Question

Say you have these two classes, Foo and Bar where Bar extends Foo and implements Serializable

class Foo {

public String name;

public Foo() {
    this.name = "Default";
}

public Foo(String name) {
    this.name = name;
}
}

class Bar extends Foo implements java.io.Serializable {

public int id;

public Bar(String name, int id) {
    super(name);
    this.id = id;
}
}

Notice that Foo doesn’t implement Serializable. So what happens when bar is serialized?

    public static void main(String[] args) throws Exception {

    FileOutputStream fStream=new FileOutputStream("objects.dat");
    ObjectOutputStream oStream=new ObjectOutputStream(fStream);
    Bar bar=new Bar("myName",21);
    oStream.writeObject(bar);

    FileInputStream ifstream = new FileInputStream("objects.dat");
    ObjectInputStream istream = new ObjectInputStream(ifstream);
    Bar bar1 = (Bar) istream.readObject();
    System.out.println(bar1.name + "   " + bar1.id);

} 

it prints “Default 21”.
The question is, why the default constructor get called when the class is not serialized?

Answer 1

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.