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I have a problem trying to decompress (in Java) a string that was compressed in Visual Basic, using DeflateStream. I did a little bit of research and saw and tried a lot of code that decompresses in Java using Inflater and InflaterOutputStream. None of the codes found succeeded. I looked over a lot of sites, and searched on google a lot of ways. I can say that the first 3-pages from google that concern java, inflater/deflater compression, visual basic, deflaterStream are already read, without success. I want to know if this thing is possible (and it should be since I have found a way of compressing/decompressing between Java and C# here : http://www.dreamincode.net/forums/topic/243808-compression-and-decompression-between-java-and-c%23/ ) I would also like to know if there are compatibility issues regarding Visual Basic DeflateStream and Java analog classes Inflater/Deflater. If someone had a similar problem I would highly appreciate also some code snippets.

Thanks, Mihai

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3 Answers 3

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Try setting the nowrap parameter in the Inflater class.

Per the documentation, the .NET DeflateStream produces the raw deflate format, RFC 1951. I presume that that's what you're using from Visual Basic.

Also per the documentation, Java's Inflater class decodes the zlib format, RFC 1950. The zlib format is the raw deflate format wrapped with a zlib header and trailer. Normally the Inflater class would fail on raw deflate data from .NET's DeflateStream. Fortunately however the Inflater class provides the nowrap parameter to the constructor (would be set to true), which allows Inflater to decode raw deflate data without the zlib wrapper.

So that should work. What are the first 20 bytes in hexadecimal of the data produced by DeflateStream?

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Thank you for the suggestion, but I have already done this, and it didn't work.
Thank you again for the suggestion. Trying to solve the problem, I gathered some methods of decompressing (gzip, bzip2, inflater, zip from apache commons, etc) and noticed that the variant of decompression that works has the nowrap parameter set. So thank you again.
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From what I've read, java's deflate/inflate uses Zlib. What does VB use to inflate to? I've failed to google this, but some references seem to indicate it uses ZIP, no zlib.

Have you tried running file compressed_file to determine the format it's compressed to?

In any case, you might want to try apache commons to decompress using the ZIP algorithm.

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Hello and thank you for the answer. I have to mention that the file that I am trying to decompress was also encrypted. So I decrypted it, and after that I arrived at the present step. I try to decompress the result of the decryption, which is held in memory, so I don't have a proper file. I tried to use apache commons in order to decompress, but with no success.
You should save the contents to disk, at least until you find out how to decompress it properly.
I tried this too, to save the contents in a file, and to decompress that file, but with no success. I ran the file command on that file, but the result was "data", which is not very conclusive.
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Ok, so I finally managed to solve this. It was a beginner error, but it took me long enough to solve it. The issue was represented by the fact that when I was reading the initial file (that was compressed and encrypted), I was keeping in memory the content of the file as a string, instead of reading directly the content as an array of bytes. And of course, some of the bytes were not supported by the string encoding and they were lost, and that is why the format of the compression was no recognized. The Inflater/Deflater classes from Java are compatible with the DeflateStream class from vb.net !

Thank you very much for your help and assistance.

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