Serialization and Deserialization in Java

Master object serialization, Serializable interface, ObjectInputStream, ObjectOutputStream, deserialization, and serialization best practices for the OCP 21 exam.

Table of Contents

• 1. Serialization Overview
• 2. Serializable Interface
• 3. ObjectOutputStream
• 4. ObjectInputStream
• 5. Serialization Control
• 6. Exam Key Points

1. Serialization Overview

Serialization is the process of converting an object into a byte stream. Deserialization is the reverse process of reconstructing an object from a byte stream.

1.1 Use Cases

• Persistence - Save object state to file
• Network communication - Send objects over network
• Caching - Store objects in cache
• Deep copying - Clone objects

2. Serializable Interface

A class must implement Serializable interface to be serializable. It's a marker interface (no methods).

2.1 Implementing Serializable

Example:

import java.io.Serializable;

class Person implements Serializable {
    private static final long serialVersionUID = 1L;
    private String name;
    private int age;
    private transient String password;  // Not serialized
    
    // Constructor, getters, setters
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

// serialVersionUID: Version control for serialization
// transient: Field not included in serialization

3. ObjectOutputStream

ObjectOutputStream is used to serialize objects to a byte stream.

3.1 Serializing Objects

Example:

import java.io.*;

Person person = new Person("Alice", 30);

try (ObjectOutputStream oos = new ObjectOutputStream(
        new FileOutputStream("person.ser"))) {
    oos.writeObject(person);
    oos.flush();
}

// Serializing multiple objects
try (ObjectOutputStream oos = new ObjectOutputStream(
        new FileOutputStream("persons.ser"))) {
    oos.writeObject(new Person("Alice", 30));
    oos.writeObject(new Person("Bob", 25));
}

4. ObjectInputStream

ObjectInputStream is used to deserialize objects from a byte stream.

4.1 Deserializing Objects

Example:

import java.io.*;

try (ObjectInputStream ois = new ObjectInputStream(
        new FileInputStream("person.ser"))) {
    Person person = (Person) ois.readObject();
    System.out.println(person.getName());
}

// Deserializing multiple objects
try (ObjectInputStream ois = new ObjectInputStream(
        new FileInputStream("persons.ser"))) {
    Person p1 = (Person) ois.readObject();
    Person p2 = (Person) ois.readObject();
}

5. Serialization Control

5.1 Custom Serialization

Example:

import java.io.*;

class Person implements Serializable {
    private String name;
    private int age;
    
    // Custom serialization
    private void writeObject(ObjectOutputStream oos) throws IOException {
        oos.defaultWriteObject();
        oos.writeObject(name.toUpperCase());  // Custom logic
    }
    
    // Custom deserialization
    private void readObject(ObjectInputStream ois) 
            throws IOException, ClassNotFoundException {
        ois.defaultReadObject();
        name = ((String) ois.readObject()).toLowerCase();  // Custom logic
    }
}

6. Exam Key Points

Critical Concepts for OCP 21 Exam:

• Serializable: Marker interface for serializable classes
• serialVersionUID: Version control for serialization
• transient: Field not included in serialization
• ObjectOutputStream: Serialize objects to byte stream
• ObjectInputStream: Deserialize objects from byte stream
• writeObject(): Custom serialization method
• readObject(): Custom deserialization method
• NotSerializableException: Thrown if class not serializable
• Static fields: Not serialized (belong to class, not instance)
• Inheritance: Parent must be serializable if child is serializable