Object Streams in Java

Java provides Object Streams to perform object-level I/O , meaning you can read/write entire Java objects to files, memory, or network streams. This is the foundation of Serialization & Deserialization.

What are Object Streams?

Object Streams are specialized streams that allow Java objects to be written as bytes and restored back as full objects.

Java provides two main classes:

• ObjectOutputStream → converts an object into a byte stream and writes it to a file.
• ObjectInputStream → reads the byte stream and converts it back into the original object.

These classes are part of the java.io package.

Requirements for Object Streams

To serialize an object:

• The class must implement Serializable
• All non-transient fields must be serializable
• serialVersionUID is recommended
• Nested objects should also be serializable

ObjectOutputStream (Writing Objects to File)

ObjectOutputStream is used to serialize Java objects and write them into a binary file.

• Writes objects using writeObject().
• Works only with classes that implement Serializable.
• Stores complete object state (fields).

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ObjectInputStream (Reading Objects from File)

ObjectInputStream is used to deserialize objects from a file and reconstruct them in memory.

• Reads objects using readObject().
• Must read in the same order they were written.
• Class definition must be available at runtime.

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Complete Example: Serialization + Deserialization

Below is the full demo with both ObjectOutputStream & ObjectInputStream.

Step 1: Create a Serializable Class

import java.io.Serializable;

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

    public Student(int id, String name) {
        this.id = id;
        this.name = name;
    }

    public String toString() {
        return "Student{id=" + id + ", name='" + name + "'}";
    }
}

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Step 2: Write Object Using ObjectOutputStream (Serialization)

import java.io.FileOutputStream;
import java.io.ObjectOutputStream;

public class WriteObject {
    public static void main(String[] args) {
        try (ObjectOutputStream oos =
                new ObjectOutputStream(new FileOutputStream("student.ser"))) {

            Student s1 = new Student(101, "Amit");
            oos.writeObject(s1);

            System.out.println("Object written successfully!");

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

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Step 3: Read Object Using ObjectInputStream (Deserialization)

import java.io.FileInputStream;
import java.io.ObjectInputStream;

public class ReadObject {
    public static void main(String[] args) {
        try (ObjectInputStream ois =
                new ObjectInputStream(new FileInputStream("student.ser"))) {

            Student s = (Student) ois.readObject();
            System.out.println("Read Object: " + s);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

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How Serialization Works Internally

• JVM converts object → byte stream.
• Writes object metadata, class name, field values.
• Deserialization reconstructs the object.
• Requires matching serialVersionUID for version control.

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Important Notes

Class must implement Serializable

class Student implements Serializable { }

Static & transient fields are not serialized

Order of readObject() must match writeObject()

serialVersionUID prevents compatibility issues

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Multiple Objects with Object Streams

Yes, you can write multiple objects:

oos.writeObject(obj1);
oos.writeObject(obj2);

And read:

obj1 = (MyClass) ois.readObject();
obj2 = (MyClass) ois.readObject();

Points to Remember

• ObjectOutputStream → writeObject() → saves objects.
• ObjectInputStream → readObject() → loads objects.
• Used for Serialization / Deserialization.
• Only works with Serializable classes.
• Stores full object state in a .ser file.