Socket programming

Java
Socket Programming
AĞer learning the contents of this chapter, the reader will be able to:
∑ understand fundamental concepts of computer communication
∑ understand sockets and ports
∑ understand java.net package features
∑ program Java Sockets
∑ create comprehensive network applications using sockets
This chapter presents key concepts of intercommunication between programs running on diﬀerent computers
in the network. It introduces elements of network programming and concepts involved in creating network
applications using sockets. The chapter introduces the java.net package containing various classes re-
quired for creating sockets and message communication using two diﬀerent protocols. It provides several
example programs demonstrating various capabilities supported by Java for creating network applications.
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13.1 INTRODUCTION
Internet and WWW have emerged as global ubiquitous media for communication and changed the way
we conduct science, engineering, and commerce. They are also changing the way we learn, live, enjoy,
communicate, interact, engage, etc. The modern life activities are getting completely centered around or
driven by the Internet.
To take advantage of opportunities presented by the Internet, businesses are continuously seeking new
and innovative ways and means for offering their services via the Internet. This created a huge demand
for software designers and engineers with skills in creating new Internet-enabled applications or porting
existing/legacy applications to the Internet platform. The key elements for developing Internet-enabled
applications are a good understanding of the issues involved in implementing distributed applications and
sound knowledge of the fundamental network programming models.
13

Socket Programming 347
13.1.1 Client/Server Communication
At a basic level, network-based systems consist of a server, client, and a media for communication
as shown in Fig. 13.1. A computer running a program that makes a request for services is called client
machine. A computer running a program that offers requested services from one or more clients is called
server machine. The media for communication can be wired or wireless network.
Fig. 13.1 Client – Server communication
Generally, programs running on client machines make requests
to a program (often called as server program) running on a server
machine. They involve networking services provided by the
transport layer, which is part of the Internet software stack, often
called TCP/IP (Transport Control Protocol/Internet Protocol)
stack, shown in Fig. 13.2. The transport layer comprises two
types of protocols, TCP (Transport Control Protocol) and UDP
(User Datagram Protocol). The most widely used programming
interfaces for these protocols are sockets.
TCP is a connection-oriented protocol that provides a reliable
ﬂow of data between two computers. Example applications that
use such services are HTTP, FTP, and Telnet.
UDP is a protocol that sends independent packets of data, called
datagrams, from one computer to another with no guarantees
about arrival and sequencing. Example applications that use such services include Clock server and Ping.
The TCP and UDP protocols use ports to map incoming data to a particular process running on a
computer. Port is represented by a positive (16-bit) integer value. Some ports have been reserved to support
common/well known services:
∑ ftp 21/tcp
∑ telnet 23/tcp
∑ smtp 25/tcp
∑ login 513/tcp
∑ http 80/tcp,udp
∑ https 443/tcp,udp
User-level process/services generally use port number value >= 1024.
Fig. 13.2 TCP/IP soĞware stack

348 Object-Oriented Programming with Java
Fig. 13.3 TCP/UDP mapping of incoming packets to appropriate port/process
Object-oriented Java technologies—Sockets, threads, RMI, clustering, Web services—have emerged as
leading solutions for creating portable, efficient, and maintainable large and complex Internet applications.
13.1.2 Hosts Identification and Service Ports
Every computer on the Internet is identified by a unique, 4-byte IP address. This is typically written in
dotted quad format like 128.250.25.158 where each byte is an unsigned value between 0 and 255. This
representation is clearly not user-friendly because it does not tell us anything about the content and then it is
difficult to remember. Hence, IP addresses are mapped to names like www.buyya.com or www.google.com,
which are easier to remember. Internet supports name servers that translate these names to IP addresses.
In general, each computer only has one Internet address. However, computers often need to communicate
and provide more than one type of service or to talk to multiple hosts/computers at a time. For example,
there may be multiple ftp sessions, web connections, and chat programs all running at the same time. To
distinguish these services, a concept of ports, a logical access point, represented by a 16-bit integer number
is used. That means, each service offered by a computer is uniquely identified by a port number. Each
Internet packet contains both the destination host address and the port number on that host to which the
message/request has to be delivered. The host computer dispatches the packets it receives to programs by
looking at the port numbers specified within the packets. That is, IP address can be thought of as a house
address when a letter is sent via post/snail mail and port number as the name of a specific individual to
whom the letter has to be delivered.
13.1.3 Sockets and Socket-based Communication
Sockets provide an interface for programming networks at the transport layer. Network communication
using Sockets is very much similar to performing file I/O. In fact, socket handle is treated like file handle.
The streams used in file I/O operation are also applicable to socket-based I/O. Socket-based
communication is independent of a programming language used for implementing it. That means, a socket
program written in Java language can communicate to a program written in non-Java (say C or C++) socket
program.
A server (program) runs on a specific computer and has a socket that is bound to a specific port. The
server listens to the socket for a client to make a connection request (see Fig. 13.4a). If everything goes
well, the server accepts the connection (see Fig. 13.4b). Upon acceptance, the server gets a new socket
bound to a different port. It needs a new socket (consequently a different port number) so that it can
continue to listen to the original socket for connection requests while serving the connected client.

