public class User implements Serializable{

    private int id;
    private String username;
    private String password;
   
    protected User(){
       
    }
   
    public User(String username, String password){
        this.setUsername(username);
        this.setPassword(password);
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }
   
}

public class Teacher implements Serializable{

    //private int id;
    private String degree;
    private String firstName;
    private String secondName;
    private String familyName;
    private String email;

    protected Teacher(){
       
    }
   
    public Teacher(String degree, String firstName,
            String secondName, String familyName, String email){
        this.setDegree(degree);
        this.setFirstName(firstName);
        this.setSecondName(secondName);
        this.setFamilyName(familyName);
        this.setEmail(email);
    }
   
    //public int getId() {
    //    return id;
    //}

    //public void setId(int id) {
    //    this.id = id;
    //}

    public String getDegree() {
        return degree;
    }

    public void setDegree(String degree) {
        this.degree = degree;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getSecondName() {
        return secondName;
    }

    public void setSecondName(String secondName) {
        this.secondName = secondName;
    }

    public String getFamilyName() {
        return familyName;
    }

    public void setFamilyName(String familyName) {
        this.familyName = familyName;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

}

<hibernate-mapping>
    <class name="pl.umk.mat.redKite.security.data.User" table="users">
        <id name="id" column="userId">
            <generator class="increment"/>
        </id>       
        <property name="username" type="string" length="20"
                  column="username" not-null="true"/>
        <property name="password" type="string" length="20"
                  column="password" not-null="true"/>
        <set name="userRoles" table="joinUsersAndUsersRoles">
            <key column="userId"/>
            <many-to-many column="userRoleId"
                          class="pl.umk.mat.redKite.security.data.UserRole"/>
        </set>
    </class>
</hibernate-mapping>

<hibernate-mapping>
    <union-subclass
        name="pl.umk.mat.redKite.main.teacher.data.Teacher"
        extends="pl.umk.mat.redKite.security.data.User"
        table="teachers">
        <property name="degree" type="string" length="10"
                  column="degree"/>
        <property name="firstName" type="string" length="20"
                  column="firstName" not-null="true"/>
        <property name="secondName" type="string" length="20"
                  column="secondName"/>
        <property name="familyName" type="string" length="50"
                  column="familyName" not-null="true"/>
        <property name="email" type="string" length="50"
                  column="email"/>
        <set name="subjects" inverse="true">
            <key column="teacherId"/>
            <one-to-many class="pl.umk.mat.redKite.main.teacher.data.TeachersForSubjects"/>
        </set>
        <set name="classes" inverse="true">
            <key column="wychId"/>
            <one-to-many class="pl.umk.mat.redKite.main.classes.data.Classes"/>
        </set>
    </union-subclass>
</hibernate-mapping>

2008-01-11 00:46:10,312 ERROR [org.hibernate.property.BasicPropertyAccessor] - <IllegalArgumentException in class: pl.umk.mat.redKite.security.data.User, getter method of property: id>
Exception in thread "main" org.hibernate.PropertyAccessException: IllegalArgumentException occurred calling getter of pl.umk.mat.redKite.security.data.User.id
        at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:171)
        at org.hibernate.engine.UnsavedValueFactory.getUnsavedIdentifierValue(UnsavedValueFactory.java:44)
        at org.hibernate.tuple.PropertyFactory.buildIdentifierProperty(PropertyFactory.java:44)
        at org.hibernate.tuple.entity.EntityMetamodel.<init>(EntityMetamodel.java:123)
        at org.hibernate.persister.entity.AbstractEntityPersister.<init>(AbstractEntityPersister.java:434)
        at org.hibernate.persister.entity.UnionSubclassEntityPersister.<init>(UnionSubclassEntityPersister.java:64)
        at org.hibernate.persister.PersisterFactory.createClassPersister(PersisterFactory.java:61)
        at org.hibernate.impl.SessionFactoryImpl.<init>(SessionFactoryImpl.java:226)
        at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1294)
        at hibernateTemp.TworzenieBazy.main(TworzenieBazy.java:39)
Caused by: java.lang.IllegalArgumentException: object is not an instance of declaring class
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:145)
        ... 9 more