Collectors in Java

Collectors in Java, it is huge. The world of collectors is very huge & one can get lost while trying its features, my own experience.

 So below I am giving a glimpse about its power in the new Java world. But I also think that it is too tricky to get the things done which we can do easily with Imperative style & that will be quite expressive to, but again many people think that Functional way is the better one.
 So check below example & you decide & please also suggest the improvements.

​Collections

 Another example about Collectors,
what if toMap() get duplicate keys?
How to collect data in Map of Map

ToMaps