Method to return function with generics-CodePudding

static <R> Function<Optional<R>,Function<R, Optional<String>>> checkPresence(String err){
        return ro -> r -> ro.map(x -> emptyStrOpt.get())
                            .orElseGet(() -> Optional.of(err));
}

//toDateOfInjury is Function<Bill,LocalDate>

static final Function<Bill,Optional<String>> dateOfInjuryNotNull = b ->
            toDateOfInjury.andThen(
                    o -> checkPresence("No date of injury").apply(o).apply(b));

IntelliJ complains that for o, it expects Optional<Object> but it's receiving Optional<LocalDate>.

Also, is there any way to declare a function using generics without returning it from a method?

EDIT: for the sake of completeness, let's just say that toDateOfInjury is

Function<Bill,Optional<LocalDate>> toDateOfInjury = b -> Optional.ofNullable(LocalDate.now()); //though it map to empty optional too of course

EDIT2: After getting past the first problem, I fixed it like so:

static final Function<Bill,Optional<String>> dateOfInjuryNotNull2 = b ->
            toDateOfInjury.andThen(
                    o -> EventValidationUtils.<LocalDate>checkPresence("No date of injury").apply(o)).apply(b);

    static <R> Function<Optional<R>,Optional<String>> checkPresence(String err){
        return ro -> ro.map(x -> emptyStrOpt.get())
                            .orElseGet(() -> Optional.of(err));
    }

CodePudding user response：

See this answer: Calling static generic methods

You need to specify generic type (here is LocaleDate) when calling a generic method. In your method call it cannot specify method type so inherit it as Object. So you need to specify it like below (Suppose Test is your class name):

static final Function<Bill,Optional<String>> dateOfInjuryNotNull = b ->
            toDateOfInjury.andThen(
                    o -> Test.<LocalDate>checkPresence("No date of injury").apply(o).apply(b));

But you face other problems need to solve.