Creating hashsets that contain different types of objects

Question

I have written a program (java sockets) where a multi-threaded server can connect to multiple clients. In order to communicate with a single client, the server program has one DataInputStream and one DataOutputStream for each client. I want to broadcast a message from the server to the clients one by one. Each client can respond back differently . Depending on the response, the server takes it next action. In fact I am trying to find which client contains a certain file. To broadcast the message to all the clients I add the DataOutputStream for each client into a HashSet.

private static HashSet<DataOutputStream> comChannels = new HashSet<DataOutputStream>();

I can iterate through the items in the HashSet to broadcast any message to the clients. I then broadcast the fileName of the file I am looking for from the server. When a client finds that file in one if its directory it sends a positive response back. If a client does not have that file it sends back a negative response.

The problem I am facing is I cannot get a reply from the clients. When I create a client thread, I create a corresponding DataInputStream for the client as well. I tried to create a HashSet of ArrayList where each ArrayList contains the DataOutputStream and DataInputStream for one single client.

HashSet<ArrayList<Object>> comChannels = new HashSet<ArrayList<Object>>();

I then create an ArrayList containing the input and output stream of a client every time a new client thread is created which I then add to the HashSet of ArrayList s. I thought i can iterate through the HashSet which would return an ArrayList in each iteration. I can then select the 1st object of the list and that would be my outputstream and the 2nd object would be my inputstream. But this does not work.

for (ArrayList<Object> client  : comChannels) {
        //select outputstream from the Arraylist and write message to client
        //select inputstream from the Arraylist and read response from client
}

How can I broadcast and get replies from clients in a way where I can identify which response came from which client? Please note that I want to send the message to one client at a time and terminate the program as soon as a positive response comes back. This saves me from sending the message to all the clients unnecessarily. So creating separate HashSets of DataInputStream s and DataOutputStream s and then writing to all the clients and reading in all the different responses is not feasible.

Answer 1

A pair of streams representing a client is a domain concept in your application and should probably be captured in a meaningful class. Something like this at a bare minimum:

public class Client {
    public DataOutputStream outStream;
    public DataInputStream  inStream; 

    public Client(DataOutputStream out, DataInputStream in) {
        this.outStream = out;
        this.inStream = in;
    }
}

Whenever you create your streams for a client stick them in an instance of this Client class. Then you can put the Client objects in a List and iterate all you want.

for (Client client : clientList) {
    // do whatever you want with client.outStream and client.inStream
}

Now you end up with a List<Client> , which is much easier for a person to grok than a HashSet<ArrayList<Object>> .