Clojure HashMap usage within a Java class

I attended the Hack (Make!) the Bank hackathon that took place on Level 39 (One Canada Square in London). My team decided to make a funds reallocating application that makes automatic payments once an account is topped up. Long story short, we got stuck at interoperability with the Java classes providing Paypal access.

We made the mistake of inserting a Clojure hash map directly to the Java class, which seemed to work but the moment we switched to paper account in the Java class (by modifying the incoming HashMap argument) the system threw an exeption. Ben from Erudine Financial realized that the structure we were passing was inmutable so we just created the proper Java HashMap instance.

(java.util.HashMap. {})

Remember that Clojure's {} is actually a clojure.core.PersistentHashMap.

This situation happens especially when clumsily using proof of concept code in production.