Question starts with

What is Singleton class? Have you used Singleton before? Singleton is a class which has only one instance in whole application and provides a getInstance() method to access the singleton instance. There are many classes in JDK which is implemented using Singleton pattern like java.lang.Runtime which provides getRuntime() method to get access of it and used to get free memory and total memory in Java.
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.

Answer: Any class which you want to be available to whole application and whole only one instance is viable is candidate of becoming Singleton. One example of this is Runtime class , since on whole java application only one runtime environment can be possible making Runtime Singleton is right decision. Another example is a utility classes like Popup in GUI application, if you want to show popup with message you can have one PopUp class on whole GUI application and anytime just get its instance, and call show() with message. 
2) Can you write code for getInstance() method of a Singleton class in Java? Most of the java programmer fail here if they have mugged up the singleton code because you can ask lots of follow-up question based upon the code they have written. I have seen many programmer write Singleton getInstance() method with double checked locking but they are not really familiar with the caveat associated with double checking of singleton prior to Java 5.

Answer: Until asked don’t write code using double checked locking as it is more complex and chances of errors are more but if you have deep knowledge of double checked locking, volatile variable and lazy loading than this is your chance to shine. I have shared code examples of writing singleton classes using enum, using static factory and with double checked locking in my recent post Why Enum Singletons are better in Java, please see there.

 

 

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 Answer: This is again related to double checked locking pattern, well synchronization is costly and when you apply this on whole method than call to getInstance() will be synchronized and contented. Since synchronization is only needed during initialization on singleton instance, to prevent creating another instance of Singleton,  It’s better to only synchronize critical section and not whole method. Singleton pattern is also closely related to factory design pattern where getInstance() serves as static factory method.
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.

Answer: As there are many ways to implement Singleton like using double checked locking or Singleton class with static final instance initialized during class loading. Former is called lazy loading because Singleton instance is created only when client calls getInstance() method while later is called early loading because Singleton instance is created when class is loaded into memory.
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 Answer: There are many classes in Java Development Kit which is written using singleton pattern, here are few of them:

6) What is double checked locking in Singleton? One of the most hyped question on Singleton pattern 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 keyword with Singleton instance and explains it then it really shows it has in depth knowledge of Java memory model and he is constantly updating his Java knowledge.

Answer: Double checked locking is a technique to prevent creating another instance of Singleton when call to getInstance() method is made in multi-threading environment. In Double checked locking pattern as shown in below example, singleton instance is checked two times before initialization.

 

 

Double checked locking should only be used when you have requirement for lazy initialization otherwise use Enum to implement singleton or simple static final variable.
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. Answer: Preferred way is not to implement Clonnable interface as why should one wants to create clone() of Singleton and if you do just throw Exception from clone() method as  “Can not create clone of Singleton class”.
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.  Answer: This is similar to previous interview question. Since constructor of Singleton class is supposed to be private it prevents creating instance of Singleton from outside but Reflection can access private fields and methods, which opens a threat of another instance. This can be avoided by throwing Exception from constructor as “Singleton already initialized”

 

 

 

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. Answer: You can prevent this by using readResolve() method, since during serialization readObject() is used to create instance and it return new instance every time but by using readResolve you can replace it with original Singleton instance.  I have shared code on how to do it in my post Enum as Singleton in Java. This is also one of the reason I have said that use Enum to create Singleton because serialization of enum is taken care by JVM and it provides guaranteed of that.
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/

Apart from these questions on Singleton pattern, some of my reader contribute few more questions, which I included here. Thank you guys for your contribution.

 

Answer: Singleton Dependency Injection: every class that needs access to a singleton gets the object through its constructors or with a DI-container.

Singleton Anti-Pattern: with more and more classes calling getInstance the code gets more and more tightly coupled, monolithic, not testable and hard to change and hard to reuse because of not configurable, hidden dependencies. Also, there would be no need for this clumsy double checked locking if you call getInstance less often (i.e. once).

 

12) How many ways you can write Singleton Class in Java? Answer:  I know at least four ways to implement Singleton pattern in Java 1) Singleton by synchronizing getInstance() method 2) Singleton with public static final field initialized during class loading. 3) Singleton generated by static nested class, also referred as Singleton holder pattern. 4) From Java 5 on-wards using Enums

13) How to write thread-safe Singleton in Java?

Answer: Thread safe Singleton usually refers to write thread safe code which creates one and only one instance of Singleton if called by multiple thread at same time. There are many ways to achieve this like by using double checked locking technique as shown above and by using Enum or Singleton initialized by class loader.

 

At last few more questions for your practice, contributed by Mansi:

14) Singleton vs Static Class? 15) When to choose Singleton over Static Class? 16) Can you replace Singleton with Static Class in Java? 17) Difference between Singleton and Static Class in java? 18) Advantage of Singleton over Static Class?