Getting result of a Future after submitting function to java.util.concurrent.ExecutorService

Getting result of a Future after submitting function to
java.util.concurrent.ExecutorService

The following works. The result is "hello world"
(def ^Callable f (fn [] "hello world"))
(let [e (java.util.concurrent.Executors/newSingleThreadExecutor)]
(try
(.get (.submit e f))
(finally (.shutdown e))))
But the following doesn't. The result of get is nil
(def e (java.util.concurrent.Executors/newSingleThreadExecutor))
(.get (.submit e f))
Why? I checked that f is getting called by replacing it with something
that has a side effect. The only difference I can see is that e is bound
using let in one and def in the other.
Another question. If I don't have the ^Callable typehint for f the first
example quietly returns nil. Shouldn't it throw an exception "more than
one matching method found" for the submit call since f is both Runnable
and Callable? If I define f using let like the following then the
exception is thrown
(let [e (java.util.concurrent.Executors/newSingleThreadExecutor)]
(let [f (fn [] "hello world2")]
(try
(.get (.submit e f))
(finally (.shutdown e)))))
Thanks