varargs ambiguity with generics

[henridf]

> var Arrays = java.import('java.util.Arrays');  
> var list = Arrays.asList(['a', 'b']);
> list.toArray();
[ [ 'a', 'b' ] ] // should be ['a', 'b'] !!

I haven't looked at the varargs handling code, but I'm assuming it's because of the ambiguity due to the asList method being generic (List<T> asList(T... a)) - there's no way to distinguish if the javascript Array being passed into Arrays.asList() is intended as a single value of type T or if it's intended as a T[].

Assuming that's the issue, we could detect this ambiguity and throw an error, requiring the javascript programmer to use apply when they want a javascript array to be 'spread' into a varargs generic function.

Does this make sense?

(Btw, ES6 compiled by babel works just fine here with Arrays.asList(...arr);).

[joeferner]

You are probably correct. We rely on a fork of commons-lang to find which method matches using MethodUtils#getMatchingAccessibleMethod. We have already patched some of the ambiguity issues that commons-lang was not handling this is just another missing case. When I have some time I'll take a look.

[henridf]

I'm happy to work on this if that's helpful -- let me know.

[joeferner]

Absolutely that would be great.

[henridf]

Here's my suggestion for the desired behavior: the javascript call Arrays.asList([a, b, c]) would result in the underlying Java call Arrays.asList(a, b, c).

If you want a single array arg to be passed to the java method, then you'd do Arrays.asList([[a, b, c]]), which results in the underlying Java call Arrays.asList([a, b, c]).

Does this behavior make sense?

[joeferner]

Can we make the rules the same as Java? How does it handle these cases?

[henridf]

Based on the little experiment below, I'd say the existing behavior is correct if we want to make things the same as Java. So this is not a bug, though we might want to add something to the README to explain this behavior.

import java.util.Arrays;                                                                                               
import java.util.List;                                                                                                 

public class testvarargs {                                                                                                   
    public static void varargs1(int... numbers) {                                                                      
        System.out.println(numbers[0]);                                                                                
    }                                                                                                                  
    public static void varargs2(Object... numbers) {                                                                   
        System.out.println(numbers[0]);                                                                                
    }                                                                                                                  
    public static <T> void varargs3(T... numbers) {                                                                    
        System.out.println(numbers[0]);                                                                                
    }                                                                                                                  

    public static void main(String[] args) {                                                                           
        int[] ints = new int[] {1, 2, 3};                                                                              

        varargs1(ints);   // prints 1                                                                                             
        varargs2(ints);   // prints [I@7852e922                                                                                
        varargs3(ints);   // prints [I@7852e922                                                           
    }                                                                                                                  
}                                                                                                                      

[joeferner]

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