Java Magic. Part 2: 0xCAFEBABE

Did you know that all java classes start with the same 4 bytes. In hex it’s CAFEBABE.

To make sure, create simple java file, call it

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hell, O'World!");

Compile it by javac, and open Hello.class with hex-editor (M-x hexl-mode for emacs)

James Gosling explained this:

We used to go to lunch at a place called St Michael’s Alley. According to local legend, in the deep dark past, the Grateful Dead used to perform there before they made it big. It was a pretty funky place that was definitely a Grateful Dead Kinda Place. When Jerry died, they even put up a little Buddhist-esque shrine. When we used to go there, we referred to the place as Cafe Dead. Somewhere along the line it was noticed that this was a HEX number. I was re-vamping some file format code and needed a couple of magic numbers: one for the persistent object file, and one for classes. I used CAFEDEAD for the object file format, and in grepping for 4 character hex words that fit after “CAFE” (it seemed to be a good theme) I hit on BABE and decided to use it. At that time, it didn’t seem terribly important or destined to go anywhere but the trash-can of history. So CAFEBABE became the class file format, and CAFEDEAD was the persistent object format. But the persistent object facility went away, and along with it went the use of CAFEDEAD - it was eventually replaced by RMI.

0xCAFEBABE is 3405691582 in decimal. If we sum all digits we get 43. One more than 42 - Ultimate Answer to the Life, the Universe, and Everything. By the way, 43 is a prime number. You see, magic everywhere. Even in the last sentence.

mishadoff 31 October 2012
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