FileSender.java

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.net.Socket;

/**
 * Sends a file to the server over the dedicated file transfer port (chatPort + 1).
 * Protocol:
 *   First line: FILE <filename> <size> <targetUserOrStar>
 *   Followed immediately by <size> raw bytes of the file.
 */
public class FileSender {
    private final String host;
    private final int filePort;

    public FileSender(String host, int filePort) {
        this.host = host;
        this.filePort = filePort;
    }

    public void sendFile(String targetUserOrStar, String path) throws IOException {
        File file = new File(path);
        if (!file.exists() || !file.isFile()) {
            throw new IOException("File not found: " + path);
        }
        String filename = file.getName();
        long size = file.length();

        try (Socket socket = new Socket(host, filePort);
             PrintWriter headerOut = new PrintWriter(socket.getOutputStream(), true);
             OutputStream dataOut = socket.getOutputStream();
             FileInputStream fis = new FileInputStream(file)) {

            headerOut.println("FILE " + filename + " " + size + " " + targetUserOrStar);
            headerOut.flush();

            byte[] buf = new byte[8192];
            long sent = 0;
            int read;
            while ((read = fis.read(buf)) != -1) {
                dataOut.write(buf, 0, read);
                sent += read;
            }
            dataOut.flush();
            if (sent != size) {
                throw new IOException("Mismatch in sent size. Expected " + size + " but sent " + sent);
            }
        }
    }
}