Java "target type of lambda conversion must be an interface"

Question

I'm trying to use lambdas and streams in java but I'm quite new to it. I got this error in IntelliJ "target type of lambda conversion must be an interface" when I try to make a lambda expression

List<Callable<SomeClass>> callList = prgll.stream()                                           .map(p->(()->{return p.funct();} )) <--- Here I get error                                           .collect(Collectors.toList()); 

Am I doing something wrong?

Answer 1

I suspect it's just Java's type inference not being quite smart enough. Try

 .map(p -> (Callable<SomeClass>) () -> p.funct()) 

Answer 2

Stream#map() is a typed method, so you can explicitly specify the type:

 .<Callable<SomeClass>>map(p -> () -> p.funct()) 

or neater, use a method reference:

 .<Callable<SomeClass>>map(p -> p::funct)