POP3Client.java

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

public class POP3Client {

    private static final int DEFAULT_PORT = 110;
    private static final int TIMEOUT_MS = 15_000;

    private Socket socket;
    private BufferedReader reader;
    private PrintWriter writer;
    private boolean connected = false;

    public void connect(String host, int port) throws IOException {
        socket = new Socket();
        socket.connect(new InetSocketAddress(host, port), TIMEOUT_MS);
        socket.setSoTimeout(TIMEOUT_MS);
        reader = new BufferedReader(new InputStreamReader(socket.getInputStream(), "UTF-8"));
        writer = new PrintWriter(new OutputStreamWriter(socket.getOutputStream(), "UTF-8"), true);

        String greeting = reader.readLine();
        if (greeting == null || !greeting.startsWith("+OK"))
            throw new IOException("Bad greeting: " + greeting);
        connected = true;
        System.out.println("[POP3] Connected: " + greeting);
    }

    public void connect(String host) throws IOException {
        connect(host, DEFAULT_PORT);
    }

    public void disconnect() {
        if (!connected)
            return;
        try {
            sendCommand("QUIT");
            socket.close();
        } catch (IOException ignored) {
        } finally {
            connected = false;
        }
    }

    public void authenticate(String username, String password) throws IOException {
        requireOK(sendCommand("USER " + username), "USER");
        requireOK(sendCommand("PASS " + password), "PASS");
        System.out.println("[POP3] Authenticated as " + username);
    }

    public int[] stat() throws IOException {
        String r = sendCommand("STAT");
        requireOK(r, "STAT");
        String[] p = r.split("\\s+");
        return new int[] { Integer.parseInt(p[1]), Integer.parseInt(p[2]) };
    }

    public Map<Integer, Integer> list() throws IOException {
        Map<Integer, Integer> result = new LinkedHashMap<>();
        sendRaw("LIST");
        requireOK(reader.readLine(), "LIST");
        String line;
        while ((line = reader.readLine()) != null && !line.equals(".")) {
            String[] p = line.split("\\s+");
            result.put(Integer.parseInt(p[0]), Integer.parseInt(p[1]));
        }
        return result;
    }

    public String top(int msgNum, int lines) throws IOException {
        sendRaw("TOP " + msgNum + " " + lines);
        requireOK(reader.readLine(), "TOP");
        return readMultiLine();
    }

    public String retr(int msgNum) throws IOException {
        sendRaw("RETR " + msgNum);
        requireOK(reader.readLine(), "RETR");
        return readMultiLine();
    }

    public void dele(int msgNum) throws IOException {
        requireOK(sendCommand("DELE " + msgNum), "DELE");
    }

    public Map<Integer, String> uidl() throws IOException {
        Map<Integer, String> result = new LinkedHashMap<>();
        sendRaw("UIDL");
        requireOK(reader.readLine(), "UIDL");
        String line;
        while ((line = reader.readLine()) != null && !line.equals(".")) {
            String[] p = line.split("\\s+");
            result.put(Integer.parseInt(p[0]), p[1]);
        }
        return result;
    }

    public void rset() throws IOException {
        requireOK(sendCommand("RSET"), "RSET");
    }

    public void noop() throws IOException {
        requireOK(sendCommand("NOOP"), "NOOP");
    }

    private String sendCommand(String cmd) throws IOException {
        sendRaw(cmd);
        return reader.readLine();
    }

    private void sendRaw(String cmd) throws IOException {
        writer.print(cmd + "\r\n");
        writer.flush();
    }

    private String readMultiLine() throws IOException {
        StringBuilder sb = new StringBuilder();
        String line;
        while ((line = reader.readLine()) != null) {
            if (line.equals("."))
                break;
            if (line.startsWith(".."))
                line = line.substring(1);
            sb.append(line).append("\r\n");
        }
        return sb.toString();
    }

    private void requireOK(String response, String cmd) throws IOException {
        if (response == null || !response.startsWith("+OK"))
            throw new IOException("Command '" + cmd + "' failed: " + response);
    }

    public boolean isConnected() {
        return connected;
    }

}