Serialization in Java

If a class is serializable, it means that object of that class can be converted into a sequence of bits so that it can be written to some storage medium or even transmitted over a network without any change in object.

Notice that for a class to be serialized successfully, two conditions must be met:

• The class must implement the java.io.Serializable interface.

• All of the fields in the class must be serializable. If a field is not serializable, it must be marked transient. - Any transient variable that was part of object will not saved to the file.

public class Employee implements java.io.Serializable
{
   public String name;
   public String address;
   public transient int SSN;
   public int number;
   public void mailCheck()
   {
      System.out.println("Mailing a check to " + name
                           + " " + address);
   }
}

Key points to be remember:

•  Any static variable cannot be serialized.

• If base class is serialized, child class would also do.

• If base class is not serialized and child class is serialized then   NonSerializableException occurs.

Serializing an Object 

The ObjectOutputStream class is used to serialize an Object. The following demo program instantiates an Employee object and serializes it to a file.

When the program is done executing, a file named employee.ser is created. The program does not generate any output, but study the code and try to determine what the program is doing.

Note: When serializing an object to a file, the standard convention in Java is to give the file a.ser extension.

import java.io.*;

public class Demo
{
   public static void main(String [] args)
   {
      Employee e = new Employee();
      e.name = "Tarun Solanki";
      e.address = "Sector-135, Noida";
      e.SSN = 1333333;
      e.number = 12;
      try
      {
         FileOutputStream fileOut =
         new FileOutputStream("/tmp/employee.ser");
         ObjectOutputStream out = new ObjectOutputStream(fileOut);
         out.writeObject(e);
         out.close();
         fileOut.close();
         System.out.printf("Serialized data is saved in /tmp/employee.ser");
      }catch(IOException i)
      {
          i.printStackTrace();
      }
   }
}

JAVA: System.out.println Explaination

System : It is a final class in the java.lang package.

out: It is a static member of System class and is of type of PrintStream class.

println: It is an overloaded member of PrintStream class(java.io.Printstream package).

Structure of  System.out.println

                   class System{

public static final PrintStream out;

}

  

          class PrintStream{  

               void println(); 

            }

JAVA: public static void main(String[] args)

class HelloWorld{ 

public static void main(String args[]){

System.out.println("Hello World");

}

}

Public:  It is an Access Modifier, which defines who can access this method(In Java World). 

Public means that this method will be accessible to any class.Its scope is global throughout the 

application.

Static: It is a keyword which is accessible without creating the instance of class.

Void: It tells the return type of the method and void means that this method will not return 

anything.

main: This is the main method which is searched by JVM as starting point for an application.

String[] args:  These are the arguments to the main method. This method must be given an array of Strings, and the array will be called 'args'. These are called command line arguments. It is 

way to give input to the application at the runtime.

Difference between MyClass.class and this

this :  It refers to  the current instance of the class where you are working on.

MyClass.class:  It refers to the Class instance that describes the class.

for example: Like you have a Test class

public class test{

static String  mode="EDIT";

}

You want to pass class instance as argument in another class.

If you pass Test.class as argument like below code then variables (like mode) set in your class will not get in another class.

Map maps = new HashMap();

maps.put("ARG_ID",Test.class);

If you use this in place of Test.class like below code then you will get variables of Test class in another class.

Map maps = new HashMap();

maps.put("ARG_ID",this);