Why I don't have there NotSerializableException?

Why I don't have there NotSerializableException cause in class A that is serialized I have private B b that is not serialized and I know that if class implement Serializable all composite classes have to implement Serializable/Externalizable as well.

import java.io.*;

public class Test
{
    public static void main(String[] args) throws IOException
    {
        FileOutputStream fileOutput = new FileOutputStream("a.dat");
        ObjectOutputStream outputStream = new ObjectOutputStream(fileOutput);
        outputStream.writeObject(new A());
        fileOutput.close();
        outputStream.close();
    }
}

class A implements Serializable
{
    private int age;
    private B b;
}

class B
{

}

When you serialise new A(), b is null so no B.

It is the runtime type that is important - for instance you could have class BDerived extends B implements java.ui.Serializable. Or an ArrayList could be serialised from List field reference.

You can see the exception if you initialise b with a non-null, non-serialisable type. For instance

    private B b = new B();