private class ImageDownloadTask extends AsyncTask<String,Integer,byte[]> {
    @Override
    protected byte[] doInBackground(String... params) {
        try {
            URL url = new URL(params[0]);
            URLConnection connection = url.openConnection();
            InputStream inputStream = connection.getInputStream();
            ByteArrayOutputStream byteBuffer = new ByteArrayOutputStream();
            int bufferSize = 1024;
            byte[] buffer = new byte[bufferSize];

            int len;
            while ((len = inputStream.read(buffer)) != -1) {
                byteBuffer.write(buffer,len);
            }
            return byteBuffer.toByteArray();
        } catch (IOException ex) {
            return new byte[0];
        }
    }
}

我想用它来下载图片.

public byte[] getProfilePicture(Context context,String id) {
    String url = context.getString(R.string.facebook_picture_url_large,id);
    ImageDownloadTask task = new ImageDownloadTask();
    return task.doInBackground(url);
}