Check the class file version of all class files within a JAR file

CheckJarClassVersion.java

import java.io.DataInputStream;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;
import java.util.zip.ZipEntry;
import java.util.zip.ZipFile;

/**
 * Check the class file version of all class files within a JAR file
 *
 * When execute this program provide the path to the JAR file as argument.
 */
public class CheckJarClassVersion {

	public static void main(String[] args) {
		Map<Integer, List<String>> map = new TreeMap<>();
		try (ZipFile jar = new ZipFile(args[0])) {
			Enumeration<? extends ZipEntry> entries = jar.entries();
			while (entries.hasMoreElements()) {
				ZipEntry entry = entries.nextElement();
				if (!entry.getName().endsWith(".class"))
					continue;

				try (DataInputStream in = new DataInputStream(jar.getInputStream(entry))) {
					int magic = in.readInt();
					if (magic != 0xCAFEBABE) {
						System.out.println("Not a class file - skipping " + entry.getName());
						continue;
					}
					int classFileVersion = in.readInt();
					List<String> files = map.get(classFileVersion);
					if (files == null) {
						files = new ArrayList<>();
						map.put(classFileVersion, files);
					}
					files.add(entry.getName());
				} catch (Exception e) {
					System.err.println("Error reading  class " + entry.getName() + ": " + e);
				}
			}
			for (int versionCode : map.keySet()) {
				List<String> files = map.get(versionCode);
				System.out.println(String.format("Version of Class File Format: %d (Java %d) - files: %d", versionCode,
						versionCode - 44, files.size()));
				// Uncomment to see all files in each category
				// files.forEach(x -> System.out.println("\t" + x));
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

}