Serializable is just a “marker interface” for a given class.
But that class must adhere to certain rules:
http://docs.oracle.com/javase/1.5.0/docs/api/java/io/Serializable.html
To allow subtypes of non-serializable classes to be serialized, the
subtype may assume responsibility for saving and restoring the state
of the supertype’s public, protected, and (if accessible) package
fields. The subtype may assume this responsibility only if the class
it extends has an accessible no-arg constructor to initialize the
class’s state. It is an error to declare a class Serializable if this
is not the case.
to answer @Sleiman Jneidi question asked in comment,
in oracle documentation mentioned above, its clearly mentioned
During deserialization, the fields of non-serializable classes will be initialized using the public or protected no-arg constructor of the class. A no-arg constructor must be accessible to the subclass that is serializable. The fields of serializable subclasses will be restored from the stream.
Thus, default no-arg constructor of class Foo called of, resulted in initialization.