code snippet:
Address addr = (Address)session.load(Address.class, new Long(1));

addr.setCity("Bangalore");
addr.setState("Karnataka");
addr.setStreet("bowee");
addr.setZipcode("560008");
session.update(addr);
transaction.commit();
============================================
there is one address record with id 1

hbm file for address pojo class is

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="javabeat.net.hibernate.Address" table="ADDRESS">
<meta attribute="class-description">This class contains the student's address
details.</meta>
<id name="addressId" type="long" column="ADDRESS_ID">
<generator class="increment" />
</id>
<property name="street" column="ADDRESS_STREET" type="string" length="250" />
<property name="city" column="ADDRESS_CITY" type="string" length="50" />
<property name="state" column="ADDRESS_STATE" type="string" length="50" />
<property name="zipcode" column="ADDRESS_ZIPCODE" type="string" length="10" />
</class>
</hibernate-mapping>

please help why am i getting this error.

please send the code, what type of error are you getting.

see whether the record in the database is commited. This could be one of the possible reasons.

code:
address.hbm.xml:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="javabeat.net.hibernate.Address" table="ADDRESS">
<meta attribute="class-description">This class contains the student's address
details.</meta>
<id name="addressId" type="long" column="ADDRESS_ID">
<generator class="increment" />
</id>
<property name="street" column="ADDRESS_STREET" type="string" length="250" />
<property name="city" column="ADDRESS_CITY" type="string" length="50" />
<property name="state" column="ADDRESS_STATE" type="string" length="50" />
<property name="zipcode" column="ADDRESS_ZIPCODE" type="string" length="10" />
</class>
</hibernate-mapping>

=======================================================================================

hibernate.configuration file:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<property name="hibernate.connection.driver_class">org.apache.derby.jdbc.ClientDriver</property>
<property name="hibernate.connection.url">jdbc:derby://localhost:1527/hkbk</property>
<property name="hibernate.connection.username">root</property>
<property name="connection.password">root</property>
<property name="connection.pool_size">1</property>
<property name="hibernate.dialect">org.hibernate.dialect.DerbyDialect</property>
<property name="show_sql">true</property>
<property name="hbm2ddl.auto">create-drop</property>

<!-- mapping resource-->
<mapping resource="javabeat/net/hibernate/Student.hbm.xml"/>
<mapping resource="javabeat/net/hibernate/Address.hbm.xml"/>
<mapping resource="javabeat/net/hibernate/Course.hbm.xml"/>
</session-factory>
</hibernate-configuration>
==========================================================================================

code to enter records in data base:
/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/

package javabeat.net.hibernate;

/**
*
* @author juahmed
*/
import java.util.HashSet;
import java.util.Set;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;



public class JavaBeatHibernateExample {

public static void main(String[] args) {
Session session = null;
SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
session =sessionFactory.openSession();
Transaction transaction = null;
try {
transaction = session.beginTransaction();
Address address = new Address("OMR Road", "Bangalore", "KARNATAKA", "560008");
//By using cascade=all option the address need not be saved explicitly when the student object is persisted the address will be automatically saved.
//session.save(address);
Set<Course> courses = new HashSet<Course>();
courses.add(new Course("Maths"));
courses.add(new Course("Computer Science"));
Student student1 = new Student("Eswar", address, courses);
Student student2 = new Student("Joe", address, courses);
session.save(student1);
session.save(student2);

transaction.commit();
} catch (Exception e) {
transaction.rollback();
e.printStackTrace();
} finally {
session.close();
}

}

}
========================================================
code to update the database record:

/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/

package javabeat.net.hibernate;

/**
*
* @author juahmed
*/
import java.util.HashSet;
import java.util.Set;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;



public class JavaBeatHibernateExampleUpdate {

public static void main(String[] args) {
Session session = null;
SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
session =sessionFactory.openSession();
Transaction transaction = null;
try {
transaction = session.beginTransaction();
Address addr = (Address)session.load(Address.class, new Long(1));

addr.setCity("Bangalore");
addr.setState("Karnataka");
addr.setStreet("bowee");
addr.setZipcode("560008");
session.update(addr);
transaction.commit();
} catch (Exception e) {
transaction.rollback();
e.printStackTrace();
} finally {
session.close();
}

}

}
==================
address record is present in database with id=1.


please help me.

I think to update you dont need to load it first. create a pojo, assign the id directly to it, assign all other values and call update on it. it should work.

This code is looking fine. I ran it by taking your code, it's running fine in Oracle.
Check your database whether it's support "increment" generate key or not.