Fig. 13.4 Establishment of path for two-way communication between a client and server
13.2 SOCKET PROGRAMMING AND JAVA.NET CLASS
A socket is an endpoint of a two-way communication link between two programs running on the network.
Socket is bound to a port number so that the TCP layer can identify the application that data is destined
to be sent. Java provides a set of classes, deﬁned in a package called java.net, to enable the rapid
development of network applications. Key classes, interfaces, and exceptions in java.net package
simplifying the complexity involved in creating client and server programs are:
The Classes
∑ ContentHandler
∑ DatagramPacket
∑ DatagramSocket
∑ DatagramSocketImpl
∑ HttpURLConnection
∑ InetAddress
∑ MulticastSocket
∑ ServerSocket
∑ Socket
∑ SocketImpl
∑ URL
∑ URLConnection
∑ URLEncoder
∑ URLStreamHandler
The Interfaces
∑ ContentHandlerFactory
∑ FileNameMap
∑ SocketImplFactory
∑ URLStreamHandlerFactory

Exceptions
∑ BindException
∑ ConnectException
∑ MalformedURLException
∑ NoRouteToHostException
∑ ProtocolException
∑ SocketException
∑ UnknownHostException
∑ UnknownServiceException
13.3 TCP/IP SOCKET PROGRAMMING
The two key classes from the java.net package used in creation of server and client programs are:
∑ ServerSocket
∑ Socket
A server program creates a speciﬁc type of socket that is used to listen for client requests (server socket),
In the case of a connection request, the program creates a new socket through which it will exchange data
with the client using input and output streams. The socket abstraction is very similar to the ﬁle concept:
developers have to open a socket, perform I/O, and close it. Figure 13.5 illustrates key steps involved in
creating socket-based server and client programs.
Fig. 13.5 Socket-based client and server programming
A simple Server Program in Java The steps for creating a simple server program are:
1. Open the Server Socket:
ServerSocket server = new ServerSocket( PORT );
2. Wait for the Client Request:
Socket client = server.accept();

3. Create I/O streams for communicating to the client
DataInputStream is = new DataInputStream(client.getInputStream());
DataOutputStream os = new DataOutputStream(client.getOutputStream());
4. Perform communication with client
Receive from client: String line = is.readLine();
Send to client: os.writeBytes(“Hellon”);
5. Close socket:
client.close();
An example program illustrating creation of a server socket, waiting for client request, and then
responding to a client that requested for connection by greeting it is given below:
Program 13.1
// SimpleServer.java: A simple server program.
import java.net.*;
import java.io.*;
public class SimpleServer {
public static void main(String args[]) throws IOException {
// Register service on port 1254
ServerSocket s = new ServerSocket(1254);
Socket s1=s.accept(); // Wait and accept a connection
// Get a communication stream associated with the socket
OutputStream s1out = s1.getOutputStream();
DataOutputStream dos = new DataOutputStream (s1out);
// Send a string!
dos.writeUTF(“Hi there”);
// Close the connection, but not the server socket
dos.close();
s1out.close();
s1.close();
}
}
A simple Client Program in Java The steps for creating a simple client program are:
1. Create a Socket Object:
Socket client = new Socket(server, port_id);
2. Create I/O streams for communicating with the server.
is = new DataInputStream(client.getInputStream());
os = new DataOutputStream(client.getOutputStream());
3. Perform I/O or communication with the server:
Receive data from the server: String line = is.readLine();
Send data to the server: os.writeBytes(“Hellon”);
4. Close the socket when done:
client.close();
An example program illustrating establishment of connection to a server and then reading a message
sent by the server and displaying it on the console is given below:

