how to define in java class to matches the proto blank field definition?

Question

how to define in java class to get runtime schema that matches the proto definition in below

message PacketHeader {
  int32 appId = 1;
  int64 uid = 2;
  int64 instanceId = 3;
  // -------------------------  4 is blank
  uint32 flags = 5;
}

a wrong case:

public class PacketHeader {
  public int appId;
  public long uid;
  public long instanceId;
  // ---------------------------- how to define the blank 4 ?
  public int flags;
}