利用线程将网络指定的图片下载到本地，将文件分四次下载，结果并未实现QAQ求大神指教下

import java.io.File;import java.io.IOException;import java.io.InputStream;import java.io.RandomAccessFile;import java.net.HttpURLConnection;import java.net.MalformedURLException;import java.net.URL;public class FileDownload extends Thread{private static String source;private static File target;private long start;private long end;static HttpURLConnection con; static URL url;public FileDownload(String source, File target, long start, long end) {super();this.source = source;this.target = target;this.start = start;this.end = end;}@Overridepublic void run() {int count = 0;InputStream input = null;RandomAccessFile output = null;long i = 0 ;try {int code = con.getResponseCode();if(code == HttpURLConnection.HTTP_OK){input = con.getInputStream();output = new RandomAccessFile(new File(target,"plmm.jpg"), "rw");//input.skip(start);output.seek(start);byte[] b = new byte[128];int len = 0;while((len = input.read(b))!=-1&&count<=(end - start)){count += len ;output.write(b, 0, len);i = output.getFilePointer();}} }catch (IOException e) {e.printStackTrace();}finally{try {if(input != null)input.close();if(output != null)output.close();} catch (IOException e) {e.printStackTrace();}}}public static void main(String[] args) throws IOException {source = "http://192.168.46.254:8888/easyBuy/images/plmm.jpg";target = new File("C:\\Documents and Settings\\Administrator\\桌面");url = new URL(source);con = (HttpURLConnection)url.openConnection();long item = con.getContentLength()/4;for(int i = 0;i < 4 ; i++){new FileDownload(source,target,i*item,(i+1)*item).start();}}}