Working with Java Arrays
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One improvement that I’d like to see in Clojure is more examples in the doc strings (or maybe in a separate :example metadata item). Still, nothing to stop me building up a set of my own.
So, here are some simple examples of working with Java arrays in Clojure…
Given some sample data:
(def my-list '(1 2 3 4 5)) (def my-vector [1 2 3 4 5]) (def my-map {:a "apple" :b "banana" :c "chopped liver"})
To convert to Java arrays:
(to-array my-list) #<Object[] [Ljava.lang.Object;@962522b> (to-array my-vector) #<Object[] [Ljava.lang.Object;@37e55794> (to-array my-map) #<Object[] [Ljava.lang.Object;@52cd19d>
Note that this always returns Object[] regardless of the contents of the collection. Note also that the map isn’t flattened (the pp function used here is in clojure.contrib.pprint):
user=> (pp) [[:a "apple"], [:b "banana"], [:c "chopped liver"]]
If the array is 2-dimensional there is a corresponding function:
user=> (def my-vec-2d [[1 2 3] [4 5 6] [7 8 9]]) #'user/my-vec-2d user=> (to-array-2d my-vec-2d) #<Object[][] [[Ljava.lang.Object;@3a42f352> user=> (pp) [[1, 2, 3], [4, 5, 6], [7, 8, 9]] nil
If you need to use a specific type of array (e.g. to pass a String[] into a Java method) or need to use an array with more than 3 dimensions it’s a little trickier:
user=> (into-array my-list) #<Integer[] [Ljava.lang.Integer;@60c0c8b5> user=> (pp) [1, 2, 3, 4, 5] nil user=> (into-array my-vector) #<Integer[] [Ljava.lang.Integer;@2151b0a5> user=> (pp) [1, 2, 3, 4, 5] nil user=> (into-array my-map) #<MapEntry[] [Lclojure.lang.MapEntry;@7ae0c7c3> user=> (into-array (vals my-map)) #<String[] [Ljava.lang.String;@731de9b> user=> (pp) ["apple", "banana", "chopped liver"] nil
There, that should serve as a handy reference for myself for when I’m feeling forgetful…
[…] with Java Arrays – Clojure (here, via @hkrnws) — This article explains how to transform Clojure data structures into java […]
How does one specify an array as the parameter to a constructor in a gen-class scenario? I’d like to extend a class which has multiple single-argument constructors, some of which are of type Class[]. What would that look like in clojure?
Hmm, I’m not actually sure. I’ve just tried running what I thought was the correct solution in a REPL and it doesn’t work, so you’re going to have to give me a day or two to come up with the correct answer.