The RxMarbles tool is available at http://rxmarbles.com/ and was created by André Staltz. It provides an interactive interface for various reactive operators. This tool was created for RxJs, but there are no conceptual differences from RxJava. The things you learn across these platforms are transferable.
For example, it is possible to play with a .map() operation, as shown in the following figure:
Also, the changes will be seen in realtime. So, for example, if we alter the order of the elements, as in the following figure, the after-transformation changes will be reflected immediately:
There isn't much to add about this tool--just ...