Is it possible to set the child of a work item programmatically?
I want to create a user story and define tasks as childrens of it.
Is this possible with a plain-Java-Application?

Yes, this is possible. Some pseudo-code:

IWorkItemClient client=...
IWorkItem story=...
IWorkItemReferences references= client.resolveWorkItemReferences(story,
monitor);
for (IWorkItem task : childTasks) {
IItemReference reference=
WorkItemLinkTypes.createWorkItemReference(task);
references.add(WorkItemEndPoints.CHILD_WORK_ITEMS, reference);
}

Please see
https://jazz.net/wiki/bin/view/Main/ProgrammaticWorkItemCreation for
more information about creating work items in a plain-Java environment.

HTH,
Patrick
Jazz Work Item Team