Empty, Error and Never overloads with type witness

[akarnokd]

Witness variants for #653.

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[zsxwing]

Is it necessary to have such overloads? I just think it's weird that passing an unused var.

[samuelgruetter]

I think this witness trick is common in C#, but I've never seen it in Java, so I would not add these overloads.

[akarnokd]

It depends on how many times you write the following pattern:

import static rx.Observable.*;

concat(from(1,2,3), empty()); // incompatible types Object vs Integer in Java < 8
concat(from(1,2,3), empty(4));
concat(from(1,2,3), Observable.<Integer>empty());

merge(just(4), error(new Exception()); // incompatible types Object vs Integer in Java < 8
merge(just(4), error(new Exception(), 5);
merge(just(4), Observable.<Integer>error(new Exception());

[benjchristensen]

Instead of adding overloads everywhere we have the cast operator that solves this issue so a user can do either of these:

Observable<String> s = Observable.empty().cast(String.class).take(1)
Observable<String> s = Observable.<String>empty().take(1);

Even if we were to use the witness patter it would be better to take Class<T> rather than T so that we don't have to pass random values in.

Observable<String> s = Observable.empty(String.class).take(1);

This is equally elegant but requires overloads for all similar use cases.

In .Net this is essential because you cannot write the type of an anonymous type (as per @headinthebox ) and this isn't a problem in Java.