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How would you add custom HTTP headers (like an Authorization bearer token) to an outgoing request using HttpRequest.Builder? How would you make an asynchronous, non-blocking HTTP request using HttpClient's sendAsync() method, and how does the returned CompletableFuture integrate with the rest of an asynchronous pipeline? How would you configure HttpClient with custom timeouts, a proxy, and connection pooling behavior, and what's the difference between a connection timeout and a request/response timeout? How would you send a POST request with a JSON request body using HttpClient, and correctly parse a JSON response using a library like Jackson? How would you send URL-encoded form data (application/x-www-form-urlencoded) using HttpClient, as opposed to a JSON request body? How would you use InetAddress to perform DNS resolution (converting a hostname to an IP address) programmatically, and what's the difference between InetAddress.getByName() and getAllByName()? How does HttpClient handle automatic following of HTTP redirects (3xx responses), and how would you configure or disable this behavior? How would you implement a custom BodySubscriber or BodyHandler to stream a very large HTTP response body incrementally, rather than buffering the entire response into memory at once? How would you implement retry logic with exponential backoff for HTTP requests made via HttpClient, given the client doesn't provide built-in retry support natively? How would you implement a WebSocket client using java.net.http.WebSocket for real-time, bidirectional communication, and how does its event-driven Listener interface work?
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What is the java.net.Socket and ServerSocket class pair, and how would you implement a basic TCP client-server communication using them at a lower level than HttpClient?

Intermediate

Answer

Socket represents a client-side TCP connection endpoint, and ServerSocket represents a server-side listener that accepts incoming client connections (accept() blocks until a client connects, returning a Socket representing that specific connection) -- both provide raw InputStream/OutputStream access for reading/writing bytes directly over the established TCP connection, representing a much lower-level networking API than HttpClient (which builds an entire HTTP protocol layer on top of raw sockets), appropriate when you need to implement a custom, non-HTTP protocol or need direct low-level control over the TCP connection itself.
// Server
ServerSocket serverSocket = new ServerSocket(8080);
Socket clientSocket = serverSocket.accept();  // blocks until a client connects
BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
String message = in.readLine();

// Client
Socket socket = new Socket("localhost", 8080);
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
out.println("Hello, server!");
Real-world example A custom, lightweight internal protocol between two in-house microservices (not needing full HTTP semantics) is implemented directly using raw Socket/ServerSocket, avoiding the overhead of a full HTTP request/response cycle for a very simple, high-frequency, low-latency internal communication need.

Common follow-ups: Why would you choose raw sockets over HttpClient or gRPC for a custom protocol, given both provide more built-in structure?;How does this classic blocking Socket API relate to NIO's non-blocking SocketChannel discussed earlier?

gRPC Services;I/O & NIO
All 15 Java Networking & HTTP Client questions