Some argue that the Optional type is worth being used in collections. It allegedly solves the problems like a HashMap that returns null if there is no mapping for a key or if null is mapped to the key. If you use a Map<Optional<Something>> then you can clearly separate a missing mapping and an absent value. And that way you are one level deeper in the rabbit hole.

First of all...

You can...

...tell if a key is mapped to null or not mapped without using Optional. There is the method containsKey(). This is one more method call to separate the non-mapped key from a mapped null value. However, calling Optional does this too. So what is the point? On the other hand...

You do not need...

...to tell if the key is mapped to null or if the mapping is missing. If there is a difference in your program code between the two cases then you coded the business logic in the wrong way. This is certainly a code smell. Think of null as “nothing,” and instead of thinking “null is assigned to the key ‘aaaaaarrghhh'” say out loud: Nothing is assigned to the key ‘aaaaaarrghhh’. You see? There is no difference.  And now everyone in your office is giving you funny looks.

Using Optional as a value in a Map...

You will...

...end up one level deeper in the rabbit hole after a while. Code lives an independent life. It is not only you, who develop it. In large organizations there are developers who are certainly drunk when coding. (This is the only reasonable explanation for some code.) They will soon populate your Map<Optional<Something>> with

• Null values,
• Absent Optional values
• and even  Optional objects that wrap something else, but not your “Something“.

From time to time, if you are lucky you may even find some non-null, non-absent Optional<Something> values.