Program 13.2
// SimpleClient.java: A simple client program.
import java.net.*;
import java.io.*;
public class SimpleClient {
public static void main(String args[]) throws IOException {
// Open your connection to a server, at port 1254
Socket s1 = new Socket(“localhost”,1254);
// Get an input file handle from the socket and read the input
InputStream s1In = s1.getInputStream();
DataInputStream dis = new DataInputStream(s1In);
String st = new String (dis.readUTF());
System.out.println(st);
// When done, just close the connection and exit
dis.close();
s1In.close();
s1.close();
}
}
Running Socket Programs Compile both server and client programs and then deploy server program
code on a machine which is going to act as a server and client program, which is going to act as a client.
If required, both client and server programs can run on the same machine. To illustrate execution of server
and client programs, let us assume that a machine called mundroo.csse.unimelb.edu.au on which we
want to run a server program as indicated below:
[raj@mundroo] java SimpleServer
The client program can run on any computer in the network (LAN, WAN, or Internet) as long as there
is no ﬁrewall between them that blocks communication. Let us say we want to run our client program on a
machine called gridbus.csse.unimelb.edu.au as follows:
[raj@gridbus] java SimpleClient
The client program is just establishing a connection with the server and then waits for a message. On
receiving a response message, it prints the same to the console. The output in this case is:
Hi there
which is sent by the server program in response to a client connection request.
It should be noted that once the server program execution is started, it is not possible for any other server
program to run on the same port until the ﬁrst program which is successful using it is terminated. Port
numbers are a mutually exclusive resource. They cannot be shared among different processes at the same
time.
13.4 UDP SOCKET PROGRAMMING
The previous two example programs used the TCP sockets. As already said, TCP guarantees the delivery of
packets and preserves their order on destination. Sometimes these features are not required and since they

do not come without performance costs, it would be better to use a lighter transport protocol. This kind of
service is accomplished by the UDP protocol which conveys datagram packets.
Datagram packets are used to implement a connectionless packet delivery service supported by the UDP
protocol. Each message is transferred from source machine to destination based on information contained
within that packet. That means, each packet needs to have destination address and each packet might be
routed differently, and might arrive in any order. Packet delivery is not guaranteed.
The format of datagram packet is:
| Msg | length | Host | serverPort |
Java supports datagram communication through the following classes:
∑ DatagramPacket
∑ DatagramSocket
The class DatagramPacket contains several constructors that can be used for creating packet object.
One of them is:
DatagramPacket(byte[] buf, int length, InetAddress address, int port);
This constructor is used for creating a datagram packet for sending packets of length length to the
specified port number on the specified host. The message to be transmitted is indicated in the first argument.
The key methods of DatagramPacket class are:
byte[] getData()
Returns the data buffer.
int getLength()
Returns the length of the data to be sent or the length of the data received.
void setData(byte[] buf)
Sets the data buffer for this packet.
void setLength(int length)
Sets the length for this packet.
The class DatagramSocket supports various methods that can be used for transmitting or receiving
data a datagram over the network. The two key methods are:
void send(DatagramPacket p)
Sends a datagram packet from this socket.
void receive(DatagramPacket p)
Receives a datagram packet from this socket.
A simple UDP server program that waits for client’s requests and then accepts the message (datagram)
and sends back the same message is given below. Of course, an extended server program can manipulate
client’s messages/request and send a new message as a response.
Program 13.3
// UDPServer.java: A simple UDP server program.
import java.net.*;
import java.io.*;
public class UDPServer {

