10 Interview questions on Singleton Pattern in Java

10 Interview questions on Singleton Pattern in Java
Singleton pattern in Java is one of the most common patterns available and it’s also used heavily in Java. This is also one of my favorite interview question and has lots of interesting follow-up to dig into details , this not only check the knowledge of design pattern but also check coding , multithreading aspect which is very important while working for a real life application.



In this short java interview tutorial I have listed some of the most common question asked on Singleton pattern during a Java Interview. I have not provided the answers of these questions as they are easily available via google search but if you guys need I can try to  modify this tutorial to include answers as well.

So question starts with what is Singleton? Have you used Singleton before?
Singleton is a class which has only one instance thought out the application and provides a getInstance() method to access the singleton instance.

1) Which classes are candidates of Singleton? Which kind of class do you make Singleton in Java?
Here they will check whether candidate has enough experience on usage of singleton or not. Does he is familiar of advantage/disadvantage or alternatives available for singleton in Java or not.


2) Can you write code for getInstance() method of a Singleton class in Java?
Most of the guys fail here if they have mugged up the singleton code because you can ask lots of follow-up question based upon the code written by candidate. In my experience I have seen mostly java interviewee right Singleton getInstance() method with double checking but they are not really familiar with the caveat associated with double checking of singleton prior to Java 5.

3) Is it better to make whole getInstance() method synchronized or just critical section is enough? Which one you will prefer?
This is really nice question and I mostly asked to just quickly check whether candidate is aware of performance trade off of unnecessary locking or not. Since locking only make sense when we need to create instance and rest of the time its just read only access so locking of critical section is always better option. read more about synchronization on How Synchronization works in Java


4) What is lazy and early loading of Singleton and how will you implement it?
This is another great Singleton interview question in terms of understanding of concept of loading and cost associated with class loading in Java. Many of which I have interviewed not really familiar with this but its good to know concept.


5) Example of Singleton in standard JAVA Development Kit?
This is open question to all, please share which classes are Singleton in JDK. Answer to this question is java.lang.Runtime


6) What is double checked locking in Singleton?
One of the most hyped question on Singleton and really demands complete understanding to get it right because of Java Memory model caveat prior to Java 5. If a guy comes up with a solution of using volatile instance of Singleton then it really shows it has in depth knowledge of Java memory model and he is constantly updating his Java knowledge.


7) How do you prevent for creating another instance of Singleton using clone() method?
This type of questions generally comes some time by asking how to break singleton or when Singleton is not Singleton in Java.


8) How do you prevent for creating another instance of Singleton using reflection?
Open to all. In my opinion throwing exception from constructor is an option.

9) How do you prevent for creating another instance of Singleton during serialization?
Another great question which requires knowledge of Serialization in Java and how to use it for persisting Singleton classes. This is open to you all but in my opinion use of readResolve() method can sort this out for you.


10) When is Singleton not a Singleton in Java?
There is a very good article present in Sun's Java site which discusses various scenarios when a Singleton is not really remains Singleton and multiple instance of Singleton is possible. Here is the link of that article http://java.sun.com/developer/technicalArticles/Programming/singletons/



Read more: http://javarevisited.blogspot.com/2011/03/10-interview-questions-on-singleton.html#ixzz1sqOgcCYO