It did not look as problem

Simple standard serialization example, i am trying serializa to file and deserialize object f class Data
public class Serial {

class Data implements java.io.Serializable{
Data () {param = 1; str = "TEST TEST TEST";};
Data (int _param) {param =_param;};
public String str;
public int param;
}

public
Serial() throws ClassNotFoundException{
Object object = new Data();//new javax.swing.JButton("push me");
ObjectOutputStream out = null;
try {

Stack stack = new Stack();

stack.push(object);
stack.push(object);
stack.push(object);

out = new ObjectOutputStream(new FileOutputStream("F://htmls//filename.ser"));

out.writeObject(stack);
out.close();

ObjectInputStream inStream = new ObjectInputStream(new FileInputStream("F://htmls//filename.ser"));
Stack content = (Stack) inStream.readObject();
System.out.println("size: "+content.size());
inStream.close();
} catch (IOException ex) {
Logger.getLogger(Serial.class.getName()).log(Level.SEVERE, null, ex);
} finally {
try {
out.close();
} catch (IOException ex) {
Logger.getLogger(Serial.class.getName()).log(Level.SEVERE, null, ex);
}
}

}
}

I got exception below, on out.writeObject(stack); File filename.ser is not empty, looking like as serialized object( at least, I am able find test string "TEST TEST TEST"; there.
What is wrong? why occurs java.io.NotSerializableException, nothing prevents class Data to be serialized..

25 бер 2011 17:49:47 javaapplication10.Serial <init>
SEVERE: null
java.io.NotSerializableException: javaapplication10.Serial
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1156)

.........................
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:326)
at javaapplication10.Serial.<init>(Serial.java:47)
at javaapplication10.ThreadSerial.main(ThreadSerial.java:128)