Add toObservable() to the common HystrixExecutable interface

[bbaetz]

All of the implementers of HystrixExecutable implement toObservable() and toObservable(Scheduler observeOn)

This patch adds those methods to the common interface, and also adds @OverRide annotations to appropriate places.

Use case is that I have a factory method that may return a HystrixCollapser or a HystrixCommand, and want to be able to call .toObservable on the result. (I'm still doing some designs on this and may not use it that way, but hopefully this technically-breaking change can make 0.40....)

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[benjchristensen]

This will break anybody who implements this interface. No one should really be doing that, but are we okay with a possible breaking change?

[bbaetz]

This will break anybody who implements this interface.

Yep.

No one should really be doing that

So the reason that I wanted this is because I am implementing this interface, as a wrapper/delegate around a set of commands that basically do:

• get from cache
• if its not there, get from the real service, then asyncrhonously write back to the cache.

This does mean that I could just create my own interface that includes HystrixExecutable+toObservable(), and I'm not sure that I acually need the lazy toObservable, so maybe this isn't worth putting upstream?

[benjchristensen]

Then you are an example of someone this would break. If a 1.x release came out that causes your code to break, wouldn't you consider that a problem since it wasn't a 2.x major version bump?

This is an example of why Java interfaces are troublesome in public APIs prior to Java 8 default methods.

Currently in the master branch I have a HystrixObservable interface that includes observe and toObservable that is in addition to HystrixExecutable. I'd appreciate your review of the API in #321.

[bbaetz]

Yeah, fair enough. I'll comment on the other ticket