Why does Map.of not allow null keys and values?

Question

With Java 9, new factory methods have been introduced for the List, Set and Map interfaces. These methods allow quickly instantiating a Ma

Answer 1

The major difference is: when you build your own Map the "option 1" way ... then you are implicitly saying: "I want to have full freedom in what I am doing".

So, when you decide that your map should have a null key or value (maybe for the reasons listed here) then you are free to do so.

But "option 2" is about a convenience thing - probably intended to be used for constants. And the people behind Java simply decided: "when you use these convenience methods, then the resulting map shall be null-free".

Allowing for null values means that

 if (map.contains(key)) 

is not the same as

 if (map.get(key) != null)

which might be a problem sometimes. Or more precisely: it is something to remember when dealing with that very map object.

And just an anecdotal hint why this seems to be a reasonable approach: our team implemented similar convenience methods ourselves. And guess what: without knowing anything about plans about future Java standard methods - our methods do the exact same thing: they return an immutable copy of the incoming data; and they throw up when you provide null elements. We are even so strict that when you pass in empty lists/maps/... we complain as well.