Aggregate: ToList, Reduce, and Collect

Aggregating

RxJava has several operators that aggregate emissions from an Observable to provide an Observer with an intended result. Operators that aggregate are typically stateful, since an internal state must be maintained between emissions. A few commonly used aggregation operators are .toList(), .reduce(), and .collect().

ToList

The .toList() operator converts an Observable<T> into an Observable<List<T>>. This is useful if the Observer ultimately cares about a List<T> but the stream itself pushes objects of type T.