public static void main(String args[]){
DatagramSocket aSocket = null;
if (args.length < 1) {
System.out.println(“Usage: java UDPServer <Port Number>”);
System.exit(1);
}
try {
int socket_no = Integer.valueOf(args[0]).intValue();
aSocket = new DatagramSocket(socket_no);
byte[] buffer = new byte[1000];
while(true) {
DatagramPacket request = new DatagramPacket(buffer,
buffer.length);
aSocket.receive(request);
DatagramPacket reply = new DatagramPacket(request.getData(),
request.getLength(),request.getAddress(),
request.getPort());
aSocket.send(reply);
}
}
catch (SocketException e) {
System.out.println(“Socket: ” + e.getMessage());
}
catch (IOException e) {
System.out.println(“IO: ” + e.getMessage());
}
finally {
if (aSocket != null)
aSocket.close();
}
}
}
A corresponding client program for creating a datagram and then sending it to the above server and then
accepting a response is listed below.
Program 13.4
// UDPClient.java: A simple UDP client program.
import java.net.*;
import java.io.*;
public class UDPClient {
public static void main(String args[]){
// args give message contents and server hostname
DatagramSocket aSocket = null;
if (args.length < 3) {
System.out.println(
“Usage: java UDPClient <message> <Host name> <Port number>”);
System.exit(1);

}
try {
aSocket = new DatagramSocket();
byte [] m = args[0].getBytes();
InetAddress aHost = InetAddress.getByName(args[1]);
int serverPort = Integer.valueOf(args[2]).intValue();
DatagramPacket request =
new DatagramPacket(m, args[0].length(), aHost, serverPort);
aSocket.send(request);
byte[] buffer = new byte[1000];
DatagramPacket reply = new DatagramPacket(buffer, buffer.length);
aSocket.receive(reply);
System.out.println(“Reply: ” + new String(reply.getData()));
}
catch (SocketException e) {
System.out.println(“Socket: ” + e.getMessage());
}
catch (IOException e) {
System.out.println(“IO: ” + e.getMessage());
}
finally {
if (aSocket != null)
aSocket.close();
}
}
}
13.5 MATH SERVER
It is time to implement a more comprehensive network application by using the socket programming APIs
you have learned so far. A sample math client-server interaction demonstrating online math server that can
perform basic math operations is shown in Fig. 13.6.
Fig. 13.6 A socket-based math server and clients
The basic math interface is shown as follows:

Program 13.5
// MathService.java: A basic math interface.
public interface MathService {
public double add(double firstValue, double secondValue);
public double sub(double firstValue, double secondValue);
public double div(double firstValue, double secondValue);
public double mul(double firstValue, double secondValue);
}
The implementation of this interface is not related to any network operation. The following code shows
a very simple implementation of this interface:
Program 13.6
// PlainMathService.java: An implementation of the MathService interface.
public class PlainMathService implements MathService {
public double add(double firstValue, double secondValue) {
return firstValue+secondValue;
}
public double sub(double firstValue, double secondValue) {
return firstValue-secondValue;
}
public double mul(double firstValue, double secondValue) {
return firstValue * secondValue;
}
public double div(double firstValue, double secondValue) {
if (secondValue != 0)
return firstValue / secondValue;
return Double.MAX_VALUE;
}
}
The implementation of the MathServer is quite straightforward, which looks pretty similar to the echo
server mentioned previously. The difference is that the MathServer have to consider the specific protocol
defined by the math server and client communication. The program uses a very simple protocol operator:
first_value:second_value. It is the math server’s responsibility to understand this protocol and delegate to
the proper methods such as add, sub, mul, or div.
Program 13.7
// MathServer.java : An implementation of the MathServer.
import java.io.*;
import java.net.*;
public class MathServer{
protected MathService mathService;
protected Socket socket;

