The United States Supreme Court has described the relationship between due process and res judicata as follows: "'The doctrine of res judicata rests at bottom upon the ground that the party to be affected, or some other with whom he is in privity, has litigated or had an opportunity to litigate the same matter in a former action in a court of competent jurisdiction. The opportunity to be heard is an essential requisite of due process of law in judicial proceedings. And as a State may not, consistently with the Fourteenth Amendment, enforce a judgment against a party named in the proceedings without a hearing or an opportunity to be heard [citations], so it cannot, without disregarding the requirement of due process, give a conclusive effect to a prior judgment against one who is neither a party nor in privity with a party therein.' " ( Richards v. Jefferson County, 517 U.S. at p. 797, fn. 4.) These due process principles apply in the case of class actions. ( Hansberry v. Lee, 311 U.S. at pp. 40-46.)