Design pattern interview question in Java Hello guys, if you are preparing for Java interviews and looking for frequently asked design pattern interview questions then you have come to the right place. In the past, I have shared the best courses for Java interviews, and today, I am going to share popular design pattern questions from Java interviews. You can use these questions to both practices and check your knowledge about OOP design patterns. Both OOP and GOF design pattern interview questions are an integral part of any good list of core Java interview questions. Java is a popular Object-oriented programming language and has lots of design patterns and design principles, contributed by many developers and open-source frameworks. As a Java programmer, it's expected from you to know OOPS concepts like Abstraction, Encapsulation, and Polymorphism; what is a design pattern in Java, Some popular Java design patterns, and, most importantly, when to use those design patterns in Java application. The purpose of asking the design pattern interview question in Java is to check whether the Java programmer is familiar with those essential design patterns or not. Design patterns in Java interviews are as crucial as multi-threading, collection, and programming questions and one should not neglect it. Good knowledge of Software design patterns goes a long way in becoming a better Java developer which every company looks for. These Software design patterns provide templates and tricks used to design and solve real-world software problems and tasks. If you know how to apply and use these time-tested design patterns then you can write better code, I mean the code which is more inextensible, maintainable, and flexible. If you are a senior or experienced Java programmer, then you can expect more complex and tough design patterns in Java interviews like Memento, Chain of Responsibility, and Builder design pattern and solving real-time software design questions. By the way, always remember, giving an example from your project creates a better impression and if you want to learn about the modern implementation of design patterns in Java, I suggest you join the Design Patterns in Java course by Dmitri Nesteruk on Udemy. In this course, you will not only learn how to recognize and apply design patterns but also how to Refactor existing designs to use design patterns 18 Best Java design pattern questions and answers Here is my list of the top 10 design pattern interview questions in Java. I have also provided an answer to those Java design pattern questions as a link. No matter which level of Java interview is you going like a programmer, software engineer, a senior software engineer in Java, you can expect a few questions from the Java design pattern. 1. What is the Decorator pattern in Java? Can you give an example of a Decorator pattern? The Decorator pattern is another popular Java design pattern question, which is common because of its heavy usage in java.io package. BufferedReader and BufferedWriter are an excellent examples of the decorator pattern in Java. If you want to learn more about how a Decorator pattern can help you to write better code, I also suggest you check out the Software Design Patterns: Best Practices for Software Developers course on Educative. This course provides a lot of real-world examples in Java programming language which makes it easy to understand and apply these design patterns and write better code. 2. When to use the Strategy Design Pattern in Java? Strategy pattern is quite useful for implementing a set of related algorithms like compression algorithms, filtering strategies, etc. The strategy design pattern allows you to create Context classes, which use Strategy implementation classes for applying business rules. One good example of a Strategy pattern from JDK itself is a Collections.sort() method and the Comparator interface, which is a strategy interface and defines a strategy for comparing objects. Because of this pattern, we don't need to modify the sort() method (closed for modification) to compare any object; at the same time, we can implement a Comparator interface to define a new comparing strategy (open for extension). This pattern is actually based upon the open-closed design principle and if you understand that principle then it's quite easy for you to understand the Strategy pattern as well. It's actually better to know about SOLID principles as many design patterns are based upon that. If you’re looking for a complete course on Object-Oriented design principles, I recommend checking out SOLID Principles of Object-Oriented Design by Steve Smith on Pluralsight. This is a useful course for anyone looking to strengthen their overall knowledge of software architecture. 3. What is the Observer design pattern in Java? When do you use the Observer pattern in Java? This is one of the most common Java design pattern interview questions. The observer pattern is based upon notification, there are two kinds of object Subject and Observer. Whenever there is a change in the subject's state observer will receive a notification. See What is Observer design pattern in Java with real-life examples for more details. 4. What is the difference between Strategy and State design Patterns in Java? This is an interesting Java design pattern interview question as both Strategy and State pattern has the same structure. If you look at the UML class diagram for both patterns, they look exactly the same, but their intent is totally different. The state design pattern is used to define and manage the state of an object, while the Strategy pattern is used to define a set of interchangeable algorithms, and let's client choose one of them. So Strategy pattern is a client-driven pattern while Object can manage their state itself. 5. When to use the Composite design pattern in Java? Have you used it previously in your project? This design pattern question is asked on Java interview not just to check familiarity with the Composite pattern but also, whether a candidate has real-life experience or not. The Composite pattern is also a core Java design pattern, which allows you to treat both whole and part object to treat similarly. Client code, which deals with a Composite or individual object, doesn't differentiate between them, it is possible because the Composite class also implements the same interface as their individual part. One of the good examples of the Composite pattern from JDK is the JPanel class, which is both Component and Container. When the paint() method is called on JPanel, it is internally called the paint() method of individual components and let them draw themselves. In the second part of this design pattern interview question, be truthful; if you have used it, then say yes, otherwise say that you are familiar with the concept and used it on your own. 6. What is the Singleton pattern in Java? Singleton pattern in Java is a pattern that allows only one instance of Singleton class available in the whole application. java.lang.Runtime is a good example of a Singleton pattern in Java. There are lots of follow-up questions on the Singleton pattern see 10 Java singleton interview question answers for those follow-ups. 7. Can you write thread-safe Singleton in Java? There are multiple ways to write thread-safe Singleton in Java, like by writing singleton using double-checked locking, by using static Singleton instance initialized during class loading. By the way, using Java enum to create a thread-safe singleton is the most simple way. See Why Enum singleton is better in Java for more details. 