RxJava2: Using Flowable with zipWith - rx-java2

Question

I'm trying to make the following code work me but something is wrong, here is a snippet:
private void myMethod() {
Flowable.fromIterable(cache)
.zipWith(this::doesExist, (record, exist) -> {
// do stuff
return true;
}).subscrib();
}
private Flowable<Boolean> doesExist(CacheRecord record) {
// Do something
return Flowable.just(true);
}
This doesn't compile, any idea?
UPDATE:
Any thoughts about the following snippet:
Flowable.fromIterable(m_cache) //
.flatMapCompletable(cachedStation -> {
return Single.zip(Single.just(cachedStation), doesIssueExist(cachedStation), (record, exist) -> {
System.out.println(cachedStation + ", " + exist);
return true;
}).toCompletable();
}).subscribe();

Answer 1

Your doesExist method requires a CacheRecord as a parameter. But the method reference you have given this::doesExist sends an instance of Subscriber<? super Object> that's why the incompatible type error is showing.
The expanded form of your method is given below.
private void myMethod() {
Flowable.fromIterable(cache)
.zipWith(new Publisher<Object>() {
#Override
public void subscribe(Subscriber<? super Object> s) {
doesExist(s);
}
}, (record, exist) -> {
// do stuff
return true;
}).subscribe();
}
Here, the first parameter to zipWith
new Publisher<Object>() {
#Override
public void subscribe(Subscriber<? super Object> s) {
doesExist(s);
}
}
is what you have shortened as this::doesExist
As you can see the zipWith requires the first parameter a Publisher, and you have created an anonymous Publisher, and in the subscribe method you are calling doesExist(s) by sending the Subscriber<? super Object> s, which is not the required type. Your method reference statement this::doesExist does exactly the above operation and that's why the incompatible type error is shown by the compiler.
If you are trying to zip the Flowable with the flowable returned by doesExist method, you can directly call it, without method reference, by passing a valid CacheRecord object as follows
Flowable.fromIterable(cache)
.zipWith(doesExist(anotherCache), (record, exist) -> {
// do stuff
return true;
}).subscribe();
Note: See method reference for more information
Update: If you are trying to pass the items emitted by fromIterable to doesExist method and get combined result boolean and cacheRecord, then
create a holder class as follows
class CacheRecordResult {
CacheRecord cacheRecord;
boolean isExist;
public CacheRecordResult(CacheRecord cacheRecord, boolean isExist) {
this.cacheRecord = cacheRecord;
this.isExist = isExist;
}
}
Then subscribe to CacheRecordResult as follows
private void myMethod() {
Flowable.fromIterable(cache)
.flatMap(cacheRecord -> doesExist(cacheRecord)
.map(exist -> new CacheRecordResult(cacheRecord, exist)))
.subscribe(cacheRecordResult -> {
CacheRecord cacheRecord = cacheRecordResult.cacheRecord;
boolean isExist = cacheRecordResult.isExist;
});
}