public void add(ID mbr) {
    hashedMembers.add(mbr);
    members.add(mbr);
    int idx = members.size() - 1;
    ensureFDCapacity(idx);
    failureDetectionPorts[idx] = -1;
  }
ensures that there is a slot at idx to store an int
geode-membership/src/main/java/org/apache/geode/distributed/internal/membership/gms/GMSMembershipView.java