Supermarket operation support system - Software Development Case Studies in Java - page 365

The class LocalProxy implements the MarketProxy interface for the single
process prototype. It has a few private attributes to keep track of the codes
of the currently connected employee, the current customer and the current
transaction. In addition the server attribute implements the association to
the Market interface.
This class implements all the methods of the interface. It ensures that the
normal operations are performed only after a connection has been made
(method assertConnected() ). In addition it initiates a new transaction when
the first product is bought by a new customer.
package Counter;
import Market.Market;
public class LocalProxy implements MarketProxy {
// code of the currently logged employee
private long employee;
// information about the ongoing transaction
private long customer;
private long transaction;
// the actual server that carries on the processing
private Market server;
// constructor
public LocalProxy(Market server) {
this.server # server;
employee # 0;
customer # 0;
transaction # 0;
}
// connects a terminal to the server and stores
// the code of the employee
public String connect( long employee, String password)
throws Exception{
String employeeName # server.authenticate(employee
,password);
if (employeeName ## null ) return null ;
this .employee # employee;
return employeeName;
}
public void disconnect() {
employee # 0;
customer # 0;
transaction # 0;
}
// verify that a connection has been succesfully
// completed, otherwise throw an exception
private void assertConnected() throws Exception {
if (employee ## 0)
throw new Exception("Not connected");
}