Add Seq<T> onEmptySwitch(Supplier<? extends Stream<? extends T>>)

[lukaseder]

As suggested by @johnmcclean-aol (https://github.com/jOOQ/jOOL/issues/177#issuecomment-170689447), an onEmptySwitch(Supplier<? extends Stream<? extends T>>) method might be useful in some situations.

Criticism:

• The current onEmptyXXX() names were inspired by Optional.orElseXXX(). We should think about the term switch, or if there is any more suitable term for this action
• Due to generic type erasure, we cannot overload a single onEmptySwitch(Supplier<E>) method with different bounds for <E>. What would be the best upper bound? Seq? Stream? Iterable?

[tlinkowski]

Sample implementation:

default Seq<T> onEmptySwitch(Supplier<? extends Stream<T>> supplier) {
    Supplier<? extends Stream<T>> lazy = () -> {
        Iterator<T> it = iterator();
        return it.hasNext() ? seq(it) : supplier.get();
    };
    return of(lazy).flatMap(Supplier::get);
}

As for your question about overloads, maybe different names like: onEmptyGetStream, onEmptyGetSeq, and onEmptyGetIterable?

[lukaseder]

Thank you for your suggestion, @tlinkowski. Hmm, I'm not sure it is correct, though. Why are you flatmapping your "lazy" stream with Supplier get? If the this stream contains 3 elements, for instance, that would call Supplier.get() 3 times.

But still, the fact that we'll need these wonky overloads to be sure we can get around type erasure, I'm still a bit hesitant to add this at all.

[tlinkowski]

Why are you flatmapping your "lazy" stream with Supplier get? If the this stream contains 3 elements, for instance, that would call Supplier.get() 3 times.

No, no - this Supplier::get is called only on the lazy variable, and not on the given supplier. So it's always called only once because there is only one lazy variable. But perhaphs I shouldn't have provided this implementation in such form, and I should have provided it using the proposed Seq.lazy [#302]:

default Seq<T> onEmptySwitch(Supplier<? extends Stream<T>> supplier) {
    return lazy(() -> {
        Iterator<T> it = iterator();
        return it.hasNext() ? seq(it) : supplier.get();
    });
}

[lukaseder]

Oh yeah, my bad. I get it. I was confused about the casing of supplier vs. Supplier :)