Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
        at org.hibernate.internal.util.collections.IdentityMap.entryArray(IdentityMap.java:164)
        at org.hibernate.internal.util.collections.IdentityMap.concurrentEntries(IdentityMap.java:75)
        at org.hibernate.engine.internal.StatefulPersistenceContext.clear(StatefulPersistenceContext.java:237)
        at org.hibernate.internal.SessionImpl.cleanup(SessionImpl.java:598)
        at org.hibernate.internal.SessionImpl.close(SessionImpl.java:379)
        at com.testcompany.testPROG.main(testPROG.java:91)

public class testPRG {
   private static SessionFactory factory; 
   public static void main(String[] args) {
      BigDecimal myAmount=new BigDecimal("0"); 
      int i = 0;
      try{
         factory = new AnnotationConfiguration().
                   configure().
                   //addPackage("com.xyz") //add package if used.
                   addAnnotatedClass(Customers.class).
                   buildSessionFactory();
      }catch (Throwable ex) { 
         System.err.println("Failed to create sessionFactory object." + ex);
         throw new ExceptionInInitializerError(ex); 
      }
      // Customers cust = new Customers();
      Date dNow = new Date( );
      SimpleDateFormat ft = 
      new SimpleDateFormat ("E yyyy.MM.dd 'at' hh:mm:ss a zzz");
      System.out.println("Current Date: " + ft.format(dNow));

      System.out.println("Started!");
      
      Session session = factory.openSession();
      Transaction tx = null;
      try{
         tx = session.beginTransaction();
         List cust_array = session.createQuery("FROM Customers").list(); 
         for (Iterator iterator = 
                           cust_array.iterator(); iterator.hasNext();){
            Customers cust = (Customers) iterator.next(); 
            i++ ;
            dNow = new Date( );
               
            if (i % 1000 == 0 ) {
              System.out.println("Row "+i+" "+ ft.format(dNow));
            }        
            
            Set<CaseCustomers> cacuList = cust.getCaseCustomerses() ;
            
            for (Iterator iterator2 = 
                              cacuList.iterator(); iterator2.hasNext();){
               CaseCustomers cacu = (CaseCustomers) iterator2.next(); 
               Set<Bookings> bookList = cacu.getBookingses() ;
                for (Iterator iterator3 = 
                                  bookList.iterator(); iterator3.hasNext();){
                   Bookings book = (Bookings) iterator3.next(); 
                   myAmount = myAmount.add(book.getAmount());
                }
            
            }      
         }
         // tx.commit();
         System.out.println("Total book Amount: " + myAmount.setScale(2).toPlainString());
         
         Date dNow2 = new Date( );
         System.out.println("Job finished: " + ft.format(dNow2));
      }catch (HibernateException e) {
         if (tx!=null) tx.rollback();
         e.printStackTrace(); 
      }finally {
         session.close(); 
      }
   }