how to read data of composite class object from file using readObject()?

how to read data of composite class object from file using readObject()?

how to read data of composite object from file using readObject()? My code
is below and i got output as
version=4
purpose is DEMO
value=4
Exception in thread "main" java.lang.NullPointerException
at Serializations.readObj(PS_Task3.java:39)
at PS_Task3.main(PS_Task3.java:46)
Press any key to continue . . .
//PS_Task3.java //this is the code i have tried to execute. i got correct
output when i remove variable of type demo in SerializationDemo . but
NullPointerException when i tried for composite class. please help
me..thanks in advance
import java.io.*;
class SerializationDemo implements Serializable //composite class
{
public int value = 4;
public demo d1;
public String purpose="DEMO";
public int getValue(){
return value;
}
}
class demo{
private int demoval;
demo(){
demoval=10;
}
public int getValue(){
return demoval;
}
}
class Serializations {
void writeObj()throws IOException{ //writes into file
FileOutputStream fos = new FileOutputStream("temp1.bin");
ObjectOutputStream oos = new ObjectOutputStream(fos);
SerializationDemo sd = new SerializationDemo();
oos.writeObject(sd);
oos.flush();
oos.close();
}
void readObj()throws IOException,ClassNotFoundException{ //reads from
file
FileInputStream fis = new FileInputStream("temp1.bin");
ObjectInputStream oin = new ObjectInputStream(fis);
SerializationDemo sd = (SerializationDemo) oin.readObject();
System.out.println("version="+sd.value);
System.out.println("purpose is "+sd.purpose);
System.out.println("value="+sd.getValue());
System.out.println("value="+sd.d1.getValue());
}
}
public class PS_Task3{
public static void main(String args[]) throws
IOException,ClassNotFoundException {
Serializations s = new Serializations();
s.writeObj();
s.readObj(); //here i got the error
}
}