public void setMathService(MathService mathService) {
this.mathService = mathService;
}
public void setSocket(Socket socket) {
this.socket = socket;
}
public void execute() {
try {
BufferedReader reader = new BufferedReader(
new InputStreamReader(socket.getInputStream()));
// read the message from client and parse the execution
String line = reader.readLine();
double result = parseExecution(line);
// write the result back to the client
BufferedWriter writer = new BufferedWriter(
new OutputStreamWriter(socket.getOutputStream()));
writer.write(“”+result);
writer.newLine();
writer.flush();
// close the stream
reader.close();
writer.close();
}
catch (Exception e) {
e.printStackTrace();
}
}
// the predefined protocol for the math operation is
// operator:first value:second value
protected double parseExecution(String line)
throws IllegalArgumentException {
double result = Double.MAX_VALUE;
String [] elements = line.split(“:”);
if (elements.length != 3)
throw new IllegalArgumentException(“parsing error!”);
double firstValue = 0;
double secondValue = 0;
try {
firstValue = Double.parseDouble(elements[1]);
secondValue = Double.parseDouble(elements[2]);
}
catch(Exception e) {
throw new IllegalArgumentException(“Invalid arguments!”);
}
switch (elements[0].charAt(0)) {
case ‘+’:
result = mathService.add(firstValue, secondValue);
break;
case ‘-’:

result = mathService.sub(firstValue, secondValue);
break;
case ‘*’:
result = mathService.mul(firstValue, secondValue);
break;
case ‘/’:
result = mathService.div(firstValue, secondValue);
break;
default:
throw new IllegalArgumentException(“Invalid math operation!”);
}
return result;
}
public static void main(String [] args)throws Exception{
int port = 10000;
if (args.length == 1) {
try {
port = Integer.parseInt(args[0]);
}
catch(Exception e){
}
}
System.out.println(“Math Server is running...”);
// create a server socket and wait for client’s connection
ServerSocket serverSocket = new ServerSocket(port);
Socket socket = serverSocket.accept();
// run a math server that talks to the client
MathServer mathServer = new MathServer();
mathServer.setMathService(new PlainMathService());
mathServer.setSocket(socket);
mathServer.execute();
System.out.println(“Math Server is closed...”);
}
}
A test client program that can access the math server is shown below:
Program 13.8
// MathClient.java: A test client program to access MathServer.
import java.io.*;
import java.net.Socket;
public class MathClient {
public static void main(String [] args){
String hostname = “localhost”;
int port = 10000;
if (args.length != 2) {
System.out.println(“Use the default setting...”);
}
else {

hostname = args[0];
port = Integer.parseInt(args[1]);
}
try {
// create a socket
Socket socket = new Socket(hostname, port);
// perform a simple math operation “12+21”
BufferedWriter writer = new BufferedWriter(
new OutputStreamWriter(socket.getOutputStream()));
writer.write(“+:12:21”);
writer.newLine();
writer.flush();
// get the result from the server
BufferedReader reader = new BufferedReader(
new InputStreamReader(socket.getInputStream()));
System.out.println(reader.readLine());
reader.close();
writer.close();
}
catch (Exception e) {
}
}
}
13.6 URL ENCODING
It is very important that the Web can be accessed via heterogeneous platforms such as Windows, Linux,
or Mac. The characters in the URL must come from a ﬁxed subset of ASCII in order to maintain the
interoperability between various platforms. Speciﬁcally, the capital letters A–Z, the lowercase letters a–z,
the digits 0–9 and the punctuation characters. Encoding is very simple, any characters that are not ASCII
numerals, letters, or the punctuation marks allowed are converted into bytes and each byte is written as a
percentage sign followed by two hexadecimal digits. For example, the following program helps encode a
query string if non-ASCII characters are present.
Program 13.9
// QueryStringFormatter.java: encodes a string with non-ASCII characters.
import java.io.UnsupportedEncodingException;
import java.net.URLEncoder;
public class QueryStringFormatter {
private String queryEngine;
private StringBuilder query = new StringBuilder();
public QueryStringFormatter(String queryEngine) {
this.queryEngine = queryEngine;
}

