Question

I have a bunch of DataOutputStreams in Hashtable outputStreams;. There is a Socket for each of them. I want to loop through them but I want to exclude one particular Socket. I’ve been trying with the following algorithm but it doesn’t seem to work.

for(Enumeration e = outputStreams.elements(); e.hasMoreElements(); ) {

    DataOutputStream dout = (DataOutputStream)e.nextElement();
    OutputStream sdout = null;
    try {
        sdout = socket.getOutputStream();
    } catch (IOException ie) {
        ie.printStackTrace();
    }

    if (dout != sdout) {
        try {
            dout.writeUTF(message);
        } catch (IOException ie) {
            ie.printStackTrace();
        }
    }
}

Answer 1

It appears that a Hashtable will insert a copy of the instance into its backing structure. Andypandy is right about your problem, but then I wondered if a table of OuputStreams (where you wouldn’t wrap the socket output stream) would fix it and it didn’t.

import java.io.ByteArrayOutputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.Socket;
import java.util.Hashtable;

public class Main
{

    public static void main(String[] args) throws IOException {

        Hashtable<Integer, OutputStream> ht = new Hashtable<Integer, OutputStream>();

        for(int i = 0; i < 5; i++)
            ht.put( i, new DataOutputStream(new ByteArrayOutputStream()) );

        Socket sock = new Socket("google.com", 80);
        ht.put( ht.size(), sock.getOutputStream() );

        for(OutputStream dos : ht.values())
            System.out.println(dos);

        System.out.println(sock.getOutputStream());
    }
}

Output:

java.net.SocketOutputStream@112b853
java.io.DataOutputStream@36428
java.io.DataOutputStream@a4b78b
java.io.DataOutputStream@e3f6d
java.io.DataOutputStream@1660d22
java.io.DataOutputStream@e84763
java.net.SocketOutputStream@1a6c368

The socket’s output stream that is inserted into the table has a different address than that of the original output stream. Since there’s no equals override, there’s no easy way to meaningfully compare streams without having knowledge of their corresponding sockets. You should re-think your design completely, perhaps using the socket descriptor as your key.