Are Custom Enums Serializable in Java? Best Practices and Requirements

Is a custom enum with non-serializable members Serializable in Java?

public enum Country {
    Australia(R.drawable.flag_au),
    Austria(R.drawable.flag_at),
    UnitedState(R.drawable.flag_us);

    Country(int icon) {
        this.icon = icon;
        nonSerializableClass = new NonSerializableClass(this.toString());
    }

    public int getIcon() {
        return icon;
    }

    public static class NonSerializableClass {
        public NonSerializableClass(String dummy) { this.dummy = dummy; }
        public String dummy;
    }

    private final int icon;

    public NonSerializableClass nonSerializableClass;
}

In Java, all enums are inherently serializable since they implement the Serializable interface. However, when you add non-serializable members to your custom enums, you need to handle serialization carefully to maintain data integrity during the serialization and deserialization processes.

private void writeObject(java.io.ObjectOutputStream out) throws IOException {
    out.defaultWriteObject(); // Serialize default fields
    out.writeInt(icon); // Serialize icon
}

private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException {
    in.defaultReadObject(); // Deserialize default fields
    icon = in.readInt(); // Deserialize icon
    nonSerializableClass = new NonSerializableClass(this.toString()); // Recreate non-serializable member
}

Causes

• Enums in Java implicitly implement Serializable, allowing standard usage without additional code for simple enums.
• Adding non-serializable member variables means the serializable state of your enum needs to be managed manually to avoid exceptions during serialization.

Solutions

• Implement custom serialization methods like writeObject and readObject in your enum class to control how enum data is serialized and deserialized.
• Mark the non-serializable member variables as transient if they do not need to be serialized and re-constructed post-deserialization.

Mistake: Not handling non-serializable members when customizing serialization.

Solution: Implement custom readObject and writeObject methods to ensure that all member variables are correctly serialized.

Mistake: Assuming default serialization will work with non-serializable member variables.

Solution: Always check and implement custom serialization when non-serializable types are present.