Question

I am trying to write a function which will accept an InputStream with zipped file data and would return another InputStream with unzipped data.

The zipped file will only contain a single file and thus there is no requirement of creating directories, etc…

I tried looking at ZipInputStream and others but I am confused by so many different types of streams in Java.

Answer 1

Concepts

GZIPInputStream is for streams (or files) zipped as gzip (".gz" extension). It doesn’t have any header information.

This class implements a stream filter for reading compressed data in the GZIP file format

If you have a real zip file, you have to use ZipFile to open the file, ask for the list of files (one in your example) and ask for the decompressed input stream.

Your method, if you have the file, would be something like:

// ITS PSEUDOCODE!!

private InputStream extractOnlyFile(String path) {
   ZipFile zf = new ZipFile(path);
   Enumeration e = zf.entries();
   ZipEntry entry = (ZipEntry) e.nextElement(); // your only file
   return zf.getInputStream(entry);
}

Reading an InputStream with the content of a .zip file

Ok, if you have an InputStream you can use (as @cletus says) ZipInputStream. It reads a stream including header data.

ZipInputStream is for a stream with [header information + zippeddata]

Important: if you have the file in your PC you can use ZipFile class to access it randomly

This is a sample of reading a zip-file through an InputStream:

import java.io.FileInputStream;
import java.util.zip.ZipEntry;
import java.util.zip.ZipInputStream;


public class Main {
    public static void main(String[] args) throws Exception
    {
        FileInputStream fis = new FileInputStream("c:/inas400.zip");

        // this is where you start, with an InputStream containing the bytes from the zip file
        ZipInputStream zis = new ZipInputStream(fis);
        ZipEntry entry;
            // while there are entries I process them
        while ((entry = zis.getNextEntry()) != null)
        {
            System.out.println("entry: " + entry.getName() + ", " + entry.getSize());
                    // consume all the data from this entry
            while (zis.available() > 0)
                zis.read();
                    // I could close the entry, but getNextEntry does it automatically
                    // zis.closeEntry()
        }
    }
}