Question

At the clojure repl, there is a unique number that pops up for any IDeref object

user=> (def a (atom 1))
#'user/a
user=> a
#<Atom@2e1c6600: 1>

where is this number – 2e1c6600– stored and how do we grab this number from a?

Answer 1

It is the hex representation of the hashCode of the underlying Java Object. If you absolutely need it, you can get it using (Integer/toHexString (.hashCode a))

user=> (def a (atom 1))
#'user/a
user=> a
#<Atom@56092666: 1>  
user=> (Integer/toHexString (.hashCode a))
"56092666"