/** * Adds an outgoing arc to the specified vertex. Also calls {@link #addIncomingArcTo} for * <tt>headVertex</tt> with <tt>this</tt> as the argument. Does nothing if <tt>headVertex</tt> is * the head vertex of an already existing outgoing arc. * * @param headVertex Head vertex to be added to establish a new outgoing arc. <tt>Null</tt> is not * allowed. */ public void addOutgoingArcTo(Vertex headVertex) { if (!_heads.contains(headVertex)) { _heads.add(headVertex); headVertex.addIncomingArcTo(this); } }