How to Deserialize Object in Java: Simple Guide with Example

To deserialize an object in Java, use ObjectInputStream to read the object from a file or stream and cast it back to its original class. This process reverses serialization, restoring the object's state from bytes.

Syntax

Deserialization in Java uses ObjectInputStream to read bytes and convert them back into an object. The main steps are:

• Create an ObjectInputStream from an InputStream (like a FileInputStream).
• Call readObject() to get the object.
• Cast the returned Object to the original class.

java

ObjectInputStream ois = new ObjectInputStream(new FileInputStream("file.ser"));
MyClass obj = (MyClass) ois.readObject();
ois.close();

Example

This example shows how to deserialize a Person object from a file named person.ser. The Person class implements Serializable to allow serialization and deserialization.

java

import java.io.*;

class Person implements Serializable {
    private static final long serialVersionUID = 1L;
    String name;
    int age;

    Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return "Person{name='" + name + "', age=" + age + "}";
    }
}

public class DeserializeExample {
    public static void main(String[] args) {
        try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream("person.ser"))) {
            Person p = (Person) ois.readObject();
            System.out.println("Deserialized: " + p);
        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

Output

Deserialized: Person{name='Alice', age=30}

Common Pitfalls

• ClassNotFoundException: Happens if the class of the serialized object is not found in the classpath during deserialization.
• serialVersionUID mismatch: If the class has changed and the serialVersionUID does not match, deserialization fails.
• Not implementing Serializable: The class must implement Serializable or deserialization will throw NotSerializableException.
• File not found or corrupted data: The file must exist and contain valid serialized data.

Wrong way: Trying to deserialize without casting or from a wrong file.

Right way: Always cast the object and handle exceptions properly.

java

try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream("wrongfile.ser"))) {
    // Wrong: no cast
    Object obj = ois.readObject();
    // Right: cast to expected class
    Person p = (Person) obj;
} catch (Exception e) {
    e.printStackTrace();
}

Quick Reference

Remember these key points for deserialization:

• Use ObjectInputStream wrapped around an InputStream.
• Call readObject() and cast the result.
• Handle IOException and ClassNotFoundException.
• Ensure the class implements Serializable and has a matching serialVersionUID.

Key Takeaways

Use ObjectInputStream and readObject() to deserialize objects in Java.

Always cast the deserialized object to its original class.

Handle IOException and ClassNotFoundException during deserialization.

Ensure the class implements Serializable and has a consistent serialVersionUID.

Deserialization restores the object's state from bytes saved during serialization.