public String getEngine() {
return this.queryEngine;
}
public void addQuery(String queryString, String queryValue)
throws Exception {
query.append(queryString+“=”+
URLEncoder.encode(queryValue,“UTF-8")+“&”);
}
public String getQueryString(){
return “?”+query.toString();
}
public static void main(String[] args) throws Exception {
QueryStringFormatter formatter =
new QueryStringFormatter(“http://www.google.com.au/search”);
formatter.addQuery(“newwindow”,“1”);
formatter.addQuery(“q”,“Xingchen Chu & Rajkumar Buyya”);
System.out.println(formatter.getEngine()+
formatter.getQueryString());
}
}
The output of this program is shown as follows:
http://www.google.com.au/search?newwindow=1&q=Xingchen+Chu+%26+Rajkumar+Buyya&
It can be seen that the whitespace has been encoded as “+” and the “&” has been encoded as “%26” the
percentage sign following by its byte value. Other characters remain the same. These conversions have been
performed by the URLEncoder class, which is part of the Java base class library and provides facilities for
encoding strings (urls) in different formats. There is also an URLDecoder class that performs the reverse
process. These two classes are useful not only for URLs but also for managing HTML form data.
13.6.1 Writing and Reading Data via URLConnection
Besides the socket and datagram introduced in the previous section, java.net package provides another
useful class that can be used to write and read data between the server and the client: URLConnetion
It is not possible to instantiate the URLConnection class directly. Instead you have to create the
URLConnection via the URL object.
URLConnection connection = new URL(“www.yahoo.com”).openConnection();
Then the URLConnection class provides the getInputStream and getOutputStream methods that
are very similar to the getInputStream and getOutputStream provided by the Socket class. The
following example shows how to use the URLConnection and the URLEncoder class to send queries to
the Yahoo search engine.
Program 13.10
// TextBasedSearchEngine.java:
import java.io.*;
import java.net.*;
public class TextBasedSearchEngine {

private String searchEngine;
public TextBasedSearchEngine(String searchEngine) {
this.searchEngine = searchEngine;
}
public void doSearch(String queryString) {
try {
// open a url connection
URL url = new URL(searchEngine);
URLConnection connection = url.openConnection();
connection.setDoOutput(true);
// write the query string to the search engine
PrintStream ps = new PrintStream(connection.getOutputStream());
ps.println(queryString);
ps.close();
// read the result
DataInputStream input =
new DataInputStream(connection.getInputStream());
String inputLine = null;
while((inputLine = input.readLine())!= null) {
System.out.println(inputLine);
}
}
catch(Exception e) {
}
}
public static void main(String[] args) throws Exception{
QueryStringFormatter formatter =
new QueryStringFormatter(“http://search.yahoo.com/search”);
formatter.addQuery(“newwindow”,“1");
formatter.addQuery(“q”,“Xingchen Chu & Rajkumar Buyya”);
// search it via yahoo
TextBasedSearchEngine search =
new TextBasedSearchEngine(formatter.getEngine());
search.doSearch(formatter.getQueryString());
}
}
Output:
The program ﬁrst creates an encoded query string that can be used by the web application, then it utilises
the URLConnection class to send/receive data from the Yahoo search engine. The output in the entire
HTML page will be something like this:
<!doctype html public “-//W3C/DTD HTML 4.01//EN” “http://www.w3.org/TR/
html4/strict.dtd”>
<html>
<head>
…
…
</html>

