How to upcast object contained in Java 8 Optional?

I would write a method like this:

@SuppressWarnings("unchecked")  // Safe. See below.
static <T> Optional<T> copyOf(Optional<? extends T> opt) {
  return (Optional<T>) opt;
}

(If you don't like the name copyOf, see my comment about Guava's ImmutableList below)

This is very efficient in terms of runtime speed: the cast gets elided at compile time:

static <T> java.util.Optional<T> copyOf(java.util.Optional<? extends T>);
    Code:
       0: aload_0  # Read the parameter.
       1: areturn  # Return the parameter.

so the only cost is that of a method call; this is easily done away with by the JIT.

You can then invoke like:

Optional<A> a = copyOf(func2());

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This is safe because Optional has the following property: it is guaranteed not to have any state changes caused by setter methods taking parameters dependent upon the type variable T. Phew. Quite a mouthful. I'll make it more concrete.

Because Optional

1. has no setter methods (of any kind, but more generally none that take parameters of type T, SomeGenericType<T> etc)
2. is final (so you can't subclass it to add a setter to violate the previous point)

there is nothing you can do to the value held by the Optional<T> (or lack thereof) that will make it not an instance of T (or lack thereof).

And because every instance of T is also an instance of its superclasses, there is nothing unsafe about:

SuperclassOfT s = optionalOfT.get();

As such, this method is type safe (it will fail if you've invoked it on a non-present optional; but that's not a type error).

You will find similar code in Guava's ImmutableList.copyOf (the inspiration for calling it "copyOf" above, even though it's not really a copy). There, there are setter methods (like add), but those methods immediately throw UnsupportedOperationExceptions, and thus do not affect the list's state.

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Note that whilst immutable types have the necessary properties described above to make such a cast safe, the type does not necessarily need to be immutable to perform the cast safely.

For example, you could have an ErasableOptional<T> type, which has an erase() method on it which, when called, converted a "present" value into an "absent" value (i.e. get() no longer succeeds). It would be safe to cast such an instance to an ErasableOptional<SupertypeOfT> because the value is either a T or absent; you can't make it not an instance of SupertypeOfT or absent.

Optional is, quote from java doc:

a container object which may or may not contain a non-null value.... Additional methods that depend on the presence or absence of a contained, such as orElse() ...

As we can see Optional container, uses generic type to define which object to contain.

Optional <T>

so you can do this:

Optional<? extends A> a = func2(); // 2. Upcasting Will work :)

and to access the optional object a an object of type A :

a.get();