May 28, 2020
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Objects and Client/Server Connections

  • By Matt Weisfeld
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This series, The Object-Oriented Thought Process, is intended for someone just learning an object-oriented language and who wants to understand the basic concepts before jumping into the code, or someone who wants to understand the infrastructure behind an object-oriented language he or she is already using. These concepts are part of the foundation that any programmer will need to make the paradigm shift from procedural programming to object-oriented programming.

Click here to start at the beginning of the series.

In keeping with the code examples used in the previous articles, Java will be the language used to implement the concepts in code. One of the reasons that I like to use Java is because you can download the Java compiler for personal use at the Sun Microsystems Web site http://java.sun.com/. You can download the standard edition, J2SE 5.0, at http://java.sun.com/j2se/1.5.0/download.jsp to compile and execute these applications. I often reference the Java J2SE 5.0 API documentation and I recommend that you explore the Java API further. Code listings are provided for all examples in this article as well as figures and output (when appropriate). See the first article in this series for detailed descriptions for compiling and running all the code examples.

In the previous column, we covered the basic Java technology required to construct a simple client/server example. The example illustrated how an object is constructed on one machine and then marshaled across a network to another machine. The process involves serializing the object so that it can be transferred across a wire and then reconstructed on the other side. In this article, we will further explore the process of moving an object across a client-server connection. Specifically, we will investigate what happens when the unexpected occurs. We will first review the client/server application that we began in the last column, and then we will throw in some wrinkles.

Running the Client/Server Example

First, let’s review the code from the last column. The examples in this article extend this code, so it is important that you reach this baseline by compiling and running the simple client server example (if you have not read the previous column or would like to review it, please visit the following URL: http://www.developer.com/db/article.php/3597071).

There are 3 files in this example, Employee.java, Server.java and Client.java.

The basic concept is this:

  • An Employee object is created
  • Attributes in the Employee object are set
  • The client sends Employee object is sent across a network
  • The server accepts the Employee object
  • The server changes the Employee object’s attributes
  • The server sends the Employee object back to the client
  • The client verifies that the attributes were indeed altered

The complete code for the Employee class is shown in Listing 1.

import java.io.*;
import java.util.*;

public class Employee implements Serializable {

   private int employeeNumber;
   private String employeeName;

   Employee(int num, String name) {
      employeeNumber = num;
      employeeName= name;

   public int getEmployeeNumber() {
      return employeeNumber ;

   public void setEmployeeNumber(int num) {
      employeeNumber = num;

   public String getEmployeeName() {
      return employeeName ;

   public void setEmployeeName(String name) {
      employeeName = name;

Listing 1: The Employee Object

The complete code for the Client class is shown in Listing 2.

import java.io.*;
import java.net.*;

public class Client {

   public static void main(String[] arg) {
      try {
         Employee joe = new Employee(150, "Joe");

         System.out.println("employeeNumber= " + joe
         System.out.println("employeeName= " + joe

         Socket socketConnection = new Socket("", 11111);

         ObjectOutputStream clientOutputStream = new 
         ObjectInputStream clientInputStream = new 


         joe= (Employee)clientInputStream.readObject();

         System.out.println("employeeNumber= " + joe
         System.out.println("employeeName= " + joe


      } catch (Exception e) {System.out.println(e); }

Listing 2: The Client

The complete code for the Server class is shown in Listing 3.

import java.io.*;
import java.net.*;

public class Server {

   public static void main(String[] arg) {

      Employee employee = null;

      try {

         ServerSocket socketConnection = new ServerSocket(11111);

         System.out.println("Server Waiting");

         Socket pipe = socketConnection.accept();

         ObjectInputStream serverInputStream = new    

         ObjectOutputStream serverOutputStream = new 

          employee = (Employee )serverInputStream.readObject();

         employee .setEmployeeNumber(256);
         employee .setEmployeeName("John");



      }  catch(Exception e) {System.out.println(e); 


Listing 3: The Server

Page 1 of 3

This article was originally published on May 8, 2006

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