When you want to write your own implementaion of serialization implement Externalizable interface which sub interface of Serialization. Whe you implement Externalizable interface you need implement readExternal() & writeExternal() methods of Externalizable interface.
When you implement Serializable, JVM will take care of the serialization but Externalizable the coder need to take care of logid of serialization by implementing Externalizable interface methods.