Client

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

public class FileClient {
    static Socket clientSock;
    static int fileSize;
    static final int MAXTHREAD = 3;
    static DownloadThread[] dlThread;
    public static void main(String[] args) {
        if(!init()) return;
        if(!getFileSize()) return;
        if(!initThread()) return;
    }

    public static boolean init() {
        try {
            clientSock = new Socket("127.0.0.1", 13267);
        } catch (IOException e) {
            System.out.println(e.getLocalizedMessage());
            return false;
        }
        dlThread = new DownloadThread[MAXTHREAD];
        System.out.println("Connecting...");
        return true;
    }

    public static boolean getFileSize(){
        try {
            BufferedReader in = new BufferedReader(new InputStreamReader(clientSock.getInputStream()));
            String line =in.readLine();
            fileSize = Integer.parseInt(line);
        } catch (IOException e) {
            System.out.println(e.getLocalizedMessage());
            disconnected();
            return false;
        }
        return true;
    }

    public static boolean initThread()
    {
        int start = 0;
        int end = 0;
        for (int i = 0; i < dlThread.length; i++) {
            if(i==0)  { start = 1; end = (fileSize/MAXTHREAD); }
            else {start = end + 1; end = end + (fileSize/MAXTHREAD);}
            try {
                dlThread[i] = new DownloadThread(new Socket("127.0.0.1", 13267), i, start, end);
            } catch (IOException e) {
                System.out.println(e.getLocalizedMessage());
                disconnected();
                return false;
            }
            new Thread(dlThread[i]).start();
        }
        return true;
    }

    public static void disconnected() {
        try {
            clientSock.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

class DownloadThread implements Runnable {
    private int BufferSize;
    private byte[] byteArr;
    private Socket client;
    private int id;
    private int start;
    private int end;
    private OutputStream out;
    private InputStream in;
    private FileOutputStream fos;
    private BufferedOutputStream bos;
    // Constructor
    DownloadThread(Socket client,int id,int start,int end) {
        this.client = client;
        this.id = id;
        this.start = start;
        this.end = end;
        this.BufferSize = (start + end)-1;
    }

    public boolean init() {
        try {
            out = client.getOutputStream();
            in = client.getInputStream();
            byteArr = new byte[BufferSize];
            fos = new FileOutputStream("buffer.p"+id);
            bos = new BufferedOutputStream(fos);
        } catch (IOException e) {
            System.out.println(e.getLocalizedMessage());
            disconnected();
            return false;
        }
        return true;
    }

    public void disconnected() {
        try {
            client.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public boolean sendFileRequest() {
        String msg = start + " " + end + "\n";

        try {
            out.write(msg.getBytes());
        } catch (IOException e) {
            System.out.println(e.getLocalizedMessage());
            disconnected();
            return false;
        }
        return true;
    }

    public boolean receiveFile() {
        int current = 0;
        try {
            int bytesRead = in.read(byteArr,0,byteArr.length);
            System.out.println("Receive file from : " + client);
            current = bytesRead;
            do {
                 bytesRead =
                 in.read(byteArr, current, (byteArr.length-current));
                 if(bytesRead > 0) current += bytesRead;
            } while(bytesRead > 0);
            in.close();
            bos.write(byteArr, 0 , current);
            bos.flush();
            bos.close();
        } catch (IOException e) {
            System.out.println(e.getLocalizedMessage());
            disconnected();
            return false;
        }
        return true;

    }

    public void run() {
        init();
        sendFileRequest();
        receiveFile();
        disconnected();
    }

}