Understanding Why `ObjectOutputStream.writeObject` Requires Serializable Types

What is the reason that `ObjectOutputStream.writeObject` does not accept non-Serializable objects?

import java.io.*;

class MyClass implements Serializable {
    private int value;
    public MyClass(int value) {
        this.value = value;
    }
}

public class SerializationExample {
    public static void main(String[] args) {
        try {
            ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("myFile.bin"));
            MyClass myObject = new MyClass(10);
            out.writeObject(myObject); // This works since MyClass is Serializable
            out.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

The `ObjectOutputStream.writeObject` method is a part of Java's serialization mechanism, which converts an object into a byte stream for storage or transmission. To utilize it, the object being serialized must implement the `Serializable` interface. This requirement ensures that Java can handle the serialization process correctly and reliably, avoiding issues with non-serializable types.

import java.io.*;

class NonSerializableClass {
    private int value;
}

class SerializableClass implements Serializable {
    private int value;
    private NonSerializableClass nonSerializableField;
}

Causes

• The `writeObject` method checks if the object implements `Serializable` interface to ensure it can be serialized.
• Non-serializable objects can contain references to resources that cannot be reliably serialized, such as threads or file handles, which would lead to data loss or corruption during serialization.

Solutions

• Ensure that the class of the object you're trying to serialize implements the `Serializable` interface.
• If a class has non-serializable fields, mark them as `transient` to skip serialization.

Mistake: Forgetting to implement `Serializable` in custom classes.

Solution: Always implement the `Serializable` interface in custom classes intended for serialization.

Mistake: Attempting to serialize an object that contains non-serializable fields without marking them as transient.

Solution: Use the `transient` keyword for fields that should not be serialized.