An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object. 

What is the difference between the Reader/Writer class hierarchy and the Input Stream/Output Stream class hierarchy?

The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented. 

By implementing either the java.io.Serializable interface, or the java.io.Externalizable interface. As long as one class in a class's inheritance hierarchy implements Serializable or Externalizable, 

There is no method in the Serializable interface. The Serializable interface acts as a marker, telling the object serialization tools that your class is serializable. 

There are two methods in the Externalizable interface. You have to implement these two methods in order to make your class externalizable. These two methods are readExternal() and writeExternal(). 

When you use Serializable interface, your class is serialized automatically by default. But you can override writeObject() and readObject()two methods to control more complex object serailization process. 

Latest Answer : Java gives you control over the serialization process through the java.io.Externalizable interface. The java.io.Externalizable interface extends the java.io.SerialiJava gives you control over the serialization process through the java.io.Externalizable ...