a simple question about Arrays.asList

[webber]

I am trying to use java.util.Arrays/asList in Clojure.

(java.util.Arrays/asList 1 2 3)

CompilerException java.lang.IllegalArgumentException: No matching method: asList, compiling:(NO_SOURCE_PATH:1:1) 

(java.util.Arrays/asList 1)

ClassCastException java.lang.Long cannot be cast to [Ljava.lang.Object;  user/eval7195 (NO_SOURCE_FILE:1)

What should I do to work it ?

[Aaron Cohen]

Since Arrays.asList is a variadic method, the Java compiler is magically creating an array behind the scenes for you when you write Java code invoking it.

Clojure does variadic functions differently for native clojure code.

To do interop with a Java variadic method have to create the array yourself:

(java.util.Arrays/asList (make-array Object 1 2 3))

Why are you trying to use Arrays#asList btw? There are probably better ways to do what you're trying to do.

--Aaron

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[Mauricio Aldazosa]

There are many things that need to be understood in a call like this.

First of all, that method receives an array as its argument:  https://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html#asList(T...)

From your first example I think you are trying to translate to clojure the following java call:

java.util.Arrays.asList(1, 2, 3);

In java this is a varargs call, which means java will change the call to one using an array as the parameter (http://docs.oracle.com/javase/8/docs/technotes/guides/language/varargs.html).

So, in order to make that call we need to wrap those arguments in an array. The easiest way to do this would be to wrap the arguments in a vector an turn that into an array:

user> (java.util.Arrays/asList (into-array [1 2 3]))
[1 2 3]

If you don't already have a sequence (the vector in the previous example) then you can use a function to do the work:

user> (defn as-list [& args] (java.util.Arrays/asList (into-array args)))

#'user/as-list

user> (as-list 1 2 3)
[1 2 3]
user> (type (as-list 1 2 3))
java.util.Arrays$ArrayList

Cheers,

Mauricio

[webber]

Hi Aaron,

Thank you for your reply. I was able to understand.

> Why are you trying to use Arrays#asList btw? There are probably better ways to do what you're trying to do.

I just wanted to test reify to implement java.util.Comarator as follows:

(java.util.Collections/sort  x 

 (reify java.util.Comparator

     (compare [this x y]

       (Integer/compare x y))))

Initially, I gave a test data like this.

 (def x 

  (doto (java.util.ArrayList.)

    (.add 3)(.add 1)(.add 2)))

 However, I thought asList is better. Are there better ways to do ?

Thanks,

MH

[webber]

Hi Mauricio,

The into-array worked fine !

Thanks,

MH

[Rafal Lewczuk]

Hi,

Try this:

(java.util.Arrays/asList (to-array [1 2 3 4]))


Regards,
rle