Clojure: working with a java.util.HashMap in an idiomatic Clojure fashion

clojure interop hashmap purely-functional clojure-java-interop
foxdonut picture foxdonut · Nov 3, 2009 · Viewed 14.6k times · Source

I have a java.util.HashMap object m (a return value from a call to Java code) and I'd like to get a new map with an additional key-value pair.

If m were a Clojure map, I could use:

(assoc m "key" "value")

But trying that on a HashMap gives:

java.lang.ClassCastException: java.util.HashMap cannot be cast to clojure.lang.Associative

No luck with seq either:

(assoc (seq m) "key" "value")

java.lang.ClassCastException: clojure.lang.IteratorSeq cannot be cast to clojure.lang.Associative

The only way I managed to do it was to use HashMap's own put, but that returns void so I have to explicitly return m:

(do (. m put "key" "value") m)

This is not idiomatic Clojure code, plus I'm modifying m instead of creating a new map.

How to work with a HashMap in a more Clojure-ish way?

Answer

Siddhartha Reddy picture Siddhartha Reddy · Nov 3, 2009

Clojure makes the java Collections seq-able, so you can directly use the Clojure sequence functions on the java.util.HashMap.

But assoc expects a clojure.lang.Associative so you'll have to first convert the java.util.HashMap to that:

(assoc (zipmap (.keySet m) (.values m)) "key" "value")

Edit: simpler solution:

(assoc (into {} m) "key" "value")

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