google protocol buffers: how to define message contains ArrayList> in proto file

Question

ArrayList corresponds to repeated string:

message m1 {
    repeated string mylist = 1;
}

How to define ArrayList< ArrayList< String> > in message? Thanks!

Answer 1

You'll need another message to represent the inner List.

message M1 {
  repeated M2 mylist = 1;
}

message M2 {
  repeated string mylist = 1;
}

Of course, you may add as many fields as you'd like to M2, and you'll need some separate conversion logic to assemble the List<M2> into an ArrayList<ArrayList<String>>.

You may even want to create a reusable message to represent a list of strings:

message M1 {
  repeated StringList mylist = 1;
}

message StringList {
  repeated string value = 1;
}