Networking in java is a concept of connecting computing devices to each other so that user can share resources.

The java.net package of the java contains a group of interfaces and classes that provide the low-level communication details.

The two common network protocols:

• TCP:  TCP(Transmission Control Protocol) allows reliable communication between two applications. TCP is typically used over Internet Protocol which is known to as TCP/IP.
• UDP: UDP(User Datagram Protocol) a connection-less protocol allows for packets of data to be transmitted between applications.

 
image source: Oracle.com

We will explore the following two topics in this tutorial

• Socket Programming: provides an option to share data between different computing devices.
• Working with URLs: URL stands for Uniform Resource Locator

Socket programming

A socket is one end-point of a two-way communication link between two programs running on the network on different machines. A network can either be LAN or WAN etc. The java.net package provides two classes

1. Socket and
2. ServerSocket 

These 2 classes implement the client side of the connection on one machine and the server side of the connection on server machine, respectively.

Socket programming is used to establish a communication between the devices running on different JRE. Socket programming can be connection-less or connection-oriented.

The client in socket programming required:

• Port number
• Server IP Address

Socket class:

A socket is an endpoint between the machines for communication. These Socket classes mainly used for socket creation, The Socket class methods are:

• public InputStream getInputStream():to get the InputStream attached with socket.
• public OutputStream getOutputStream():to get the OutputStream attached with socket.
• public synchronized void close():to close the socket

ServerSocket class:

This class can be used for a server socket creation. To establish communication with the clients by usingthis object

The ServerSocket class methods are:

• public Socket accept(): which returns the establish connection between server and client and also socket.
• public synchronized void close(): to close the server socket
• publicintgetLocalPort()
  Returns the port that the server socket is listening on.

Example:

Java Socket Programming

ServerExample.java

import java.io.DataInputStream;
import java.net.ServerSocket;
import java.net.Socket;

public class ServerExample {

public static void main(String[] args) {
try {
ServerSocket soc = newServerSocket(6666);
Socket sa = soc.accept(); //To establishes connection
DataInputStream dis = newDataInputStream(sa.getInputStream());
String msg = (String) dis.readUTF();
System.out.println("The message= " + msg);
soc.close();
} catch (Exception e) {
System.out.println(e);
}
}
}

ClientExample.java

import java.io.DataOutputStream;
import java.net.Socket;

public class ClientExample {

public static void main(String[] args) {
try {
Socket soc = new Socket("localhost", 6666);
DataOutputStream dos = new DataOutputStream(soc.getOutputStream());
dos.writeUTF("Server...");
dos.flush();
dos.close();
soc.close();
} catch (Exception e) {
System.out.println(e);
}
}
}

Open two command prompts to execute this program. Compile the program using “javac” command, and run the program by using “java” command as shown below. After executing the client application, on the server console a message will be displayed.

URL myURL = new URL("https://www.subjectcoach.com/");

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

public class URLExample
{
public static void main(String [] args)
{
try
{
URL url = new URL("https://www.subjectcoach.com#testref");
System.out.println("URL value " + url.toString());
System.out.println("Protocol value "
+ url.getProtocol());
         System.out.println("Default Port: "+url.getDefaultPort());
System.out.println("Authority is "
+ url.getAuthority());
System.out.println("Resource Name: " + url.getFile());
System.out.println("Host: " + url.getHost());
System.out.println("Port: " + url.getPort());
System.out.println("Query String: " + url.getQuery());
System.out.println("Ref: " + url.getRef());
}catch(IOException e)
{
e.printStackTrace();
}
}
}

import java.net.*;
import java.io.*;
public class file
{
public static void main(String [] args)
{
try
{
URL url = new URL("http://www.google.com");
URLConnection urlConnection = url.openConnection();
HttpURLConnection connection = null;
if(urlConnection instanceof HttpURLConnection)
{
connection = (HttpURLConnection) urlConnection;
}
else
{
throw new Exception("Invalid URL");
}
BufferedReader in = new BufferedReader(
new InputStreamReader(connection.getInputStream()));
String pageContents = "";
String line;
while((line = in.readLine()) != null)
{
pageContents += line;
}
System.out.println(pageContents);
}catch(Exception e)
{
e.printStackTrace();
}
}
}