Design Pattern

1. When to use Strategy Design Pattern in Java?

Strategy pattern in quite useful for implementing set of related algorithms e.g. compression algorithms, filtering strategies etc. Strategy design pattern allows you to create Context classes, which uses Strategy implementation classes for applying business rules. This pattern follow open closed design principle and quite useful in Java. One example of Strategy pattern from JDK itself is a Collections.sort() method and Comparator interface, which is a strategy interface and defines strategy for comparing objects. Because of this pattern, we don't need to modify sort() method (closed for modification) to compare any object, at same time we can implement Comparator interface to define new comparing strategy (open for extension).

2. What is Observer design pattern in Java? When do you use Observer pattern in Java?

This is one of the most common Java design pattern interview question. Observer pattern is based upon notification, there are two kinds of object Subject and Observer. Whenever there is change on subject's state observer will receive notification. See What is Observer design pattern in Java with real life example for more details.

3. Difference between Strategy and State design Pattern in Java?

This is an interesting Java design pattern interview questions as both Strategy and State pattern has same structure. If you look at UML class diagram for both pattern they look exactly same, but there intent is totally different. State design pattern is used to define and mange state of object, while Strategy pattern is used to define a set of interchangeable algorithm and let's client to choose one of them. So Strategy pattern is a client driven pattern while Object can manage there state itself.

4. What is decorator pattern in Java? Can you give an example of Decorator pattern?

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 good example of decorator pattern in Java. See How to use Decorator pattern in Java for more details.

5. When to use Composite design Pattern in Java? Have you used previously in your project?

This design pattern question is asked on Java interview not just to check familiarity with Composite pattern but also, whether candidate has real life experience or not. Composite pattern is also a core Java design pattern, which allows you to treat both whole and part object to treat in similar way. Client code, which deals with Composite or individual object doesn't differentiate on them, it is possible because Composite class also implement same interface as there individual part. One of the good examples of Composite pattern from JDK is JPanel class, which is both Component and Container.  When paint() method is called on JPanel, it internally called paint() method of individual components and let them draw themselves. On second part of this design pattern interview question, be truthful, if you have used then say yes, otherwise say that you are familiar with concept and used it by your own. By the way always remember, giving an example from your project creates better impression.

6. What is Singleton pattern in Java?

Singleton pattern in Java is a pattern which allows only one instance of Singleton class available in whole application. java.lang.Runtime is good example of Singleton pattern in Java. There are lots of follow up questions on Singleton pattern see 10 Java singleton interview question answers for those followups

7. Can you write thread-safe Singleton in Java?

There are multiple ways to write thread-safe singleton in Java e.g by writing singleton using double checked locking, by using static Singleton instance initialized during class loading. By the way using Java enum to create thread-safe singleton is most simple way. See Why Enum singleton is better in Java for more details.

8. When to use Template method design Pattern in Java? Template pattern is another popular core Java design pattern interview question. I have seen it appear many times in real life project itself. Template pattern outlines an algorithm in form of template method and let subclass implement individual steps. Key point to mention, while answering this question is that template method should be final, so that subclass cannot override and change steps of algorithm, but same time individual step should be abstract, so that child classes can implement them.

9. What is Factory pattern in Java? What is advantage of using static factory method to create object?

Factory pattern in Java is a creation Java design pattern and favorite on many Java interviews.Factory pattern used to create object by providing static factory methods. There are many advantage of providing factory methods e.g. caching immutable objects, easy to introduce new objects etc. See what is Factory pattern in Java and benefits for more details.

10. Difference between Decorator and Proxy pattern in Java? Another tricky Java design pattern question and trick here is that both Decorator and Proxy implements interface of the object they decorate or encapsulate. As I said, many Java design pattern can have similar or exactly same structure but they differ in there intent. Decorator pattern is used to implement functionality on already created object, while Proxy pattern is used for controlling access to object. One more difference between Decorator and Proxy design pattern is that, Decorator doesn't create object, instead it get object in its constructor, while Proxy actually creates objects.

11. When to use Setter and Constructor Injection in Dependency Injection pattern?

Use Setter injection to provide optional dependencies of an object, while use Constructor injection to provide mandatory dependency of an object, without which it can not work. This question is related to Dependency Injection design pattern and mostly asked in context of Spring framework, which is now become an standard for developing Java application. Since Spring provides IOC container, it also gives you way to specify dependencies either by using setter methods or constructors. You can also take a look my previous post on same topic.

12. What is difference between Factory and Abstract factory in Java

see  here to answer this Java design pattern interview question.

13. When to use Adapter pattern in Java? Have you used it before in your project?

Use Adapter pattern when you need to make two class works with incompatible interfaces. Adapter pattern can also be used to encapsulate third party code, so that your application only depends upon Adapter, which can adapt itself when third party code changes or you moved to a different third party library. By the way this Java design pattern question can also be asked by providing actual scenario.

14. Can you write code to implement producer consumer design pattern in Java?

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 its better to use Concurrency util to implement producer consumer pattern instead of plain old wait and notify in Java.  Here is a good example of implementing producer consumer problem using BlockingQueue in Java.

15. What is Open closed design principle in Java?

Open closed design principle is one of the SOLID principle defined by Robert C. Martin, popularly known as Uncle Bob. This principle advices that a code should be open for extension but close for modification. At first this may look conflicting but once you explore power of polymorphism, you will start finding patterns which can provide stability and flexibility of this principle. One of the key example of this is State and Strategy design pattern, where Context class is closed for modification and new functionality is provided by writing new code by implementing new state of strategy. See this article to know more about Open closed principle.

16. What is Builder design pattern in Java? When do you use Builder pattern ?

Builder pattern in Java is another creational design pattern in Java and 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 

17. Can you give an example of  SOLID design principles in Java?

There are lots of SOLID design pattern which forms acronym SOLID, read this list of SOLID design principles for Java programmer  to answer this Java interview question.

18. What is difference between Abstraction and Encapsulation in Java?

I have already covered answer of this Java interview question in my previous post as Difference between encapsulation and abstraction in Java. See there to answer this question.