Reproducing the problem:

Set your JAVA_OPTS to "-Xms256m -Xmx2048m" to allow plenty of Heap Space 
(remove any 
other setting to be able to reproduce the error). Then run the following code:

InMemoryTopicMapStore store = new InMemoryTopicMapStore();
TopicMapIF tm = store.getTopicMap();
TopicMapBuilderIF builder = tm.getBuilder();

int i = 0;
while (true) {
    TopicIF t = builder.makeTopic();
    String oid = t.getObjectId();
    i++;
    if (i % 10000 == 0) {
        System.out.println("topics: " + i + ", last oid: " + oid);
    }
}

This should create around 1.000.000 topics (no problem, plenty of heap 
available), but crash on a 
OutOfMemory Error (PermGen space), with a trace like:

Caused by: java.lang.OutOfMemoryError: PermGen space
        at java.lang.String.intern(Native Method)
        at 
net.ontopia.topicmaps.impl.basic.SubjectIdentityCache.registerObject(SubjectIden
tityCache.java:1
39)
        at 
net.ontopia.topicmaps.impl.utils.AbstractSubjectIdentityCache$TopicAddedHandler.
processEvent(
AbstractSubjectIdentityCache.java:176)
        at 
net.ontopia.topicmaps.impl.utils.ObjectTreeManager$EventHandler.treeAddEvent(Obj
ectTreeMana
ger.java:126)
        at 
net.ontopia.topicmaps.impl.utils.ObjectTreeManager$EH01.processEvent(ObjectTreeM
anager.java
:154)
        at 
net.ontopia.topicmaps.impl.utils.ObjectTreeManager.processEvent(ObjectTreeManage
r.java:105)
        at net.ontopia.topicmaps.impl.basic.TopicMap.processEvent(TopicMap.java:343)
        at net.ontopia.topicmaps.impl.basic.TopicMap.fireEvent(TopicMap.java:289)
        at net.ontopia.topicmaps.impl.basic.TopicMap.addTopic(TopicMap.java:131)
        at net.ontopia.topicmaps.impl.basic.TopicMapBuilder.createTopic(TopicMapBuilder.java:31)
        at net.ontopia.topicmaps.impl.basic.TopicMapBuilder.makeTopic(TopicMapBuilder.java:36)
        at nl.tweedekamer.ipk.harvest.vip.Crasher.main(Crasher.java:17)


Cause:
The PermGen space is used by JVM's to store class meta data (definitions, 
constants etc) AND 
Strings that were created using String.intern() (see 
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/String.html#intern() ). 
Within Ontopia, Object 
ID's (oid) are created using String.intern(), probably to enhance speed when 
looking up id's 
trough the SubjectIdentityCache. Eventually, the String.intern() generated 
strings will take up all 
available PermGen space, and crash. Most JVM's have a cmline option to increase 
the PermGen 
space, but this masks the cause of the problem instead of fixing it. 
Note: Maven and Netbeans don't seem to use the PermGen cmdline option.

Problem:
As stated, the SubjectIdentityCache is used to cache the oid's of objects in a 
topicmap. It 
maintains a mapping of oid to objects. However, this uses a HashMap to store 
the oid-object 
mapping, which in turn uses object.hash() == object2.hash() comparison. 
Therefore, the speed 
increase intended by the use of String.intern() is not active. 

Solution:
Either NOT use String.intern() for oid's, and thus remove the limit of topics 
one can have in a 
topicmap; or use a Mapping that will utilize the speed increase gained from 
String.intern().