s a structural pattern because it gives you ways to reduce the number of objects in an application, which makes the object structure better.

The flyweight pattern stores similar objects that can be used again and again, and when no matching object is found, it makes a new one. What is Proxy pattern?.

In proxy pattern, a class represents functionality of another class. This type of design pattern comes under structural pattern.

In proxy pattern, we create object having original object to interface its functionality to outer world. What is Chain of Responsibility pattern?.

As the name suggests, the chain of responsibility pattern creates a chain of receiver objects for a request. This pattern decouples sender and receiver of a request based on type of request. This pattern comes under behavioral patterns.

In this pattern, normally each receiver contains reference to another receiver. The request is sent to the next receiver if the first one can’t handle it, and so on. What is Command pattern?.

Command pattern is a data driven design pattern and falls under behavioral pattern category. A request is wrapped under an object as command and passed to invoker object. The invoker object searches for the right object that can handle this command and sends it to the right object, which then runs the command. What is Interpreter pattern?.

Interpreter pattern provides a way to evaluate language grammar or expression. This type of pattern comes under behavioral pattern. This pattern involves implementing an expression interface which tells to interpret a particular context. Give an example where Interpreter pattern is used?.

Iterator pattern is very commonly used design pattern in Java and . Net programming environment. You can use this pattern to get to the items in a collection object in a certain order without having to know how it is represented. Iterator pattern falls under behavioral pattern category. What are the entities of Service Locator pattern?.

Mediator pattern is used to reduce communication complexity between multiple objects or classes. This pattern gives you a mediator class that usually handles all the communication between classes. Loose coupling makes it easy to maintain the code. Mediator pattern falls under behavioral pattern category. What is Memento pattern?.

Memento pattern is used to restore state of an object to a previous state. Memento pattern falls under behavioral pattern category. Name the actor classes used in Memento pattern.

Memento pattern uses three actor classes. Memento contains state of an object to be restored. Originator objects make states and store them in Memento objects, and Caretaker objects are in charge of restoring object states from Memento objects. What is Observer pattern?.

This pattern is used when there is a one-to-many relationship between objects, like when one object is changed and all of its dependent objects should be automatically notified. Observer pattern falls under behavioral pattern category. Name the actor classes used in Observer pattern.

Observer pattern uses three actor classes. Subject, Observer and Client. Subject is an object having methods to attach and detach observers to a client object. We have created an abstract class Observer and a concrete class Subject that is extending class Observer. What is state pattern?.

In State pattern a class behavior changes based on its state. This type of design pattern comes under behavior pattern. In the State pattern, we make objects that stand for different states and a context object that changes how it acts when its state object changes. What is Null Object pattern?.

In Null Object pattern, a null object replaces check of NULL object instance. Instead of putting if check for a null value, Null Object reflects a do nothing relationship. Such Null object can also be used to provide default behaviour in case data is not available.

In the Null Object pattern, we make an abstract class that describes the operations that need to be carried out, concrete classes that extend this class, and a null object class that implements this class in a way that does nothing. This class is used to check for null values. What is Strategy pattern?.

In Strategy pattern, a class behavior or its algorithm can be changed at run time. This type of design pattern comes under behavior pattern.

In the Strategy pattern, we make objects that stand for different strategies and a context object that acts differently depending on the strategy object it is attached to. The strategy object changes the executing algorithm of the context object. What is Template pattern?.

In Template pattern, an abstract class exposes defined way(s)/template(s) to execute its methods. Its subclasses can change how the method is implemented if they need to, but the way it is called must be the same as what an abstract class says. This pattern comes under behavior pattern category. What is Visitor pattern?.

In Visitor pattern, we use a visitor class which changes the executing algorithm of an element class. By this way, execution algorithm of element can vary as and when visitor varies. This pattern comes under behavior pattern category. The pattern says that the element object must accept the visitor object so that the visitor object can work on the element object. What is MVC pattern?.

Business Delegate Pattern is used to decouple presentation tier and business tier. Its main purpose is to make it easier for business-tier code to communicate or look things up remotely in presentation-tier code. In business tier we have following entities.

Composite Entity pattern is used in EJB persistence mechanism. A Composite entity is an EJB entity bean which represents a graph of objects. When a composite entity is changed, object beans that depend on it are automatically changed because they are managed by an EJB entity bean. Following are the participants in Composite Entity Bean.

The Data Access Object Pattern, or DAO pattern, separates low-level operations or APIs for accessing data from high-level business services. Following are the participants in Data Access Object Pattern.

You can use the front controller design pattern to set up a centralized way to handle requests so that only one handler handles all of them. It is possible for this handler to authenticate, authorize, log, or track requests and then send them to the right handlers. Following are the entities of this type of design pattern.

We use the intercepting filter design pattern when we need to do some work before or after the application’s request or response. Filters are defined and applied on the request before passing the request to actual target application. Requests can be authenticated, authorized, logged, or tracked by filters, which then send the requests to the right handlers. What are the entities of Intercepting Filter pattern?.

The service locator design pattern is used when we want to locate various services using JNDI lookup. Considering high cost of looking up JNDI for a service, Service Locator pattern makes use of caching technique. A service is looked up in JNDI and cached by Service Locator the first time it is needed. A second lookup for the same service through Service Locator is done in its cache, which greatly speeds up the application. What is Transfer Object pattern?.

When we want to send data from the client to the server with many attributes at once, we use the Transfer class pattern. Transfer object is also known as Value Object. Transfer Object is a simple POJO class with getter and setter methods. It can be serialized so it can be sent over the network. It does not have any behavior. Most of the time, a server-side business class gets data from the database, fills in the POJO, and either sends it to the client or passes it by value. For client, transfer object is read-only. The client can make its own transfer object and send it to the server to update the database all at once. Following are the entities of this type of design pattern.

Software Architecture and Design Patterns Interview Questions

FAQ

Which design patterns are asked in interviews?

What are the 4 basic categories for design patterns?

Do pattern questions be asked in an interview?

What are design patterns?

Gain a competitive edge by understanding key concepts and techniques in design patterns. Design patterns are like blueprints that software professionals use to solve common problems when designing an application or system. They represent tried and tested solutions to recurring design issues, making them invaluable tools for developers.

How do you answer a design pattern interview?

Your approach to answering this question may show the interviewer how much experience you have in the field. You can start by briefly explaining what you know about design patterns. Example: “Design patterns are functional codes that solve common problems of software development processes.

How many advanced design patterns interview questions are there?

Check 27 Advanced Design Patterns Interview Questions For Senior Developers and Land Your Next Six-Figure Job Offer! 100% Tech Interview Success!

How do you answer a factory pattern interview question?

When interviewers ask this question, they want to evaluate your knowledge and understanding of the many common design patterns. Hiring managers may also note how fast you answer questions given to you and how well you construct your answers. Briefly explain what you know about the factory pattern when answering this question.