Unquestioningly, the most used operator in RxJava is map, which has the following signature:

<R> Observable<R> map(Func1<T, R> func)

The preceding method declaration means that the func function can transform the T object type to the R object type, and applying map transforms Observable<T> into Observable<R>. However, a signature does not always describe the operator's behavior well, especially if the operator is doing a complex transformation. For these purposes, Marble diagrams were invented. Marble diagrams visually present stream transformations. They are so effective for describing the operator's behavior that almost all RxJava operators contain the image with a marble diagram in Javadoc. The map operator is represented by the following diagram:

From looking at the preceding diagram, it should be clear that the map makes a one-to-one transformation. Furthermore, the output stream has the same number of elements as the input stream.