Calling a list of methods on a Java object using Clojure

Question

I have a list of values I want to convert to an immutable Java POJO. The Java POJO gets generated using the builder pattern.

The list I have is:

[49.0 11.1 150.0]

In Java my POJO would get constructed like this:

Position.builder().latitude(49.0).longitude(11.1).altitude(150.0).build()

I thought to create it in Clojure I would first zipmap the values with the methods that need to be used and then I could reduce-kv everything to the Java value I need:

(defn poslist->map [list]
      (zipmap ['.latitude '.longitude '.altitude]
              list))

(.build (reduce-kv #(%2 %1 %3) 
                   (pkg.Position/builder)
                   (poslist->map list)))

My problem now is that I don't get what I expect. The function poslist->map returns the expected value:

{.latitude 49.0, .longitude 11.1, .altitude 150.0}

But the reduce-kv just returns the last value I have:

150.0

Why don't I get back the builder on which I then can apply the .build method? I had expected that the reduce-kv to have the same result as the following code that returns what I expect:

(.build (.altitude (.longitude (.latitude (pkg.Position/builder) 150.0) 11.1) 49.0))

And as the reduce-kv function returns a double the following call of .build failes with

No matching field found: build for class java.lang.Double

BTW: The reason I am mapping the methods to the list values is, that the actual list of fields I have to set is longer as in the example. I stripped down the example to be more readable while retaining my basic problem.

Community · Accepted Answer · 2017-05-23 11:44:10Z

4

While Clojure symbols act as functions, they are not connected with functions with the same name in any way:

> ('.add (java.util.ArrayList.) 1)
1
> ('first [1 2 3])
nil
> ('conj [1 2] 3)
3

So the issue, basically, is that java methods are not called at all. To fix it wrap your iterop methods with memfn:

['.latitude '.longitude '.altitude]

->

[(memfn latitude val) (memfn longitude val) (memfn altitude val)]

or, as @noisesmith suggested, with clojure functions:

->

[#(.latitude %1 %2) #(.longitude %1 %2) #(.altitude %1 %2)]

edited May 23, 2017 at 11:44

CommunityBot

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answered Jul 11, 2016 at 15:31

OlegTheCat

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2 Comments

noisesmith Over a year ago

there's no situation currently where (memfn foo) is preferable to #(.foo %)

Matthias Wimmer Over a year ago

Thank you both, that works! Just one note: to match exactly my question, the created function takes two parameters (the builder instance and the value to set). Therefore it's either (memfn latitude val) or #(.latitude %1 %2).

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