Using a socket array (Java)

I'm brand new to Java and have been working with it for, at the max, 3 hours. I've got a C background.
Anyway, I'm using it for a multi-threaded socket server and need to be able to access all the connected clients' sockets. I figured this would be achieved through creating a socket array and then assigning each socket object, as it's created, to whatever spot in the array is open. This part goes fine. However, when I try to access the methods of the socket, I get thrown a screenful of errors:
java.net.SocketException: socket closed
        at java.net.SocketInputStream.socketRead0(Native Method)
        at java.net.SocketInputStream.read(Unknown Source)
        at sun.nio.cs.StreamDecoder.readBytes(Unknown Source)
        at sun.nio.cs.StreamDecoder.implRead(Unknown Source)
        at sun.nio.cs.StreamDecoder.read(Unknown Source)
        at java.io.InputStreamReader.read(Unknown Source)
        at java.io.BufferedReader.fill(Unknown Source)
        at java.io.BufferedReader.readLine(Unknown Source)
        at java.io.BufferedReader.readLine(Unknown Source)
        at SocketThreader.run(SocketThreader.java:41)

I'm trying to use the objects with this:
for (int i = 0; i < MultiSocket.maxClients; i++)
{
    if (MultiSocket.clientsockets[i] != null && i != clientid)
    {
        PrintWriter tmpout = new PrintWriter(MultiSocket.clientsockets[i].getOutputStream(), true);
        tmpout.println("Remote msg: " + outputLine);
        tmpout.close();
    }
}

Any help on this would be appreciated, as I can't find any documentation on socket arrays.

Neat.
I've got another question.
How the hell can I check to see if a string is null?