13.7
Developing network applications is made possible in Java by using sockets, threads, RMI,
clustering, and Web services. These technologies allow for the creation of portable, efficient, and
maintainable large and complex Internet applications. The java.net package provides a powerful
and flexible set of classes for implementing network applications.
Typically, programs running on client machines make requests to programs on a server
machine. These involve networking services provided by the transport layer. The most widely
used transport protocols on the Internet are TCP (Transmission control Protocol) and UDP
(User Datagram Protocol). TCP is a connection-oriented protocol providing a reliable flow of data
between two computers. It is used by applications such as the World Wide Web, e-mail, Ğp, and
secure shell. On the other hand, UDP is a simpler message-based connectionless protocol which
sends packets of data known as datagrams from one computer to another with no guarantees of
arrival. Network applications using UDP include Domain Name Systems (DNS), streaming
media applications as IPTV, VoIP, and online games. Both protocols use ports for application-
to-application communication. A port is a logical access point represented by a positive 16-bit
integer value. Each service offered by a computer is uniquely identified by a port number. Each
Internet packet contains both destination IP address and the port to which the request is to be
delivered. Some ports are reserved for common services such as Ğp, telnet, smtp, hĴp, hĴps, and
login. Port numbers >=1024 are generally used for user-level services.
Sockets provide an interface for programming networks at the transport layer. Using sockets,
network communication is very much similar to performing file I/O. A socket is an endpoint
of a two-way communication link between two programs running on the network. The source
and destination IP address, and the port numbers constitute a network socket. Two key classes
from java.net package used in creation of server and client programs are ServerSocket,
which represents a server socket, and Socket, an instantiation of which performs the
actual communication with the client. Datagram communication is also supported through
DatagramPacket and DatagramSocket classes. Writing and reading data between server and
the client is also supported through the URLconnection class.
13.8 EXERCISES
Objective Questions
13.1 ______ is a connection-oriented and reliable protocol, ______ is a less reliable protocol.
13.2 The TCP and UDP protocols use ______ to map incoming data to a particular process running on a
computer.
13.3 Datagrams are normally sent by ______ protocol.
13.4 Java uses ______ class representing a server and ______ class representing the client that uses TCP
protocol.
13.5 ______ is used to wait for accepting client connection requests.
13.6 Class ______ is used to create a packet of data for UDP protocol.
13.7 If something goes wrong related to the network, ______ will be thrown when dealing with TCP/
UDP programming in Java.
13.8 The main purpose of URL encoding is to maintain the ______ between various platforms.
13.9 Based on the URL encoding mechanism, “www.test.com/test me&test you” becomes ______.

13.10 ______ method is used to instantiate a URLConnection instance.
13.11 UDP is more reliable than TCP protocol: True or False.
13.12 The same port number can be reused many times when binding with sockets simultaneously: True
or False.
13.13 In order to create a client socket connecting to a particular server, the IP address must be given to
the client socket, otherwise, it cannot connect to the server: True or False.
13.14 Sockets provide an interface for programming networks at the transport layer: True or False.
13.15 Call Socket.close() method will close the TCP server that socket connects to: True or False.
13.16 The socket instance does not need to be explicitly closed to release all the resources it occupies, as
the Java Garbage Collection mechanism will do it automatically: True or False
13.17 The following line of code
Socket socket = new Socket(“localhost,” 1254);
will create a TCP server at localhost port 1254: True or False.
13.18 Java TCP Socket uses the InputStream/OutputStream to read/write data to the network channel:
True or False.
13.19 Java UDP sends/receives data as raw Java primitive types: True or False.
13.20 URL Encoding is useful for translating Chinese to English that presents in a particular URL: True
or False.
Review Questions
13.21 Explain the client-server architecture via a simple example.
13.22 Explain the TCP/IP stacks. Brieﬂy explain the TCP and UDP protocols and the difference between
the two protocols.
13.23 What is port? List some well-known ports and explain the applications associated with them.
13.24 Discuss the process of creation of server and client sockets with exceptions handled explicitly with
a suitable example.
13.25 What are URN, URL, and URI. Explain the differences between them.
13.26 Explain why the URL encoding is very important to web applications.
Programming Problems
13.27 Write a simple program that can read a host name and convert it to an IP address.
13.28 Write a URL-based program that pulls content from www.buyya.com.
13.29 Write a ping-pong client and server application. When a client sends a ping message to the server,
the server will respond with a pong message. Other messages sent by the client can be safely
dropped by the server.
13.30 Rewrite the math server application, instead of using the TCP socket, use the UDP datagram
socket.
13.31 Write an online dictionary application, users are able to search online dictionary and get the
meaning of the words. The online dictionary should maintain its indexing in order to enhance the
performance and scalability.
13.32 Write a simple crawler program that can parse certain HTML information and ﬁnd out the
hyperlinks within the HTML. (Use the URLConnection class to implement this application).
13.33 Write a Socket-based Java server program that responds to client messages as follows: When it
receives a message from a client, it simply converts the message into all uppercase letters and sends
back the same to the client. Write both client and server programs demonstrating this.

Socket programming

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Socket programming