以http下载为例
1.建立连接
String urlStr = "下载地址url"
URL url = new URL(urlStr);
HttpURLConnection con = (HttpURLConnection)url.openConnection();
2.设置连接属性
String sProperty = "bytes=" + [int]开始下载的字节处（bytes） + "-";
con.setRequestProperty("RANGE", sProperty);
3.随机写入
File file = new File("下载路径+下载文件名");
RandomAccessFile randomAccessFile = new RandomAccessFile(file, "rwd");
randomAccessFile.seek(int[上一次下载的字节处])
InputSream is = con.getInputStream();
if(206 == con.getResponseCode()){//206是http断点续传返回的正常结果码，不再是200
//读入
byte[] buffer = new byte[1024];
int len = 0;
while(len = is.read(buf) != -1){
randomAccessFile.write(buf, 0, len);
//下载进度等等
}
}
randomAccessFile.close();
con.disconnect();
至于开子线程下载，保存上次下载的字节大小等就不列出了，上面写的连伪代码都算不上呵呵