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

Allowing nulls in maps has been an error. We can see it now, but I guess it wasn't clear when HashMap was introduced. NullpointerException is the most common bug seen in production code.

I would say that the JDK goes in the direction of helping developers fight the NPE plague. Some examples:

• Introduction of Optional
• Collectors.toMap(keyMapper, valueMapper) doesn't allow neither the keyMapper nor the valueMapper function to return a null value
• Stream.findFirst() and Stream.findAny() throw NPE if the value found is null

So, I think that disallowing null in the new JDK9 immutable collections (and maps) goes in the same direction.