8. When to use the Template method design pattern in Java? The Template pattern is another popular core Java design pattern interview question. I have seen it appear many times in real-life projects themselves. The template pattern outlines an algorithm in the form of a template method and lets the subclass implement individual steps. The critical point to mention, while answering this question, is that the template method should be final so that the subclass can not override and change the steps of the algorithm. Still, the same time individual steps should be abstract, so that child classes can implement them. 9. What is the Factory pattern in Java? What is the advantage of using a static factory method to create an object? The Factory pattern in Java is a creation Java design pattern and a favorite on many Java interviews. A factory pattern is used to create an object by providing static factory methods. There are many advantages of providing factory methods like caching immutable objects, easy to introduce new objects, etc. See What is Factory pattern in Java and benefits for more details. 10. What is the difference between Decorator and Proxy patterns in Java? Another tricky Java design pattern question and trick here is that both Decorator and Proxy implement the interface of the Object they decorate or encapsulate. As I said, many Java design patterns can have similar or exactly the same structure, but they differ in their intent. The Decorator pattern is used to implement functionality on an already created object, while a Proxy pattern is used for controlling access to an object. One more difference between the Decorator and the Proxy design pattern is that the Decorator doesn't create an object; instead, it gets the Object in its constructor, while Proxy actually creates objects. You can also read the Head First Analysis and Design to understand the difference between them. 11. When to use Setter and Constructor Injection in the Dependency Injection pattern? Use Setter injection to provide optional dependencies of an object while use Constructor injection to provide a mandatory dependency of an object, without which it can not work. This question is also related to the Dependency Injection design pattern and mostly asked in the context of the Spring framework, which is now become a standard for developing Java applications. Since Spring provides an IOC container, it also gives you a way to specify dependencies either by using setter methods or constructors. You can also take a look at my previous post on the same topic. 12. What is the difference between Factory and Abstract Factory in Java I have already answered this question in detail with my article with the same title. The main difference is that the Abstract Factory creates a factory while the Factory pattern creates objects. So both abstract the creation logic, but one abstract is for factory and the other for items. You can see here to answer this Java design pattern interview question. 13. When to use the Adapter pattern in Java? Have you used it before in your project? Use the Adapter pattern when you need to make two class works with incompatible interfaces. The Adapter pattern can also be used to encapsulate third-party code so that your application only depends upon the Adapter, which can adapt itself when third-party code changes or you move to a different third-party library. By the way, this Java design pattern question can also be asked by providing an actual scenario. You can further read Head First Design Pattern to learn more about the Adapter pattern and its real-world usage. The book is updated for Java 8 as well, so you will learn new Java 8 ways to implement these old design patterns. 14. Can you write code to implement a producer-consumer design pattern in Java? The Producer-consumer design pattern is a concurrency design pattern in Java, which can be implemented using multiple ways. If you are working in Java 5, then it's better to use Concurrency util to implement producer-consumer pattern instead of plain old wait and notify in Java. Here is an excellent example of implementing the producer-consumer problem using BlockingQueue in Java. 15. What is the Builder design pattern in Java? When do you use the Builder pattern? Builder pattern in Java is another creational design pattern in Java and is often asked in Java interviews because of its specific use when you need to build an object which requires multiple properties, some optional and some mandatory. See When to use Builder pattern in Java for more details. 16. What is the Open closed design principle in Java? The Open closed design principle is one of the SOLID principles defined by Robert C. Martin, popularly known as Uncle Bob in his most popular book, Clean Code. This principle advises that code should be open for extension but closed for modification. At first, this may look conflicting, but once you explore the power of polymorphism, you will start finding patterns that can provide stability and flexibility to this principle. One of the critical examples of this is the State and Strategy design pattern, where the Context class is closed for modification, and new functionality is provided by writing new code by implementing a new state of strategy. See this article to know more about Open closed principle. 17. Can you give an example of SOLID design principles in Java? There are lots of SOLID design pattern which forms acronym SOLID like: 1. Single Responsibility Principle or SRP 3. Open Closed Design Principle or OCD 3. Liskov Substitution Principle 4. Interface Segregation Principle 5. Dependency Inversion Principle. You can further read this list of SOLID design principles for Java programmers to answer this Java interview question. 18. What is the difference between Abstraction and Encapsulation in Java? Even though both Abstraction and Encapsulation look similar because both hide complexity and make the external interface simpler, there is a subtle difference between them. Abstraction hides logical complexity while Encapsulation hides Physical Complexity. Btw, I have already covered the answer to this Java interview question in my previous post as the difference between Encapsulation and abstraction in Java, here are some more differences from that post: This was my list of 10 popular design pattern interview questions in Java. I have not included MVC (Model View Controller) design pattern because that is more specific to J2EE and Servlet JSP interview. Still, if you are going for any Java interview, which demands experience in J2EE, then you must prepare the MVC design pattern. That's all on Java design pattern interview questions and answers. Please let us know if you have any other interesting questions on the Java design pattern. 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If you like these design pattern interview questions, then please share it with your friends and colleagues. If you have any questions or feedback, then please drop a note. P. S. - If you are looking for some free courses to learn Design Pattern and Software Architecture, I also suggest you check out the free Java Design Patterns and Architecture course by John Purcell on Udemy. It's completely free, all you need to do is create an Udemy account to access this course.