I have two pojo classes Student and Address. The respective tables for those classes are 'student' and 'address'. There is one-to-one mapping from address to student. I'm getting the following error during the execution.

The Address.java code:::::
package onetoone;;
public class Address {

private int addressId;
private String city;
private String state;
private Student s;

public Student getS() {
return s;
}
public void setS(Student s) {
this.s = s;
}
public int getAddressId() {
return addressId;
}
public void setAddressId(int addressId) {
this.addressId = addressId;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
public String getState() {
return state;
}
public void setState(String state) {
this.state = state;
}

}

The Student.java code is as follows:::::
package onetoone;

public class Student {

private int studentId;
private String studentName;

public int getStudentId() {
return studentId;
}
public void setStudentId(int studentId) {
this.studentId = studentId;
}
public String getStudentName() {
return studentName;
}
public void setStudentName(String studentName) {
this.studentName = studentName;
}

}

I inserted the values in the database but while retrieving the data using the following code I'm getting an error at the line a.getCity()

The Application class code is as follows:::::
package onetoone;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.util.*;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;

public class OneToOne{


public static void main(String args[])
{
SessionFactory factory=null;
Session session=null;
try
{
factory=new Configuration().configure().buildSessionFactory();
session=null;
session=factory.openSession();
Object o = session.get(Address.class, new Integer(100));
Address a = (Address)o;
System.out.println(a.getCity());
Student s=a.getS();
System.out.println(s.getStudentName());
}
catch (HibernateException he)
{
System.out.println(he.getMessage());
}
finally
{
//SessionFactory close
factory.close();
//Session close
session.close();
}
}
}

The error is as follows::::::::
Apr 17, 2013 2:28:26 PM org.hibernate.impl.SessionFactoryImpl close
Hibernate: select address0_.address_id as address1_0_1_, address0_.city as city0_1_, address0_.state as state0_1_, student1_.student_id as student1_1_0_, student1_.student_name as student2_1_0_ from address address0_ left outer join student student1_ on address0_.address_id=student1_.student_id where address0_.address_id=?
INFO: closing
Apr 17, 2013 2:28:26 PM org.hibernate.connection.DriverManagerConnectionProvider close
INFO: cleaning up connection pool: jdbc:mysql://localhost:3306/database1?zeroDateTimeBehavior=convertToNull
Exception in thread "main" java.lang.NullPointerException
at onetoone.OneToOne.main(OneToOne.java:24)
Java Result: 1
BUILD SUCCESSFUL (total time: 40 seconds)

tells us clearly that variable

o == null

which means that the there is no persistent instance on database with id set to 100.
I suggest you to log the jdbc-activities with a JDBCLogger like p6spy,
so you will see the exact sql which is executed against your database and so you will hopeful also understand why it returns 0 records.