design pattern interview question are integral part of any good list of
core Java interview questions. Java is a popular Object oriented
programming language and have lots of design pattern and design
principles, contributed by many developers and open source framework. As
a Java programmer its expected from you to know OOPS concept like Abstraction, Encapsulation and polymorphism, What is design pattern in Java, Some popular Java design pattern and most importantly when to use those design pattern in Java application. purpose of asking design pattern
interview question in Java is to check whether Java programmer are
familiar to those essential design patterns or not. Design patterns in
Java interviews are as important as multi-threading, collection and programming questions. If you are senior or experienced Java programmer than expect more complex and tough design pattern in Java interview e.g. Chain of responsibility design pattern and solving real time software design questions.
Top Java design pattern questions and answers
Here is my list of top 10 design pattern interview question in Java. I have also provided answer of those Java design patternquestion as link. no matter which level of Java interview are you going
e.g. programmer, software engineer, senior software engineer in Java,
you can expect few question from Java 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 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 herewith 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 Java. 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.
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