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Best way to see if a file is compressed

ryno71 asked
Medium Priority
Last Modified: 2010-08-05

If I want to see if a file is compressed , what is the best way?  If see if the zipentry is null... might work but if the the file is corrupted, that wouldn't be accurate

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Test for an extension .zip or .ZIP

If a zip-file is named another way somebody is cheating.



Thats just it, if I dont know... say it was sent to me in a byte array I wouldn't know.
File f;   // Got it from a directory listing, then you don't know its name.

 //    compressed



If I am the one naming the new file I receive and don't know what the original name was this won't work.  
OK then, if the first 8 bytes are (octal):

 0120, 0113, 003, 004, 024, 000, 002, 000

then you may assume it's a ZIP compressed file.

You can read those 8 bytes from a raw InputStream from the file.

Correction: I did some research on other zip files:

only 4 bytes are enough:

0120, 0113, 003, 004



Something like this?

I keep getting false

public class BytesFromFile

public static byte[] getBytesFromFile(File file) throws IOException {
        InputStream is = new FileInputStream(file);
        // Get the size of the file
        long length = file.length();
        // You cannot create an array using a long type.
        // It needs to be an int type.
        // Before converting to an int type, check
        // to ensure that file is not larger than Integer.MAX_VALUE.
        if (length > Integer.MAX_VALUE) {
            // File is too large
        // Create the byte array to hold the data
        byte[] bytes = new byte[4];
            byte[] bytes1= new byte[] {0120,0113,003,004};

            boolean bo = Arrays.equals(bytes, bytes1);
            boolean bo1 = bytes.equals(bytes1);
            System.out.println("result is "+ result);
            System.out.println("bo is "+ bo);
            System.out.println("bo1 is "+ bo1);
        // Read in the bytes
        int offset = 0;
        int numRead = 0;
        while (offset < bytes.length
               && (numRead=is.read(bytes, offset, bytes.length-offset)) >= 0) {
            offset += numRead;
        // Ensure all the bytes have been read in
        if (offset < bytes.length) {
            throw new IOException("Could not completely read file "+file.getName());
        // Close the input stream and return bytes
        return bytes;

      public static void main(String args[]) {
                    BytesFromFile da = new BytesFromFile();
                    System.out.println(" ");
                              System.out.println("Zip1 ");
                              System.out.println(" ");
                              String fileName=args[0];
                              byte[] bytes =null;
                              File temp=null;
                              temp = new File(fileName);
                              System.out.println("bytes are "+bytes);
               catch (Exception e1)

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;

public class TestForCompression
    * Program entry point.
    * @param commandLine program command line vector.
    * @throws IOException
   public static void main(String[] commandLine) throws IOException
      byte[] bytes = new byte[] {0120,0113,003,004};
      byte[] data = new byte[4];
      RandomAccessFile ra;

      if(commandLine.length > 0)
         ra = new RandomAccessFile(new File(commandLine[0]), "r");
         if(ra.read(data) != data.length)
            System.out.println("Short file, not compressed.");
         for(int i = 0;  i < data.length;  ++i)
            if(data[i] != bytes[i])
               System.out.println("Not compressed.");

Always test your code!


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Thanks.  that will work better!



Where did you find that you need to look at four bytes for a compressed file (pkzip or Gzip), couldn't it be two?

From the SUN JAVA sources:

ZIP      "PK\003\004"
GZIP    0x8B1F

seems to be 4 bytes all the time.



where did you find this?  Been looking and can't sem to find it!

Thanks alot!
The JDK is accompanied by "src.zip"

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