Why is explicit object casting needed?

In order to assign a superclass object in a variable to a subclass,one needs to do explicit casting.

For example:Person person=null;

Man man = (Man)person;

An automatic casting takes place when we typecast a object in subclass as parent class object.

Define Externalizable.

Externalizable is coined as an Interface

It extends the Serializable Interface.

It also sends data into the Streams.

Externalizable sends data in a Compressed Format.

Externalizable is having two methods,for e.g. writeExternal(ObjectOuput out) & readExternal(ObjectInput in)