Question

I am writing an application to do some distributed calculations in a peer to peer network. In defining the network I have two class the P2PNetwork and P2PClient. I want these to be generic and so have the definitions of:

P2PNetwork<T extends P2PClient<? extends P2PNetwork<T>>>

P2PClient<T extends P2PNetwork<? extends T>>

with P2PClient defining a method of setNetwork(T network). What I am hoping to describe with this code is:

1. A P2PNetwork is constituted of
   clients of a certain type
2. A P2PClient may only belong to a
   network whose clients consist of the
   same type as this client (the
   circular-reference)

This seems correct to me but if I try to create a non-generic version such as

MyP2PClient<MyP2PNetwork<? extends MyP2PClient>> myClient;

and other variants I receive numerous errors from the compiler. So my questions are as follows:

1. Is a generic circular reference even
   possible (I have never seen anything explicitly forbidding it)?
2. Is the above generic definition a
   correct definition of such a circular
   relationship?
3. If it is valid, is it the “correct”
   way to describe such a relationship
   (i.e. is there another valid
   definition, which is stylistically
   preferred)?
4. How would I properly define a
   non-generic instance of a Client and
   Server given the proper generic
   definition?

Answer 1

Circular generic references are indeed possible. Java Generics and Collections includes several examples. For your case, such a specimen would look like this:

public interface P2PNetwork<N extends P2PNetwork<N, C>,
                            C extends P2PClient<N, C>> {
  void addClient(C client);
}

public interface P2PClient<N extends P2PNetwork<N, C>,
                            C extends P2PClient<N, C>> {
  void setNetwork(N network);
}

class TorrentNetwork implements P2PNetwork<TorrentNetwork, TorrentClient> {
  @Override
  public void addClient(TorrentClient client) {
    ...
  }
}

class TorrentClient implements P2PClient<TorrentNetwork, TorrentClient> {
  @Override
  public void setNetwork(TorrentNetwork network) {
    ...
  }
}

...

TorrentNetwork network = new TorrentNetwork();
TorrentClient client = new TorrentClient();

network.addClient(client);