What's the difference between curly braces and normal brackets in RxJava with Kotlin

Question

I don't understand the real difference between the curly braces and and the normal brackets in Kotlin when using RxJava. For example, I have the following code which works as expected:

someMethodThatReturnsCompletable()     .andThen(anotherMethodThatReturnsACompletable())     .subscribe(...) 

But the following does NOT work:

someMethodThatReturnsCompletable()     .andThen { anotherMethodThatReturnsACompletable() }     .subscribe(...) 

Note the difference in the andThen() part of the chain with the curly braces. I can't understand what the difference between the two is. I've had a look at some articles but unfortunately I am still having difficulty in understanding this subtle difference.

Answer 1

The first code segment executes anotherMethodThatReturnsACompletable() and passes the return value to andThen(), where a Completable is accepted as parameter.

In the second code segment, you are writing a function literal as a lambda expression. It passes a function of type () -> Unit to andThen(), which it also a valid statement, but the code inside the lambda may not be called.

In Kotlin, there is a convention that if the last parameter to a function is a function, and you're passing a lambda expression as the corresponding argument, you can specify it outside of parentheses:

lock (lock) {     sharedResource.operation() } 

Since Kotlin support SAM conversion,

This means that Kotlin function literals can be automatically converted into implementations of Java interfaces with a single non-default method, as long as the parameter types of the interface method match the parameter types of the Kotlin function.

Looking back to Completable, there are several overloaded andThen() functions:

andThen(CompletableSource next) andThen(MaybeSource<T> next) andThen(ObservableSource<T> next) andThen(org.reactivestreams.Publisher<T> next) andThen(SingleSource<T> next) 

Here you can specify the SAM Type by calling:

andThen( CompletableSource {     //implementations })