If we don't implement serializable interface, what will it happen?
Easy question to answer: you can't serialize the object. Most people producing code for use in the "real world" would implement the Serializable interface on spec, because you can get a NonSerializableException if you try to serialize an instance whose fields are not all serializable.
There is another mechanism for (long-term) object persistence: use of JavaBeans. I hardly know anything about them