Java Networking Basics

+--------+                    +--------+
| CLIENT | <--- Network --->  | SERVER |
| Socket |                    | Socket |
+--------+                    +--------+
   Sends request           Listens, responds

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

public class SimpleServer {
    public static void main(String[] args) throws IOException {
        ServerSocket server = new ServerSocket(5000);
        System.out.println("Server listening on port 5000...");

        Socket client = server.accept(); // Waits for connection
        System.out.println("Client connected!");

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

        String message = in.readLine();
        System.out.println("Client says: " + message);
        out.println("Server received: " + message);

        client.close();
        server.close();
    }
}

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

public class SimpleClient {
    public static void main(String[] args) throws IOException {
        Socket socket = new Socket("localhost", 5000);

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

        out.println("Hello Server!");
        System.out.println("Server replied: " + in.readLine());

        socket.close();
    }
}

import java.net.*;

URL url = new URL("https://api.example.com/users?page=1");
System.out.println("Protocol: " + url.getProtocol()); // https
System.out.println("Host: " + url.getHost());           // api.example.com
System.out.println("Path: " + url.getPath());           // /users
System.out.println("Query: " + url.getQuery());         // page=1

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

URL url = new URL("https://jsonplaceholder.typicode.com/posts/1");
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");

BufferedReader reader = new BufferedReader(
    new InputStreamReader(conn.getInputStream()));
String line;
StringBuilder response = new StringBuilder();
while ((line = reader.readLine()) != null) {
    response.append(line);
}
reader.close();
System.out.println(response.toString());