Networking with Sockets - SCJD Exam with J2SE 5 - page 220

DVDSocketClient makes use of the DVDCommand object in each of its DBClient methods. For
example, in the find method, the command object is constructed with the FIND enum value of
SocketCommand . Next, the regex attribute is set using the query parameter of the find method.
These two steps are shown again here:

DvdCommand cmdObj = new DvdCommand(SocketCommand.FIND);
cmdObj.setRegex(query);

The Result Object
The DVDResult object is used to encapsulate the result of the request. When the DVDDatabase
has completed a request, the result is sent back to DVDSocketServer , which in turn wraps
the result in a DVDResult object and sends it back to the socket client. As in the case of the
command object, DVDResult must be serializable since it is sent across the network. Even
exceptions are wrapped in DVDResult and sent back to the client.
The result object is similar to the DVDCommand object in its operation. There are a number
of constructors that take the varying types of return values from the DBClient methods. You
can find the code in the project download, but the following is an example of its usage in the
find method of DVDSocketClient :

DVDResult serialReturn = (DVDResult)ois.readObject();
if (!serialReturn.isException()) {
retVal = serialReturn.getCollection();
}

The DVDResult is received from the socket server. The result object is checked for excep-
tions, and if no exceptions are detected, the Collection is extracted from DVDResult . We
know it must be a Collection since our protocol ensures that for DBClient 's find method,
a Collection is always returned.

■ Note Our application protocol was separated from our socket server, based on good design principles.

Thus, changes in our protocol, or even additional protocols, can now be handled without affecting the socket
server directly.

USING ENUM CONSTANTS

The Application protocol is implemented by specifying the command type. This was performed with the fol-
lowing code taken from the constructor of the DvdCommand class:

public DvdCommand(SocketCommand command, DVD dvd) {
setCommandId(command);
this.dvd = dvd;

}