Having connected, you wish to transfer serialized object data.

Construct an ObjectInputStream or ObjectOutputStream from the socket’s getInputStream( ) or getOutputStream( ).

Object serialization is the ability to convert in-memory objects to an external form that can be sent serially (a byte at a time). This is discussed in Section 9.17.

This program (and its server) operate one service that isn’t normally provided by TCP/IP, as it is Java-specific. It looks rather like the DaytimeBinary program in the previous recipe, but the server sends us a Date object already constructed. You can find the server for this program in Section 16.4; Example 15-7 shows the client code.

/**
 * DaytimeObject - connect to the Daytime (ascii) service.
 */
public class DaytimeObject {
    /** The TCP port for the object time service. */
    public static final short TIME_PORT = 1951;

    public static void main(String[] argv) {
        String hostName;
        if (argv.length == 0)
            hostName = "localhost";
        else
            hostName = argv[0];

        try {
            Socket sock = new Socket(hostName, TIME_PORT);
            ObjectInputStream is = new ObjectInputStream(new 
                BufferedInputStream(sock.getInputStream(  )));

            // Read and validate the Object
            Object o = is.readObject(  );
            if (!(o instanceof Date))
                throw new IllegalArgumentException("Wanted Date, got " + o);

            // Valid, so cast to Date, and print
            Date d = (Date) o;
            System.out.println("Time on " + hostName + " is " + d.toString(  ));
        } catch (ClassNotFoundException e) {
            System.err.println("Wanted date, got INVALID CLASS (" + e + ")");
        } catch (IOException e) {
            System.err.println(e);
        }
    }
}

I ask the operating system for the date and time, and then run the program, which prints the date and time. The server and my desktop are within about ten seconds of agreement:

C:javasrc
etwork>date 
Current date is Sun 01-23-2000 
Enter new date (mm-dd-yy):
C:javasrc
etwork>time 
Current time is  2:52:35.43p 
Enter new time:
C:javasrc
etwork>java DaytimeObject 
Time on localhost is Sun Jan 23 14:52:25 GMT 2000
C:javasrc
etwork>