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Design - Interview Questions and Answers for 'Singleton' - 27 question(s) found - Order By Newest

Q1. Difference between Static and Singleton Class ?

Ans. 1. Static class is a class which cannot be instantiated and all its members are static whereas Singleton is the class that only permit creation of single object and then the object is reused.

2. As there is no object in Static class, it cannot participate in runtime Polymorphism.

3. As Static class doesnt allow creating objects and hence it cannot be serialized.

4. Static class body is initialized eagerly at application load time whereas Singleton object can be initiated eagerly using static blocks or lazily on first need.

5. Its not recommended to use pure static class as it fails to use many OOPs concepts.

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Q2. What is the difference between Singleton and Prototype Bean scope in Spring ?

Ans. Prototype scope - A new object is created each time it is injected/looked up. It will use new SomeClass() each time.

Singleton scope - It is the default scope. The same object is returned each time it is injected/looked up. Here it will instantiate one instance of SomeClass and then return it each time.

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Q3. serialization and deserialization of singleton class

Ans. The problem with serialized singleton class is that whenever we deserialize it, it will create a new instance of the class. To overcome this scenario all we need to do is to provide the implementation of readResolve() method.

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Q4. What is the difference between these two approaches of creating singleton Class ?

//Double Checked Locking Code
public static Singleton createInstance() {
   if(singleton == null){
      synchronized(Singleton.class) {
         if(singleton == null) {
            singleton = new Singleton();
         }
      }
   }
   return singleton;
}

//Single checked locking code
public static Singleton createInstance() {
   synchronized(Singleton.class) {
      if(singleton == null) {
         singleton = new Singleton();
      }
   }
   return singleton;
}

Ans. In First Case , Lock for the synchronized block will be received only if singleton == null whereas in second case every thread will acquire the lock before executing the code.
The problem of synchronization with singleton will only happen when the object has not be instantiated. Once instantiated , the check singleton == null will always generate true and the same object will be returned and hence no problem. First condition will make sure that synchronized access ( acquiring locks ) will only take place if the object has not been created so far.

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Q5. What are design situations to use Singleton and Prototype Design Pattern ?

Ans. Both are creational design patterns.

Singleton is used when we would like to reuse an object if object is not supposed to hold request or thread specific information. Inversely Prototype is used in situations where we would like to reuse the object information but the request / thread may require it own data to be persisted.

In short, Singleton is used in situations where we can live with single object being shared across multiple requests or threads whereas Prototype is used when we need duplicate copies of objects.

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Q6. How to avoid cloning, serialization in the singleton class ?

Ans. For Cloning-exception,For deserialization-read.resolve()

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Q7. When singleton can break and how to resolve it

Ans. Deserialization. In serialization, we can save the object of a byte stream into a file or send over a network. Suppose if you serialize the Singleton class, and then again de-serialize that object, it will create a new instance, hence deserialization will break the Singleton pattern.

To overcome this issue, we need to override readResolve() method in the Singleton class and return the same Singleton instance.

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Q8. If you are given choice to either load object eagerly or lazily , which one would you use in case of singleton pattern ?

Ans. I would choose Eager as the cost for 1 additional object is too minute for any such consideration.

Eager Loading results in Faster access ( Object available at load time) at the cost of additional space.

Lazy loading results in space saving ( Object available at first use ) at the cost of access speed.

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Q9. When should we use prototype scope and singleton scope for beans ?

Ans. We should use singleton scope for beans when they are stateless and prototype when they are stateful.

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Very frequently asked. Usually followed by questions related to private constructor and synchronized access. Frequently asked in JPMorgan and TCS (Based on 2 feedback)

Q10. Explain Singleton Design Pattern ?

Ans. http://www.buggybread.com/2014/03/java-design-pattern-singleton-interview.html

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Q11. Shouldn't we make a class with all static members is its just expected to be executed as a standalone program with just one thread. Moreover Lets assume that there is no runtime Polymorphism required and there is no need for serialization ?

Ans. Still No in case we are making use of inheritance. we may have problem wherein we have program flow moving across common inherited method and specific methods of the derived class. call made to another static method in the parent class will only access the static class of the Parent class irrespective of the call from any of the derived class.

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Q12. Should we have instance variables in the Spring Bean which has been scoped as Singleton ?

Ans. No, if required we should only have final variables.Bean scoped singleton means that only one instance of the bean will be created and will be shared among different requests and hence instance variables will get shared too.

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Q13. What are stateless objects ? How are they different from immutable objects ? Which of these two is thread safe ?

Object Oriented Programming

Ans. Stateless objects are the objects without instance fields (instance variables). The class may have compile time constants i.e static final fields.Immutable objects are the objects which have state but the state cannot be changed after initialization. Both are Thread safe.

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Q14. Which of the following is not the difference between Singleton and Static class ( Class with static members only ) ?

a. Only one object can be created for Singleton class whereas No objects are created for static class.
b. Singleton class instance is initiated using new keyword whereas static class instance is created using static method.
c. Singleton class can be serialized whereas Static class cannot be.
d. Singleton Class can participate in runtime Polymorphism whereas Static class cannot.

Ans. Singleton class instance is initiated using new keyword whereas static class instance is created using static method.

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Q15. Which of the following do you think is the primary reason you would never use a static class even the application doesn't need multiple requests or threads ?

a. Serialization
b. Runtime Polymorphism
c. Lazy Loading
d. Memory

Ans. Runtime Polymorphism

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Q16. Write code for singleton class

Ans. http://javasearch.buggybread.com/CodeSnippets/searchCodeSamples.php?keyword=singleton+class&category=code

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Q17. How to make sure that only one instance is created in Singleton Pattern ?

Ans. public class SingleTon {
private SingleTon() {
if (singleTon != null) {
throw new RuntimeException("cant not create the object");
}
}
public Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException("can not be create");
}

static private volatile SingleTon singleTon;
public static SingleTon getInstance() {
SingleTon singleTon = this.sample;
if (singleTon == null) {
synchronized (this) {
singleTon = this.singleTon;
if (singleTon == null) {
singleTon = this.singleton = new SingleTon();
}
}
}
return singleTon;
}
}

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Q18. Difference between Singleton and Factory Design Pattern ?

Ans. Both are creational design patterns but singleton facilitates in creation and reuse of single object whereas Factory deals with creation of multiple objects.

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Q19. In singleton Pattern framework, Web application deployed on multiple cluster server will have how many objects?

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Q20. How one dao Implementation object (singleton) can handle multiple requests?

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Q21. Can you write critical section code for singleton design pattern ?

^{This question is still unanswered. Can you please provide an answer.}

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Q22. Should we have instance variables in the Spring Bean which has been scoped as Singleton ?

Ans. No, if required we should only have final variables. Bean scoped singleton means that only one instance of the bean will be created and will be shared among different requests and hence instance variables will get shared too.

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Q23. What are the ways to break singleton pattern ?

Ans. If we don't have double checked locking, it can be broken easily through multi threaded access.

Through Reflection.

If multiple class loaders are loading the class.

If the class is serializable or cloneable.

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Q24. Why the singleton class should never implement Cloneable interface ?

Ans. Because Singleton implementation doesn't restrict it from cloning and hence we can have multiple objects when we actually don't intend it to have multiple objects.

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Q25. How serialization breaks Singleton pattern ?

Ans. It's not serialization but de serialization that breaks purpose of singleton as new objects can be brought to life using the serialized object.

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Q26. How can we make sue that only 1 object gets created ?

Ans. We can use Singleton Design Pattern.

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Q27. Draw uml for singleton pattern.

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