Serializing an object which has a non-serializable parent class

How does the below code work?

     class A {
         int a = 10;
     }


     class B extends A implements Serializable{

      }



     public class Test {
       public static void main(String[] args){
        B obj = new B();
        obj.a = 25;


        //Code to serialize object B  (B b= new B()),
         // deserialize it and print the value of 'a'. 
      }
    }

The code prints 10 even though I have changed the value of 'a' in the code.

Any explanation for this behaviour ?