<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
  <class name="EPOS.Database.Invoices" table="Invoices">
    <id column="ID" name="ID" type="long">
      <generator class="increment"/>
    </id>
    <property name="Date" column="InvDate"/>
    <property name="Ref"/>
    <property name="Supplier"/>
    <idbag name="Rows" table="Invoice_rows">
        <collection-id type="long" column="Invoice_row_ID's">
            <generator class="sequence"/>
        </collection-id>
        <key column="ID"/>
        <composite-element class="Row">
            <property name="ItemID" />
            <property name="Name" />
            <property name="Catergory" />
            <property name="BoxQuantity" />
            <property name="BoxNumber" />
            <property name="BoxPrice" />
        </composite-element>
    </idbag>

  </class>
</hibernate-mapping>

package EPOS.Database;

import EPOS.Amount;
import java.util.ArrayList;
import java.util.Collection;

public class Invoices {

    private long ID;
    private String Date = "";
    private String Ref = "";
    private String Supplier = "";

    private Collection Rows = new ArrayList();
   
    public Invoices() {}

    public void setID       (Long   ID)       {this.ID       = ID;}
    public void setDate     (String Date)     {this.Date     = Date;}
    public void setRef      (String Ref)      {this.Ref      = Ref;}
    public void setSupplier (String Supplier) {this.Supplier = Supplier;}
    public void setRows     (Collection Rows) {this.Rows     = Rows;}
   
    public long        getID       () {return ID;}
    public String      getDate     () {return Date;}
    public String      getRef      () {return Ref;}
    public String      getSupplier () {return Supplier;}
    public Collection  getRows     () {return Rows;}
}

Exception in thread "AWT-EventQueue-0" org.hibernate.MappingException: class Rows not found while looking for property: ItemID

package EPOS.Database;

import EPOS.Amount;
import java.util.ArrayList;
import java.util.Collection;

public class Invoices {

    private long ID;
    private String Date = "";
    private String Ref = "";
    private String Supplier = "";

    private Collection Rows = new ArrayList();
   
    public Invoices() {}

    public void setID       (Long   ID)       {this.ID       = ID;}
    public void setDate     (String Date)     {this.Date     = Date;}
    public void setRef      (String Ref)      {this.Ref      = Ref;}
    public void setSupplier (String Supplier) {this.Supplier = Supplier;}
    public void setRows     (Collection Rows) {this.Rows     = Rows;}
   
    public long        getID       () {return ID;}
    public String      getDate     () {return Date;}
    public String      getRef      () {return Ref;}
    public String      getSupplier () {return Supplier;}
    public Collection  getRows     () {return Rows;}

    public class Row{
        private long ItemID;
        private String Name;
        private String Catergory;
        private String BoxQuantity;
        private String BoxNumber;
        private String BoxPrice;

        public Row() {}

        public void setItemID      (Long   ItemID)     {this.ItemID      = ItemID;}
        public void setName        (String Name)       {this.Name        = Name;}
        public void setCatergory   (String Catergory)  {this.Catergory   = Catergory;}
        public void setBoxQuantity (String BoxQuantity){this.BoxQuantity = BoxQuantity;}
        public void setBoxNumber   (String BoxNumber)  {this.BoxNumber   = BoxNumber;}
        public void setBoxPrice    (String BoxPrice)   {this.BoxPrice    = BoxPrice;}

        public long   getItemID      () {return ItemID;}
        public String getName        () {return Name;}
        public String getCatergory   () {return Catergory ;}
        public String getBoxQuantity () {return BoxQuantity;}
        public String getBoxNumber   () {return BoxNumber;}
        public String getBoxPrice    () {return BoxPrice;}
    }
}