javac compilation error when passing method reference to method accepting interface.

[Vicente-Arturo Romero-Zaldivar]

Hi again,

By [1] I meant: https://bugs.openjdk.java.net/browse/JDK-8044546 :)

Thanks,
Vicente


On 11/06/2014 09:30 AM, Vicente-Arturo Romero-Zaldivar wrote:
> Hi Tomasz,
>
> The bug you are referring to has already been fixed, please see [1]. 
> The bits are not in any public release yet but you can build one from 
> the development repos:
>
> for 8: http://hg.openjdk.java.net/jdk8u/jdk8u-dev/langtools
> for 9: http://hg.openjdk.java.net/jdk9/dev/langtools/
>
> Thanks,
> Vicente
>
> [1]
>
> On 11/03/2014 03:15 AM, Tomasz Kowalczewski wrote:
>> Hi,
>>
>> please excuse my ignorance if this is a known bug, was discussed here 
>> or is plain silly.
>>
>> I have a curious problem when calling a method with lambda or method 
>> reference as an argument. This happens on several versions of Java 8 
>> including latest (u25). Test case is:
>>
>> package test;
>>
>> import java.util.LinkedHashMap;
>> import java.util.Map;
>> import java.util.concurrent.Callable;
>>
>> public class CompilationProblem {
>>
>>     public void shouldCompile() {
>>         toMap(() -> new LinkedHashMap()); // Compilation error
>>         toMap(LinkedHashMap::new); // Compilation error
>>     }
>>
>>     public <K, V> Map<K, V> toMap(Func0<? extends Map<K, V>> 
>> mapFactory) {
>>         return null;
>>     }
>>
>>     interface Func0<R> extends Callable<R> {
>>
>>         public R call();
>>     }
>> }
>>
>> The errors are:
>>
>> Error:(10, 17) java: method toMap in class 
>> pl.codewise.voluum.server.db.CompilationProblem cannot be applied to 
>> given types;
>>   required: pl.codewise.voluum.server.db.CompilationProblem.Func0<? 
>> extends java.util.Map<K,V>>
>>   found: ()->new Li[...]Map()
>>   reason: cannot infer type-variable(s) K,V
>>     (argument mismatch; bad return type in lambda expression
>>       java.util.LinkedHashMap cannot be converted to ? extends 
>> java.util.Map<K,V>)
>>
>> Error:(11, 22) java: incompatible types: no instance(s) of type 
>> variable(s) K,V exist so that java.util.LinkedHashMap<K,V> conforms 
>> to ? extends java.util.Map<K,V>
>>
>> This test case compiles if Func0 does not extend Callable. How does 
>> Callable influence type inference here? What does it change?
>>
>> -- 
>> Regards,
>> Tomasz Kowalczewski
>