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Input/Output: Exploring java.io > Serialization >
Siva Nookala - 06 Oct 2016
Serialization is the process of storing object data in the form of bytes. The data converted can be stored in a file or transmitted over network. This is very useful and important to save users data and retrieve at a later time. Serialization can be achieved using two interfaces - Serializable and Externalizable.

Here we will discuss Serializable and for more details about Externalizable see Externalizable In Java with Example. Serializable is a marker interface and so it does not have any methods. A class can be serialized if we simply implement this interface. For e.g., the Student class below can be serialized since it implements Serializable interface.
class Student implements Serializable
{
    String name;
    int rollNumber;
    char section;
}
Notice that there are no additional methods to be implemented.

We need to understand the following important points about Serializable.
  • Any class which extends from Student will automatically be Serializable. For e.g., the Engineer class below is Serializable since Student implements Serializable interface.
class Engineer extends Student
{
    String branch;
}
  • If a class contains objects of other classes, then those classes should also implement Serializable interface, otherwise the containing class can not be serialized. For e.g., Student class contains Address and if Address does not implement Serializable interface, then it throws a NotSerializableException if we try to serialize an object of Student class.
/*
// WON'T WORK SINCE NOT IMPLEMENTING Serializable INTERFACE
class Address
{
    String doorNumber;
    String line1;
}
*/
class Address implements Serializable
{
    String doorNumber;
    String line1;
}
class Student implements Serializable
{
    String name;
    int rollNumber;
    char section;
    Address address; // ANY CLASS INCLUDED SHOULD ALSO IMPLEMENT Serializable
}
  • Any variables which are declared as static or transient will not be serialized. Which means the value of those variables will be lost if we serialize and de-serialize. For e.g., below the values of allStudentsCount and percentage will be lost and they will set to their default values 0 and 0.0 after the serialization/de-serialization.
class Student implements Serializable
{
    static int allStudentsCount; // WON'T BE SERIALIZED
    String name;
    int rollNumber;
    char section;
    transient double percentage; // WON'T BE SERIALIZED
}

Student.allStudentsCount = 25;
Student ravi = new Student();
ravi.name = "Ravi";
ravi.rollNumber = 35;
ravi.section = 'B';
ravi.percentage = 79.34;

// AFTER SERIALIZATION AND DESERIALIZATION THE VALUES OF allStudentsCount AND percentage WILL BE LOST
How exactly the serialization and deserialization is performed is explained in Java Serialization Process -. Note that we can also implement serialization using Externalizable In Java with Example interface.

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