18 Java design pattern questions.

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
fore 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 example 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 lot's 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 can not 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 it's 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 work 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 way. 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.

This was my list of
10 popular design pattern interview question in Jav
a. I have not included MVC (Model View Controller) design pattern because that is more specific to J2EE and
Servlet JSP interview
, but if you are going for any Java interview which demands experience in J2EE than you must prepare MVC design pattern. That's all on Java design pattern interview question and answers. Please let us know if you have any other interesting question on Java design pattern.