我正在开发一个类似idm的下载程序，我读过这篇文章。我已经实现了我的第一步代码。
下面是downloader类的代码：

package download.manager;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import java.nio.channels.ReadableByteChannel;
import java.util.logging.Level;
import java.util.logging.Logger;

public class Downloader implements Runnable{

    private String url;
    private int num;
    private long start;
    private long end;
    ReadableByteChannel rbc;

    public Downloader(String url, int num, long start, long end, ReadableByteChannel rbc) {
        this.url = url;
        this.num = num;
        this.start = start;
        this.end = end;
        this.rbc = rbc;
    }        

    @Override
    public void run() {
        download();
    }

    private void download(){
        try {
            System.out.println(num + " is executing");
            URL file = new URL(url);
            FileOutputStream stream = new FileOutputStream("tmp"+num);                        
            stream.getChannel().transferFrom(rbc, start, end);
        } catch (MalformedURLException ex) {
            Logger.getLogger(Downloader.class.getName()).log(Level.SEVERE, null, ex);
        } catch (IOException ex) {
            Logger.getLogger(Downloader.class.getName()).log(Level.SEVERE, null, ex);
        }

    }

}

这是我的主要职责：

package download.manager;

import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import java.nio.channels.Channels;
import java.nio.channels.ReadableByteChannel;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 *
 * @author Behzad
 */
public class DownloadManager {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {

        URL file = null;
        ReadableByteChannel rbc = null;
        try {                       
            String url = "http://dl1.video.varzesh3.com/video/clip93/12/video/havashi/top5_save_derby_dortmond.mp4";
            file = new URL(url);
            rbc = Channels.newChannel(file.openStream());
            int size = file.openConnection().getContentLength(); 
            ExecutorService pool = Executors.newFixedThreadPool(4);
            int partSize = size / 4;
            pool.submit(new Downloader(url, 1, 0, partSize, rbc));
            pool.submit(new Downloader(url, 2, partSize, partSize, rbc));
            pool.submit(new Downloader(url, 3, 2 * partSize, partSize, rbc));
            pool.submit(new Downloader(url, 4, 3 * partSize, partSize, rbc));            
            pool.shutdown();
            pool.awaitTermination(Long.MAX_VALUE, TimeUnit.MILLISECONDS);
        } catch (MalformedURLException | InterruptedException ex) {
            Logger.getLogger(DownloadManager.class.getName()).log(Level.SEVERE, null, ex);
        } catch (IOException ex) {
            Logger.getLogger(DownloadManager.class.getName()).log(Level.SEVERE, null, ex);
        }

    }

}

但是当我运行这个代码时，下载程序只下载文件的第一部分。正如你在照片上看到的

我该怎么办？
以下是更新的下载方法：

private void download(){
        try {
            System.out.println(num + " is executing");
            URL file = new URL(url);
            ReadableByteChannel rbc = Channels.newChannel(file.openStream());
            FileOutputStream stream = new FileOutputStream("tmp"+num);                        
            stream.getChannel().transferFrom(rbc, start, end);
        } catch (MalformedURLException ex) {
            Logger.getLogger(Downloader.class.getName()).log(Level.SEVERE, null, ex);
        } catch (IOException ex) {
            Logger.getLogger(Downloader.class.getName()).log(Level.SEVERE, null, ex);
        }

    }