Packing the same bytes twice

ZipUtilPackMd5.java

import java.io.File;
import java.math.BigInteger;
import java.nio.file.Files;
import java.security.MessageDigest;

import org.zeroturnaround.zip.ZipUtil;

public class ZipUtilPackMd5 {
  public static void main(String[] args) throws Exception {
    String fileName = "target/classes/ZipUtilPackMd5.class";
    File file = new File(fileName);
    if (!file.exists())
      throw new RuntimeException(fileName + " does not exist");

    byte[] bytes = Files.readAllBytes(file.toPath());
    System.out.println("Input file " + md5(bytes));

    byte[] zippedBytes = ZipUtil.packEntry(file);
    System.out.println(md5(zippedBytes));
    zippedBytes = ZipUtil.packEntry(file);
    System.out.println(md5(zippedBytes));
  }

  private static String md5(byte[] bytes) throws Exception {
    MessageDigest m = MessageDigest.getInstance("MD5");
    m.update(bytes, 0, bytes.length);
    return "md5: " + new BigInteger(1, m.digest()).toString(16);
  }
}