Network programming in Java involves three core components: IP addresses, port numbers, and communication protocols.

IP Address

An IP address uniquely identifies a device on a network. The loopback address 127.0.0.1 represents the local machine.

Port Number

Port numbers distinguish between different applications on a device. Valid ports range from 0 to 65535, with 0–1024 typically reserved for system use.

Communication Protocols

Protocols define rules for data exchenge. Key transport layer protocols are TCP and UDP.

TCP and UDP Protocols

TCP (Transmission Control Protocol)

• Connection-oriented: requires establishing a connection before data transfer.
• Reliable: uses a three-way handshake for connection setup and four-way handshake for termination.
• Supports large data volumes but has higher overhead due to connection management.

UDP (User Datagram Protoocl)

• Connectionless: sends data in packets without prior connection establishment.
• Unreliable but faster, with each datagram limited to 64 KB.

TCP Programming with Sockets

Sockets enable communication between applications over a network, treating the connection as a stream.

Key Socket Methods

Method	Description
void close()	Closes the socket.
InputStream getInputStream()	Retrieves input stream for reading data.
OutputStream getOutputStream()	Retrieves output stream for writing data.

ServerSocket Methods

Method	Description
Socket accept()	Listens for and accepts client connections.
void close()	Closes the server socket.

Client-Side Proces

1. Create a Socket object with server IP and port.
2. Obtain input/output streams using getInputStream() and getOutputStream().
3. Read from and write to the streams.
4. Close the socket.

Example client code:

Socket clientSocket = new Socket("192.168.1.10", 8080);
OutputStream output = clientSocket.getOutputStream();
output.write("data".getBytes());
clientSocket.close();

Server-Side Process

1. Create a ServerSocket bound to a port.
2. Call accept() to wait for and accept client connections.
3. Use the returned Socket to get streams for data exchange.
4. Close sockets after communication.

Example server code:

ServerSocket server = new ServerSocket(8080);
Socket connection = server.accept();
InputStream input = connection.getInputStream();
byte[] buffer = new byte[1024];
int bytesRead = input.read(buffer);
String message = new String(buffer, 0, bytesRead);
connection.close();
server.close();

UDP Programming with Datagrams

UDP uses DatagramSocket and DatagramPacket for packet-based communication.

Process

• Create sender and receiver DatagramSocket instances.
• Package data into DatagramPacket objects with target address and port.
• Send or receive packets.
• Close the socket.

Example sender:

DatagramSocket senderSocket = new DatagramSocket();
byte[] dataBytes = "message".getBytes();
DatagramPacket packet = new DatagramPacket(dataBytes, dataBytes.length, InetAddress.getByName("localhost"), 9000);
senderSocket.send(packet);
senderSocket.close();

Example receiver:

DatagramSocket receiverSocket = new DatagramSocket(9000);
byte[] buffer = new byte[1024];
DatagramPacket receivedPacket = new DatagramPacket(buffer, buffer.length);
receiverSocket.receive(receivedPacket);
String content = new String(receivedPacket.getData(), 0, receivedPacket.getLength());
receiverSocket.close();