code: public static DnsServerAddresses singleton(final InetSocketAddress address) {
          if (address == null) {
              throw new NullPointerException("address");
          }
          if (address.isUnresolved()) {
              throw new IllegalArgumentException("cannot use an unresolved DNS server address: " + address);
          }
  
          return new SingletonDnsServerAddresses(address);
      }
docs: Returns the {@link DnsServerAddresses} that yields only a single {@code address}.