joiner.setParent(this);
    public void setJoiner(SplitJoinType type)    {        assert joiner == null && type != null;        joiner = type.getJoiner();        ListIterator iter;        iter = childrenStreams.listIterator();        while (iter.hasNext())            {                Stream s = (Stream) iter.next();                assert s != null;                joiner.add(s);            }        joinType = type;    }