Implement 'toString' method for some Emitters

[strekha]

When use .create method it's unclear why emitter is null (if call toString or observe object via debugger).

[codecov]

Codecov Report

Merging #5995 into 2.x will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5995      +/-   ##
============================================
- Coverage     98.25%   98.24%   -0.02%     
+ Complexity     6051     6047       -4     
============================================
  Files           656      656              
  Lines         44078    44085       +7     
  Branches       6118     6118              
============================================
+ Hits          43311    43312       +1     
  Misses          231      231              
- Partials        536      542       +6

Impacted Files	Coverage Δ	Complexity Δ
...ctivex/internal/operators/single/SingleCreate.java	95.45% <100%> (+0.1%)	2 <0> (ø)	⬇️
...ex/internal/operators/flowable/FlowableCreate.java	97.41% <100%> (+0.01%)	6 <0> (ø)	⬇️
...nternal/operators/observable/ObservableCreate.java	99.14% <100%> (+0.88%)	2 <0> (ø)	⬇️
...eactivex/internal/operators/maybe/MaybeCreate.java	94.11% <100%> (+0.11%)	2 <0> (ø)	⬇️
...ernal/operators/completable/CompletableCreate.java	95.23% <100%> (+0.11%)	2 <0> (ø)	⬇️
...erators/flowable/FlowableOnBackpressureLatest.java	94.59% <0%> (-5.41%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	94.44% <0%> (-4.17%)	54% <0%> (-3%)
...l/operators/observable/ObservableFlatMapMaybe.java	90.84% <0%> (-3.93%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
... and 22 more

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[akarnokd]

Please add unit tests that check these toString()s to some extent.

[akarnokd]

Please add unit tests.

[akarnokd]

If this fails, the test still passes because the error is turned into onError. Please add the .assertEmpty() after test(). In addition, please do the test for the serialized emitter as well.

[akarnokd]

The Maybe and Completable variants are missing.

[akarnokd]

Please add the serialized test here as well.

[vanniktech]

Aren't there more cases where we should do this then?

I'm still unsure how much the benefit is worth it.

[akarnokd]

Aren't there more cases where we should do this then?

Maybe on the emitters of generators. Any user-facing API object could be a candidate.

I'm still unsure how much the benefit is worth it.

There is occasionally a question where the OP states the emitter is null and he is getting an NPE. Then I had to explain that when debugging, the toString value returns "null" and not the emitter reference is null.

[vanniktech]

Should we cover the generators too?

[strekha]

Think we should. So I will add this soon.
Are there any other places except the generators? Because I don't know the API so well.

[akarnokd]

The generate methods. Please put them into a separate PR.

[vanniktech]

Alright. Separate PR it is.