Easy but bewildered game of Singleton Pattern and Class

This concept will include all aspects of singleton classes which you never have come across.

  Singleton classes: Singleton pattern is often implemented either using lazy loading or by using double-checked locking, which was not safe and broker in Java 1.4, In short before Java 5 writing Singleton was very difficult and one of the tricky interview question asked on Java interviews, but with introduction of Java memory model in Java 5 and change in volatile variable in Java its possible to write thread-safe singleton using double checked locking. Java 5 also introduced Enum in Java, which is the best choice to implement thread-safe Singleton in Java. Creating simple singleton class:This is one of my favorite method to impelemnt Singleton pattern in Java, Since Singleton instance is static and final variable it initialized when class is first loaded into memeory so creation of instance is inherently thread-safe.

public class A{
   public static A a;

 private A(){
  system.out.println(“Constructor is private ”)
}

public static A getInstance(){
  if( a == null )
  a = new A();

 return a;
}
}

Singleton classes using ENUM- Though Singleton pattern in Java exists from long time Enum Singletons are relatively new concept and in practice from Java 5 onwards after introduction of Enum as keyword and feature. Benifits:
 1) Singleton instance is thread-safe because JVM guarantees that Enum instances are created in thread-safe manner.

/**
* Singleton pattern example using Java Enumj
*
public enum EasySingleton{

 INSTANCE;

}

 2) JVM also guarantee to maintains Singleton status when Singleton class implements Serializable which is still possible without Enum by using readResolve() method but tedious and complicated. Another problem with conventional Singletons are that once you implement serializable interface they are no longer remain Singleton because readObject() method always return a new instance just like constructor in Java. you can avoid that by using readResolve() method and discarding newly created instance by replacing with Singeton as shwon in below example

public class A{
 
 //readResolve to prevent another instance of Singleton
    private Object readResolve(){

        return INSTANCE;
    }

This can become even more complex if your Singleton Class maintain state, as you need to make them transient, but witn Enum Singleton, Serialization is guarnateed by JVM

Singleton Using double-Check locking:Below code is an example of double checked locking in Singleton pattern, here getInstance() method checks two times to see whether INSTANCE is null or not and that’s why it’s called double checked locking pattern, remember that double checked locking is broker before Java 5 but with the guranteed of volatile variable in Java 5 memory model, it should work perfectly.

 
/**
* Singleton pattern example with Double checked Locking
*/
public class DoubleCheckedLockingSingleton{
     private volatile DoubleCheckedLockingSingleton INSTANCE;
 
     private DoubleCheckedLockingSingleton(){}
 
     public DoubleCheckedLockingSingleton getInstance(){
         if(INSTANCE == null){
            synchronized(DoubleCheckedLockingSingleton.class){
                //double checking Singleton instance
                if(INSTANCE == null){
                    INSTANCE = new DoubleCheckedLockingSingleton();
                }
            }
         }
         return INSTANCE;
     }
}

Interview questions on Singleton pattern: 
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: Java.lang.Runtime with getRuntime() method Java.awt.Toolkit with getDefaultToolkit() Java.awt.Desktop with getDesktop()
 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. /*code is shown above article on double-check locking*/ 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 Singleto 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 guaranted 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 to that article Apart from these questions on Singleton pattern, some of my reader contributer few more questions, which I included here. Thank you guys for your contribution.
 11) Why you should avoid the singleton anti-pattern at all and replace it with DI?
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 atleast 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-Saffe 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 classloader. At last few more questions for your practice, contributed by some other topics please contribute for finding out the answers and edit here:
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?
 19. Since Singleton class has private constructor, can the class be extended? my answer: Since the constructor is private, it can not be extended Follow up question
20. Since it can not be extended is it a final class?
  my answer: yes, it is a final class. not sure about this answer needs discussion so be careful. Strictly speaking, non-final classes with private constructors can be extended using inner classes inside them (provided you're allowed to make changes to the source code):

 
 public class Singleton {
 private Singleton() {
     System.out.println("Singleton ctor");
 }

 public static synchronized Singleton getInstance() {
     if (_instance == null) {
         _instance = new Singleton();
     }
     return _instance;
 }

 public static class Extend extends Singleton {
     public Extend() {
         System.out.println("Extend ctor");
     }
     public void method() {}
 }
 private static Singleton _instance;

}
class Invoker {
 public static void main(String[] args) {
     System.out.println("main");
     Singleton.Extend e = new Singleton.Extend();
 }
}