rompetroll rompetroll - 8 days ago 7
Java Question

Java8 Lambdas and Exceptions

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

This looks like a case of bug JDK-8054569, which doesn't affect Eclipse.

I was able to narrow it down by replacing Function with Supplier and extracting the orElseThrow method:

abstract <T> void f(Supplier<T> s);

abstract <T, X extends Throwable> T g(Supplier<? extends X> x) throws X;

void bug() {
    f(() -> g(() -> new RuntimeException("foo")));
}

and then further by removing the suppliers and lambdas altogether:

abstract <T> void f(T t);

abstract <T, X extends Throwable> T g(X x) throws X;

void bug() {
    f(g(new RuntimeException("foo")));
}

which is actually a cleaner example than the one in the bug report. This shows the same error if compiled as Java 8, but works fine with -source 1.7.

I guess something about passing a generic method return type to a generic method parameter causes the type inference for the exception to fail, so it assumes the type is Throwable and complains that this checked exception type isn't handled. The error disappears if you declare bug() throws Throwable or change the bound to X extends RuntimeException (so it's unchecked).