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Core Java - Interview Questions and Answers for 'Serialization' - 21 question(s) found - Order By Newest

Frequently asked in Cognizant (CTS)
  Q1. What is Serialization ? Why do we need it ?Core Java
Ans. Storing the state of an object in a file or other medium is called serialization.

Classes can communicate only if they are built together ( as they need Byte code for communication ). What if we need to enable communication between different applications ( i.e they have been built independently or even they reside at different locations ), We need a mechanism that will transfer the Bean state to a Medium than can be transferred to the receiving application.

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 Q2. Can we declare static variables as transient ?Core Java
Ans. It's weird that compiler doesn't complain if we declare transient with static variable because it makes no sense. At least a warning message saying "transient is useless in this situation" would have helped with code cleaning.

Static variables are never serialized and transient is an indication that the specified variable shouldn't be serialized so its kind of double enforcement not to serialize.

It could be that as it makes no different to the variable behavior and hence using both keywords with a variable are permitted.

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  Q3. What is the use of Transient Keyword ?Core Java
Ans. It in Java is used to indicate that a field should not be serialized.

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 Q4. Which elements of a class are ignored during serialization ?Core Java
Ans. 1. Objects are serialized and not classes and hence Static variables are ignored.

2. Transient is an explicit declaration to ignore the variable during serialization and hence transient instance variables are ignored too.

3. Base class instance variables if the base class hasn't been declared serializable.

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 Q5. Can we serialize static variables ?Core Java
Ans. No. Only Object and its members are serialized. Static variables are shared variables and doesn't correspond to a specific object.

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 Q6. What one should take care of, while serializing the object?
Ans. One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializable Exception.

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 Q7. serialization and deserialization of singleton classCore Java
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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 Q8. Difference between Serialization and Deserialization ?Core Java
Ans. Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.

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 Q9. What is an Externalizable interface ?
Ans. Externalizable interface is used to write the state of an object into a byte stream in compressed format.

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 Q10. Difference between serializable and externalizable interface ?
Ans. Serializable is a marker interface whereas externalizable is not.

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 Q11. What is Externalizable interface?Core Java
Ans. Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism.

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 Q12. How to avoid cloning, serialization in the singleton class ?Design
Ans. For Cloning-exception,For deserialization-read.resolve()

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 Q13. What are transient variables in java?Core Java
Ans. Transient variables are variable that cannot be serialized.

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 Q14. What is the problem with this code ?

class BuggyBread1 {

private BuggyBread2 buggybread2;

public static void main(String[] args){
try {
BuggyBread1 buggybread1 = new BuggyBread1();
ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream(new File("newFile.txt")));
objectOutputStream.writeObject(buggybread1);
} catch (Exception e) {
e.printStackTrace();
}
}
}		Core Java
Ans. Though we are trying to serialize BuggyBread1 object but we haven't declared the class to implement Serializable.

This will throw java.io.NotSerializableException upon execution.

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 Q15. Will this code run fine if BuggyBread2 doesn't implement Serializable interface ?

class BuggyBread1 implements Serializable{
private BuggyBread2 buggybread2 = new BuggyBread2();

public static void main(String[] args){
try {
BuggyBread1 buggybread1 = new BuggyBread1();
ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream(new File("newFile.txt")));
objectOutputStream.writeObject(buggybread1);
} catch (Exception e) {
e.printStackTrace();
}
}
}		Core Java
Ans. No, It will throw java.io.NotSerializableException.

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 Q16. Will this code work fine if BuggyBread2 doesn't implement Serializable ?

class BuggyBread1 extends BuggyBread2 implements Serializable{
private int x = 5;

public static void main(String[] args){
try {
BuggyBread1 buggybread1 = new BuggyBread1();
ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream(new File("newFile.txt")));
objectOutputStream.writeObject(buggybread1);
} catch (Exception e) {
e.printStackTrace();
}
}
}		Core Java
Ans. Yes.

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 Q17. Which of the following is true ?

a. We can serialize static variables
b. We can serialize transient variables
c. We can serialize final variables
d. We can serialize instance methods
Ans. We can serialize final variables

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 Q18. What is serialVersionUID ?Core Java
Ans. Everytime an object is serialized the java serialization mechanism automatically computes a hash value by passing the meta information for the class. This id is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization

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 Q19. Write code for serialization.Core Java
Ans. // Java code for serialization and deserialization
// of a Java object
import java.io.*;

class Demo implements java.io.Serializable
{
   public int a;
   public String b;

   // Default constructor
   public Demo(int a, String b)
   {
      this.a = a;
      this.b = b;
   }

}

class Test
{
   public static void main(String[] args)
   {
      Demo object = new Demo(1, "geeksforgeeks");
      String filename = "file.ser";
      
      // Serialization
      try
      {
         //Saving of object in a file
         FileOutputStream file = new FileOutputStream(filename);
         ObjectOutputStream out = new ObjectOutputStream(file);
         
         // Method for serialization of object
         out.writeObject(object);
         
         out.close();
         file.close();
         
         System.out.println("Object has been serialized");

      }
      
      catch(IOException ex)
      {
         System.out.println("IOException is caught");
      }


      Demo object1 = null;

      // Deserialization
      try
      {
         // Reading the object from a file
         FileInputStream file = new FileInputStream(filename);
         ObjectInputStream in = new ObjectInputStream(file);
         
         // Method for deserialization of object
         object1 = (Demo)in.readObject();
         
         in.close();
         file.close();
         
         System.out.println("Object has been deserialized ");
         System.out.println("a = " object1.a);
         System.out.println("b = " object1.b);
      }
      
      catch(IOException ex)
      {
         System.out.println("IOException is caught");
      }
      
      catch(ClassNotFoundException ex)
      {
         System.out.println("ClassNotFoundException is caught");
      }

   }
}

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 Q20. Can we serialize objects with only private variables in Java ?Core Java
Ans. It depends on how we are serializing. The Serialization API doesn't worry about private variables and convert it into binary representation.

If we are using a library to map it to JSON / XML using XML Mappers, it may create trouble.

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 Q21. Can you give me a use case where you utilized serialization in your project code?Core Java
Ans. we use serialization when we send response to client in JSON format.(the process of converting model object into json is nothing but serialization

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 Q22. Which of following are serialized ?Core Java
a. static variables
b. transient variables
c. instance variables
d. method local variables

Ans.c. instance variables

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