jad expects class file versions (major.minor) to be 45.3 or 46.0. Sources compiled with 1.5 have the file version 48.0. The following error message is displayed when trying to decompile a 1.5 .class file:

Parsing RequestForEnhancement.class...The class file version is 48.0, expected 45.3 or 46.0
JavaClassFileParseException: Class file version mismatch

To be able to use JAD, we can simply change the class file version of a 1.5 compiled class into 46. The following small application sets the magic number of a .class file or all classes in a jar file to whatever you desire.

ChangeMagicNumber.java:

import java.io.*;
import java.util.*;
import java.util.jar.*;
import java.util.zip.*;
 
public class ChangeMagicNumber
{
   public static void main(String []args) throws Exception {
      if (args.length != 2) {
         System.out.println("Usage: java ChangeMagicNumber [file.class | file.jar] magicNumber (eg. 46)");
         System.exit(1);
      }
       
      String filename = args[0];
      byte desiredMagicNumber = (byte) Integer.parseInt(args[1]);
      if (filename.toLowerCase().endsWith(".jar")) {
         processJar(filename, desiredMagicNumber);
      }
      else {
         processFile(filename, desiredMagicNumber);
      }
   }
    
   public static void processFile(String filename, byte desiredMagicNumber) throws Exception {
      RandomAccessFile raf = new RandomAccessFile(filename, "rw");
      raf.seek(7);
      byte b = raf.readByte();
      if (b != desiredMagicNumber) {
         raf.seek(7);
         raf.writeByte((byte) desiredMagicNumber);
      }
      raf.close();
   }
    
   public static void processJar(String filename, byte desiredMagicNumber) throws Exception {
      JarFile jf = new JarFile(filename);
      JarOutputStream jos = new JarOutputStream(new FileOutputStream(filename + "." + desiredMagicNumber));
      CRC32 crc = new CRC32();
      Enumeration enum = jf.entries();
      while (enum.hasMoreElements()) {
         ZipEntry entry = (ZipEntry) enum.nextElement();
         BufferedInputStream bis = new BufferedInputStream(jf.getInputStream(entry));
         int length = bis.available();
         byte[] buffer = null;
         if (length > 0) {
            buffer = new byte[length];
            bis.read(buffer, 0, length);
            
            buffer[7] = desiredMagicNumber;
            crc.reset();
            crc.update(buffer, 0, length);
            entry.setCrc(crc.getValue());
         }
         
         String e = entry.toString();
         jos.putNextEntry(entry);
         if (length > 0) {
            jos.write(buffer, 0, length);
         }
      }
      jf.close();
      jos.close();
   }
}

For example:

java ChangeMagicNumber rt.jar 46  

produces rt.jar.46, all classes in JAR file can be decompiled with JAD

Now the decompilation of the following 1.5 enum-example source works fine, after changing its file class version to 46.

Main.java:

enum DestinationType
{
   queue,
   topic
}
 
public class Main
{
   public static void main(String []args) {
      DestinationType dt = DestinationType.queue;

      // Outputting an enumerated type is user-friendly
      System.out.println(dt);   // prints out "queue"
      
      // Switch can be used on an enumerated type, no need to qualify the name of the constant
      switch (dt) {
         case queue:  System.out.println("Using a queue");
                      break;
         case topic:  System.out.println("Using a topic");
                      break;
         default:
      }
   }
}

Execute:

   java ChangeMagicNumber DestinationType.class 46
   jad DestinationType.class

results in:

DestinationType.jad:

// Decompiled by Jad v1.5.6d. Copyright 1997-99 Pavel Kouznetsov.
// Jad home page: http://www.geocities.com/SiliconValley/Bridge/8617/jad.html
// Decompiler options: packimports(3) 
// Source File Name:   Main.java
 
native class DestinationType extends Enum
{
    public static final DestinationType[] values()
    {
        return (DestinationType[])_2B_VALUES.clone();
    }
 
    public static DestinationType valueOf(String s)
    {
        DestinationType adestinationtype[] = _2B_VALUES;
        int i = adestinationtype.length;
        for(int j = 0; j < i; j++)
        {
            DestinationType destinationtype = adestinationtype[j];
            if(destinationtype.name().equals(s))
                return destinationtype;
        }
 
        throw new IllegalArgumentException(s);
    }
 
    DestinationType(String s, int i)
    {
        super(s, i);
    }
 
    public volatile int compareTo(Enum enum)
    {
        return super.compareTo((DestinationType)enum);
    }
 
    public static final native DestinationType queue;
    public static final native DestinationType topic;
    private static final DestinationType _2B_VALUES[] = {
        queue, topic
    };
 
    static 
    {
        queue = new DestinationType("queue", 0);
        topic = new DestinationType("topic", 1);
    }
}