package com.entity;
import java.util.*;

public class Department {
   private String deptName;
   private int deptId;
   private Set<String> toolsUsed = new HashSet<String>();
   private Set<Employee> employees = new HashSet<Employee>();
   
   public Set<Employee> getEmployees() {
      return employees;
   }
   public void setEmployees(Set<Employee> employees) {
      this.employees = employees;
   }
   public String getDeptName() {
      return deptName;
   }
   public int getDeptId() {
      return deptId;
   }
   public Set<String> getToolsUsed() {
      return toolsUsed;
   }
   public void setDeptName(String deptName) {
      this.deptName = deptName;
   }
   public void setDeptId(int deptId) {
      this.deptId = deptId;
   }
   public void setToolsUsed(Set<String> toolsUsed) {
      this.toolsUsed = toolsUsed;
   }
   
   
}

<?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="com.entity.Department" table="Department">
      <id name="deptId" column="DEPT_ID">
         <generator class="sequence">
            <param name="dept_sequence_206819"/>
         </generator>
      </id>
      <property name="deptName" column="DEPT_NAME" length="32" />
      <set name="toolsUsed" table="TOOLS_USED_DEPARTMENT">
         <key column="DEPT_ID" />
         <element type="string" column="TOOL_NAME" not-null="true" />
      </set>
      <set name="employees" cascade="all">
         <key column="DEPT_ID" />
         <one-to-many class="com.entity.Employee"/>
      </set>
   </class>
</hibernate-mapping>

package com.entity;
import java.util.HashSet;
import java.util.Set;

public class Employee {

   private int empId;
   private int deptId;
   private String empName;
   private Set<String> computerLanguage = new HashSet<String>();

   public int getEmpId() {
      return empId;
   }

   public int getDeptId() {
      return deptId;
   }

   public String getEmpName() {
      return empName;
   }

   public Set<String> getComputerLanguage() {
      return computerLanguage;
   }

   public void setComputerLanguage(Set<String> computerLanguage) {
      this.computerLanguage = computerLanguage;
   }

   public void setEmpId(int empId) {
      this.empId = empId;
   }

   public void setDeptId(int deptId) {
      this.deptId = deptId;
   }

   public void setEmpName(String empName) {
      this.empName = empName;
   }



}

<?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="com.entity.Employee" table="EMPLOYEE">
      <id name="empId" column="EMP_ID">
         <generator class="sequence">
            <param name="emp_sequence_206819"/>
         </generator>
      </id>
      <property name="empName" column="EMP_NAME" length="32" />
      <set name="computerLanguage" table="EMP_LANG">
         <key column="EMP_ID" />
         <element type="string" column="COMP_LANG" not-null="true" />
      </set>
      <many-to-one name="deptId" class="com.entity.Department" cascade="all" column="DEPT_ID" not-null="true"/>
   </class>
</hibernate-mapping>

package com.entity;
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 MainClass {

   private static Session session = null;

   private static Session getSession() {
      SessionFactory sf = new Configuration().configure()
            .buildSessionFactory();
      session = sf.openSession();
      return session;
   }
   
   public static void main(String[] args) {
      
      Session session = getSession();
      Transaction trans = session.beginTransaction();
      
      Department dept = new Department();
      dept.setDeptName("NSS");
      Set<String> tools = new HashSet<String>();
      tools.add("Putty");
      tools.add("VMWare");
      tools.add("NetStat");
      dept.setToolsUsed(tools);

      Set<Employee> employees = new HashSet<Employee>();
      
      Employee emp1 = new Employee();
      emp1.setEmpName("Tirtha");
      emp1.setDeptId(1);
      Set<String> emp1Lang = new HashSet<String>();
      emp1Lang.add("Core Java");
      emp1Lang.add("Hibernate");
      emp1Lang.add("Spring");
      emp1.setComputerLanguage(emp1Lang);
      
      Employee emp2 = new Employee();
      emp2.setEmpName("Sourav");
      Set<String> emp2Lang = new HashSet<String>();
      emp2.setDeptId(1);
      emp2Lang.add("Core Java");
      emp2Lang.add("EJB 3");
      emp2Lang.add("Spring");
      emp2.setComputerLanguage(emp2Lang);
      
      employees.add(emp1);
      employees.add(emp2);
      
      dept.setEmployees(employees);
      session.save(dept);
      trans.commit();
      
      System.out.println("Done");

   }

}

Exception in thread "main" org.hibernate.PropertyAccessException: IllegalArgumentException occurred calling getter of com.entity.Department.deptId
   at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:171)
   at org.hibernate.tuple.entity.AbstractEntityTuplizer.getIdentifier(AbstractEntityTuplizer.java:183)
   at org.hibernate.persister.entity.AbstractEntityPersister.getIdentifier(AbstractEntityPersister.java:3591)
   at org.hibernate.persister.entity.AbstractEntityPersister.isTransient(AbstractEntityPersister.java:3307)
   at org.hibernate.engine.ForeignKeys.isTransient(ForeignKeys.java:181)
   at org.hibernate.event.def.AbstractSaveEventListener.getEntityState(AbstractSaveEventListener.java:512)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:80)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
   at org.hibernate.impl.SessionImpl.fireSaveOrUpdate(SessionImpl.java:507)
   at org.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:499)
   at org.hibernate.engine.CascadingAction$5.cascade(CascadingAction.java:218)
   at org.hibernate.engine.Cascade.cascadeToOne(Cascade.java:268)
   at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:216)
   at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:169)
   at org.hibernate.engine.Cascade.cascade(Cascade.java:130)
   at org.hibernate.event.def.AbstractSaveEventListener.cascadeBeforeSave(AbstractSaveEventListener.java:431)
   at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:265)
   at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:181)
   at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:121)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:187)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:172)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:94)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
   at org.hibernate.impl.SessionImpl.fireSaveOrUpdate(SessionImpl.java:507)
   at org.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:499)
   at org.hibernate.engine.CascadingAction$5.cascade(CascadingAction.java:218)
   at org.hibernate.engine.Cascade.cascadeToOne(Cascade.java:268)
   at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:216)
   at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:169)
   at org.hibernate.engine.Cascade.cascadeCollectionElements(Cascade.java:296)
   at org.hibernate.engine.Cascade.cascadeCollection(Cascade.java:242)
   at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:219)
   at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:169)
   at org.hibernate.engine.Cascade.cascade(Cascade.java:130)
   at org.hibernate.event.def.AbstractSaveEventListener.cascadeAfterSave(AbstractSaveEventListener.java:456)
   at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:334)
   at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:181)
   at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:121)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:187)
   at org.hibernate.event.def.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:33)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:172)
   at org.hibernate.event.def.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:27)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
   at org.hibernate.impl.SessionImpl.fireSave(SessionImpl.java:535)
   at org.hibernate.impl.SessionImpl.save(SessionImpl.java:523)
   at org.hibernate.impl.SessionImpl.save(SessionImpl.java:519)
   at com.entity.MainClass.main(MainClass.java:60)
Caused by: java.lang.IllegalArgumentException: object is not an instance of declaring class
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
   at java.lang.reflect.Method.invoke(Unknown Source)
   at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:145)
   ... 46 more

<many-to-one name="department" class="com.entity.Department" ...

<set name="employees" inverse="true" ...

private Department department;
public Department getDepartment()
{
  return department;
}
public void setDepartment(Department department)
{
  this.department = department;
}