Looping the one to many: ConcurrentModificationException

Question

In slow steps my BM gets shape. I now run into the following issue:
I have a OneToMany / ManyToOne relationship between two classes: standversion contains many standassigns. The classes have the following mappings:
Standassign:
     @ManyToOne(fetch = FetchType.LAZY, targetEntity = nl.reinders.bm.Standversion.class ) @JoinColumn(name="standversionnr")
     private nl.reinders.bm.Standversion iStandversion;
StandVersion:
     @OneToMany(mappedBy = "iStandversion", fetch = FetchType.LAZY, targetEntity = nl.reinders.bm.Standassign.class)
     private java.util.Collection<nl.reinders.bm.Standassign> iStandassignsWhereIAmStandversion = new java.util.ArrayList<nl.reinders.bm.Standassign>();
This works perfectly, lazy loading and all.
So the next step is to define accessor methods:
- StandAssign has a get/setStandversion
- StandVersion has a add/remove/getStandassignsWhereIAmStandversion
Works perfectly.
However, if I execute a "setStandversion" on the Standassign, followed by a "getStandassignsWhereIAmStandversion" on the corresponding Standversion, the standassign is not present in de collection. This relation needs to be wired into the BM.
In order to implement this, the setStandversion does a:
     if (value == iStandversion) return; // optimalisation and prevent looping
     if (lValue != null) lValue.removeStandassignsWhereIAmStandversion( (nl.reinders.bm.Standassign)this );
     iStandversion = value;
     if (value != null) value.addStandassignsWhereIAmStandversion( (nl.reinders.bm.Standassign)this );
In effect: when a value is set, remove me from the old value's collection and add me to the new values collection.
And correspondingly, the addStandassignsWhereIAmStandversion does:
     if (value != null && !iStandassignsWhereIAmStandversion.contains(value))
          iStandassignsWhereIAmStandversion.add(value);
          value.setStandversion( (nl.reinders.bm.Standversion)this);
And the remove ofcourse the opposite. In effect: if a standassign is added to a standversion, make sure the standversion in the standassign is changed also.
Logically this works. But if some testcode is executed, this exception occurs:
Exception in thread "main" java.util.ConcurrentModificationException
     at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:449)
     at java.util.AbstractList$Itr.next(AbstractList.java:420)
     at oracle.toplink.essentials.indirection.IndirectList$1.next(IndirectList.java:418)
     at oracle.toplink.essentials.indirection.IndirectList$1.next(IndirectList.java:418)
     at nl.reinders.bm.BMTestToplink.main(BMTestToplink.java:71)
What is where and why concurrent modified???

Answer 1

Found it!
The testcode fetched all Standassigns from one Standversion, looped over them and make a shallow clone of each standversion. Then the newly clone Standassign was reassigned to a new Standversion. So:
create new standversion
loop over all standassigns of the old standversion
create a clone of the standassign
assign the new standversion to the cloned stand assign
The creation of the clone basically is a whole list of "new.setXXX( getXXX )", so it initially also copies the value for "Standversion". So the cloned Standassign is initially bound to the old standversion and then after the clone the Standversion is replaced with the new one.
This means that the collection that is iterated over is altered by the clone method. So what needs to be done is full fetch the collection of standassigns prior to looping over it.