Externalizable

Externalizable is an interface in Java programming that provides a mechanism for custom serialization and deserialization of objects. It is part of the java.io package. When a class implements Externalizable, it takes full control over how its state is written to and read from a stream. This differs from the default serialization provided by the Serializable interface, which Java handles automatically based on the class's fields.

To implement Externalizable, a class must declare two methods: writeExternal and readExternal. The writeExternal method is responsible for writing the object's state to the provided ObjectOutput stream. The readExternal method is responsible for reading the object's state from the provided ObjectInput stream. The order in which data is written in writeExternal must exactly match the order in which it is read in readExternal.

Implementing Externalizable offers greater flexibility and control over the serialization process. This can be useful for optimizing the size of serialized data, handling complex object graphs, or ensuring compatibility with older or different serialization formats. However, it also requires more effort from the developer, as they must explicitly manage the serialization logic. If a class implements Externalizable, its superclasses' state is still serialized using the default mechanism unless explicitly handled within the writeExternal and readExternal methods. Therefore, it is common practice for Externalizable classes to call the superclass's writeExternal and readExternal methods if they exist and are also Externalizable, or to handle their superclass's state manually if the superclass is only Serializable.