Determine Java class file version

/**
 * Mon Nov 19 00:32:13 GMT 2007
 * This simple application is released under GPL 3.0
 * See enclosed licence file
 */
package com.technojeeves.util;

import java.io.InputStream;
import java.io.FileInputStream;
import java.io.DataInputStream;
import java.io.File;
import java.io.FilenameFilter;
import java.io.IOException;

import java.util.Stack;
import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.List;
import java.util.jar.JarEntry;
import java.util.jar.JarFile;
import java.util.zip.ZipException;

/**
 * @author Charles Johnson
 */
public class JVersion implements FilenameFilter {
    private Map<File, Short> versions;

    public JVersion() {
        versions = new TreeMap<>();
    }

    public Map<File, Short> getVersions() {
        return versions;
    }

    public Map<File, String> getVersionsFriendlyNamed() {
        Map<File, String> result = new HashMap<>();
        for (Map.Entry<File, Short> entry : versions.entrySet()) {
            int version = entry.getValue() - 44;
            String v = Integer.toString(version);
            String newValue = version <= 8 ? "1." + v : v;
            result.put(entry.getKey(), newValue);
        }
        return result;
    }

    public static void main(String[] args) {
        if (args.length < 1) {
            System.err.println("Usage: zip.JVersion <root dir>");
            System.exit(-1);
        }

        try {
            JVersion jver = new JVersion();
            jver.list(args[0]);
            Map<File, Short> versions = jver.getVersions();
            if (versions.size() > 0) {
                for (Map.Entry<File, String> e : jver.getVersionsFriendlyNamed().entrySet()) {
                    System.out.printf("%s=%s%n", e.getKey(), e.getValue());
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void list(String rootDir) throws IOException {
        List<File> jars = new ArrayList<File>();
        getJars(new File(rootDir), jars);
        // listEntries(jars, searchString);
        getVersions(jars);
    }

    private void getVersions(List<File> files) throws IOException {
        JarFile jar = null;
        Stack<Short> versionStack = new Stack<>();
        File f = null;

        for (int i = 0; i < files.size(); i++) {
            versionStack.clear();
            f = (File) files.get(i);
            if (f.getName().endsWith(".class")) {
                // class file
                try (InputStream in = new FileInputStream(f)) {
                    processBinary(in, versionStack);
                }
            } else {

                jar = new JarFile(f);

                Enumeration entries = jar.entries();

                for (; entries.hasMoreElements();) {
                    JarEntry entry = (JarEntry) entries.nextElement();
                    String entryName = entry.getName();
                    if (entryName.endsWith(".class")) {
                        try (InputStream in = jar.getInputStream(entry)) {
                            processBinary(in, versionStack);
                        }
                    }
                }
            }
            // end if class
            // Make a Map entry
            versions.put(f, versionStack.pop());
            // end for entries
        }
        // end for jars
    }

    private void processBinary(InputStream in, Stack<Short> versionStack) throws IOException {
        // Make sure the highest version is on the stack
        Short majorVersion = getMajorVersion(in);
        if (versionStack.isEmpty()) {
            versionStack.push(majorVersion);
        } else {
            if (majorVersion.compareTo(versionStack.peek()) > 0) {
                versionStack.pop();
                versionStack.push(majorVersion);
            }
        }
    }

    private short getMajorVersion(InputStream in) throws IOException {
        short result = -1;
        DataInputStream din = new DataInputStream(in);
        din.skip(6L);
        result = din.readShort();
        return result;
    }

    public boolean accept(File file, String name) {
        // System.out.printf("Is directory: %b, Name: %s\n", file.isDirectory(),
        // file.getName());
        return file.isDirectory() || name.toLowerCase().endsWith(".jmod") || name.toLowerCase().endsWith(".jar");
    }

    private void getJars(File f, List<File> jars) {
        if (f.isDirectory()) {
            File[] files = f.listFiles(this);

            for (int i = 0; i < files.length; i++) {
                getJars(files[i], jars);
            }
        } else if (f.isFile() && (f.getName().toLowerCase().endsWith(".jmod"))
                || f.getName().toLowerCase().endsWith(".class") || f.getName().toLowerCase().endsWith(".jar")) {
            jars.add(f);
        }
    }
}