java - Java8 Lambdas and Exceptions

Question

I wonder if someone could explain the following weirdness to me. I'm using Java 8 update 11.

Given this method

private <F,T> T runFun(Function<Optional<F>, T> fun, Optional<F> opt) {
   return fun.apply(opt) ;
}

If I first construct a function Object, and pass that in to the method above, things compile.

private void doesCompile() {
    Function<Optional<String>, String> fun = o -> o.orElseThrow(() -> new RuntimeException("nah"));
    runFun(fun, Optional.of("foo"));

}

But, if I inline the function as a lambda, the compiler says

unreported exception X; must be caught or declared to be thrown

private void doesNotCompile () {
    runFun(o -> o.orElseThrow(() -> new RuntimeException("nah")), Optional.of("foo"));
}

---

Update: Turns out the error message was abbreviated by maven. If compiled directly with javac, the error is:

error: unreported exception X; must be caught or declared to be thrown
            runFun(o -> o.orElseThrow(() -> new RuntimeException("nah")), Optional.of("foo"));
                                     ^
  where X,T are type-variables:
    X extends Throwable declared in method <X>orElseThrow(Supplier<? extends X>)
    T extends Object declared in class Optional

Also see here for runnable test code.

Answer 1

This is what solved the problem for me:

instead of writing

optional.map(this::mappingFunction).orElseThrow(() -> new BadRequestException("bla bla"));

I wrote:

optional.map(this::mappingFunction).<BadRequestException>orElseThrow(() -> new BadRequestException("bla bla"));

Adding the explicit <BadRequestException> helps with these lambda edge cases (which are quite annoying...)

UPDATE: This is in case you can't update to the latest JDK version, if you can you should...