HashMap< String, Socket > users = new HashMap< String, Socket >( );

//when a user connects

HashMap.put( ip, sock );

//To iterate

Iterator it = users.entrySet( ).iterator( );
HashMap.Entry item;
String ip;
Socket con;

while ( it.hasNext( ) )
{
    item = ( HashMap.Entry ) it.next( );

    ip = item.getKey( );
    con = item.getValue( );

    //Check if open, etc
}