Object Serialization Interfaces in Java

Java provides two interfaces for object serialization:

1. Serializable
2. Externalizable

Both help convert objects into a byte stream, but they work very differently.

What is Serializable Interface

Serializable is a marker interface (no methods) that enables default serialization.

Example (Automatic):

public class Student implements Serializable {
    int id;
    String name;
}

Purpose

When a class implements Serializable, the JVM automatically:

• Saves all non-transient, non-static fields
• Handles the object graph (nested objects)
• Restores it during deserialization

Features

• No methods to implement
• JVM handles serialization internally
• Supports default serialization
• Works with ObjectOutputStream / ObjectInputStream
• Supports transient keyword
• Uses serialVersionUID for versioning

When to Use Serializable?

• When you want quick object saving
• Perfect for simple Java objects (DTOs, Entities)
• When performance/custom control is not required

What is Externalizable Interface

Externalizable is a subinterface of Serializable but gives full manual control over serialization.

Purpose

Used when you want to customize how objects are saved and restored.

It has two methods:

• writeExternal()
• readExternal()

public void writeExternal(ObjectOutput out) throws IOException;
public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException;

Example (Custom):

class Employee implements Externalizable {
    int id;
    String name;

    public void writeExternal(ObjectOutput out) throws IOException {
        out.writeInt(id);
        out.writeUTF(name);
    }

    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
        id = in.readInt();
        name = in.readUTF();
    }
}

Features

• Manual serialization
• Programmer controls what to save and restore
• Faster in some cases because only required data is stored
• Requires a public no-arg constructor

When to Use Externalizable?

• When you need complete and efficient control
• For large objects where performance matters
• When you want to skip some fields without using transient
• For custom binary formats / compact serialization

Serializable vs Externalizable