Monads in Java in 20 minutes

There is an entire literature genre formed from the plethora of fascinating monad tutorials, related articles and videos. Nevertheless, it is often a challenge for a Java developer to unravel this topic starting from zero, since one can easily spend several days studying how a burrito is different from a shawarma, or how proper classes are different from sets in the Von Neumann–Bernays–Gödel set theory, without getting anywhere closer to an understanding what the monad is.

In this short "tutorial on tutorials” we will define a functor and a monad, give some basic examples in Java, including the enigmatic IO monad, with an ultimate goal to find a key to understanding of more advanced functional programming resources.