Android Code Snippet Download file via HTTP with resume

'it's FTP structure just for clear understanding the code
Type FTPEntry2(Name As String, Timestamp As Long, Size As Long, FTPpath As String, LocalPath As String, status As String, DeleteAfter As Boolean)    'writable file info structure (FTP FTPEntry structure is read-only)
...

Sub Global_Download_File(f As FTPEntry2)
    If Not(Starter.InternetConnected) Then Return
    If f.Size < BIGFILESIZE/10 And Not(f.status.ToLowerCase.Contains("error")) Then   'small file is downloaded via FTP as main way
        FTP1.DownloadFile(f.FTPpath, False, others.GetPath(f.LocalPath), f.Name)
        Return     
    End If
    'dowloading big files over HTTP with resume
    Dim su As StringUtils
    Dim APIURL As String = "https://webserver/api.php?username=" & su.EncodeUrl(Starter.DeviceID, "UTF8") & "&download=" & su.EncodeUrl(f.FTPpath, "UTF8")
    Dim j As HttpJob
    j.Initialize(f.Name, Me)
 
    Dim fsize As Long
    If File.Exists(f.LocalPath, "") Then
        fsize = File.Size(f.LocalPath, "")
    End If
 
    'fistly request the HEAD to get the file size to compare later after downloading
    j.Head(APIURL)
    Wait For (j) JobDone(j As HttpJob)
    If Not(j.Success) Then
        Dim StatusCode As String = j.Response.StatusCode
        others.AddToLog("HTTP HEAD error: " & StatusCode & "; " & j.JobName)
        j.Release
        CallSubDelayed(Me, "Check_AndDownload")
        Return
    End If
    Dim headers As Map = j.Response.GetHeaders
    Dim L As List = headers.Get("content-length")
    Dim head_size As Long = L.Get(0)    'full size of the file to compare after downloading
    If head_size = 0 Then
        others.AddToLog("HTTP HEAD zero size: " & f.FTPpath)
        CallSubDelayed(Me, "Check_AndDownload")
        Return
    End If
 
    'now GETting the file---------------------------
    If fsize > head_size Then    're-downloading
        File.Delete(f.LocalPath, "")
        fsize = 0
    End If
    j.Download(APIURL): j.GetRequest.SetHeader("Range", "bytes=" & fsize & "-")
    Log("Downloading via HTTP: " & f.FTPpath)
    Wait For (j) JobDone(j As HttpJob)
    If Not(j.Success) Then
        Dim StatusCode As String = j.Response.StatusCode
        others.AddToLog("HTTP GET error: " & StatusCode & "; " & f.FTPpath)
        f.status = "download_error"
        Dim out As OutputStream = File.OpenOutput(others.GetPath(f.LocalPath), f.Name, True)   'True !!!
        File.Copy2(j.GetInputStream, out)
        out.Close '<------ very important
        j.Release
        If Starter.InternetConnected Then
            If Not(StatusCode.StartsWith("4")) Then    'client errors
                Log("resume HTTP downloading: " & f.FTPpath)
                CallSubDelayed2(Me, "Global_Download_File", f)    'resume download
                Return
            End If
        End If
        CallSubDelayed(Me, "Check_AndDownload")
        Return
    End If
    f.status = "OK"
    Dim out As OutputStream = File.OpenOutput(others.GetPath(f.LocalPath), f.Name, True)   'True !!!
    File.Copy2(j.GetInputStream, out)
    out.Close '<------ very important
    j.Release

    fsize = File.Size(f.LocalPath, "")    'current local file's size after download
    If fsize = head_size Then    'fully downloaded
        f.status = "downloaded_ok"
        CallSubDelayed3(Me, "FTP1_DownloadCompleted", f.FTPpath, True)    'finish processing file
    Else    'resume downloading
        f.status = "error_interrupted_download"
        If Starter.InternetConnected Then
            CallSubDelayed2(Me, "Global_Download_File", f)    'resume downloading
        End If
    End If
End Sub