Networking

What you'll learn

Apps rarely work alone — they fetch data from APIs, talk to servers, and stream content. This module covers networking in Java: the concepts, the modern HttpClient for calling web APIs, and raw sockets for custom protocols.

By the end you'll be able to:

• Understand IP addresses, ports, TCP, and UDP
• Call REST APIs with the Java HttpClient (GET and POST)
• Handle JSON responses
• Build TCP client/server apps with sockets
• Know when UDP is the right tool

Networking fundamentals

Every device on a network has an IP address; each program communicates through a port number. Two protocols carry the data, and choosing between them is a fundamental trade-off of reliability versus speed:

On top of TCP sits HTTP — the protocol of the web and of REST APIs. A URL identifies a resource, and an HTTP request/response carries a method (GET, POST…), headers, a status code, and a body.

The Java HTTP Client

Since Java 11, the built-in HttpClient makes calling web APIs clean and modern (no external library needed). Build a request, send it, and read the response's status code and body:

import java.net.http.*;
import java.net.URI;

HttpClient client = HttpClient.newHttpClient();

HttpRequest request = HttpRequest.newBuilder()
    .uri(URI.create("https://api.example.com/users/1"))
    .GET()
    .build();

HttpResponse<String> response =
    client.send(request, HttpResponse.BodyHandlers.ofString());

System.out.println("Status: " + response.statusCode());
System.out.println(response.body());

POST & JSON

To send data, use POST (or PUT) with a body and the right Content-Type header. A text block keeps the JSON readable:

String body = """
    {"name": "Sara", "email": "sara@example.com"}
    """;

HttpRequest request = HttpRequest.newBuilder()
    .uri(URI.create("https://api.example.com/users"))
    .header("Content-Type", "application/json")
    .POST(HttpRequest.BodyPublishers.ofString(body))
    .build();

HttpResponse<String> res =
    client.send(request, HttpResponse.BodyHandlers.ofString());
System.out.println(res.statusCode());   // 201 Created

Sockets (TCP)

For custom protocols or direct machine-to-machine communication, drop down to sockets. A ServerSocket listens on a port and accept()s incoming connections; a client Socket connects to a host and port. Once connected, you read and write through ordinary streams:

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

// --- SERVER: listens on port 5000 ---
try (ServerSocket server = new ServerSocket(5000)) {
    System.out.println("Waiting for a client...");
    Socket client = server.accept();          // blocks until someone connects

    var in  = new BufferedReader(
        new InputStreamReader(client.getInputStream()));
    var out = new PrintWriter(client.getOutputStream(), true);

    out.println("Hello from the server!");
    System.out.println("Client said: " + in.readLine());
}

UDP

When speed matters more than guaranteed delivery — live video, online games, telemetry — UDP sends independent packets (DatagramPacket) through a DatagramSocket with no connection setup. Packets may arrive out of order or not at all, and that's an acceptable trade for the lower latency.

Recap & quick check

Great — your programs can now talk to the wider world. Next up: Module 25 — JSON, XML & Data Formats.