Java Sockets

// Server side: bind to a port and wait for a connection
ServerSocket serverSocket = new ServerSocket(3000);
Socket connection = serverSocket.accept(); // blocks until a client connects

// Client side: connect to the server
Socket socket = new Socket("localhost", 3000);

ServerSocket server1 = new ServerSocket(3000); // ok - port 3000 is now claimed
ServerSocket server2 = new ServerSocket(3000); // throws java.net.BindException: Address already in use

Socket socket = new Socket("localhost", 3000);

// Text-based: good for simple line-by-line communication
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
out.println("Hello!");
String response = in.readLine(); // blocks until a full line arrives

// Object-based: used from Part 1 of the MCCS series onward
// Always create ObjectOutputStream first on both sides to avoid deadlock
ObjectOutputStream objOut = new ObjectOutputStream(socket.getOutputStream());
ObjectInputStream objIn = new ObjectInputStream(socket.getInputStream());
objOut.writeObject("Hello!");
String received = (String) objIn.readObject(); // blocks until an object arrives

// All three of these pause the current thread until they complete:
Socket client = serverSocket.accept();  // waits for a client to connect
String data   = (String) in.readObject(); // waits for incoming data
String line   = scanner.nextLine();     // waits for the user to type

// Solution: run each blocking call in its own thread
new Thread(() -> {
    String msg = (String) in.readObject();
    System.out.println("Received: " + msg);
}).start();
// the main thread continues here immediately