Actions under and by virtue of this part, shall be commenced by filing a complaint and issuing a summons thereon. All persons who are owners or claimants of the property sought to be condemned shall be joined as defendants. In case the owner or claimant is unknown to plaintiff, it shall be sufficient if the complaint includes a statement of that fact, and the defendant may be joined in the complaint under a fictitious name. In the event there is a class of persons owning or claiming some interest in the property sought to be condemned, who are too numerous to be served by personal service of the summons, or who would have to be ascertained by examination into the possible claims of a group of persons too numerous to permit identification of the defendants, the complaint may include a statement of that fact and the defendants may be joined in the complaint by describing them as a class, and in such case it shall be unnecessary to state in the complaint that the owners or claimants are unknown to the plaintiff.