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

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With Java 9, new factory methods have been introduced for the List, Set and Map interfaces. These methods allow quickly instantiating a Map object with values in one line. Now, if we consider: 

Map<Integer, String> map1 = new HashMap<Integer, String>(Map.of(1, "value1", 2, "value2", 3, "value3"));
map1.put(4, null);

The above is allowed without any exception while if we do:

Map<Integer, String> map2 = Map.of(1, "value1", 2, "value2", 3, "value3", 4, null );

It throws:

Exception in thread "main" java.lang.NullPointerException
    at java.base/java.util.Objects.requireNonNull(Objects.java:221)
..

I am not able to get, why null is not allowed in second case.

I know HashMap can take null as a key as well as a value but why was that restricted in the case of Map.of?

The same thing happens in the case of java.util.Set.of("v1", "v2", null) and java.util.List.of("v1", "v2", null).

java-9 hashmap null dictionary java
8 Answers

In my opinion non-nullability makes sense for keys, but not for values.

  1. The Map interface becomes a weak contract, you can't trust it's behaviour without looking at the implementation, and that is assuming you have access to see it. Java11's Map.of() does not allow null values while HashMap does, but they both implement the same Map contract - how come?
  2. Having a null value or no value at all could not, and arguably should not be considered as a distinct scenario. When you get the value for a key and you get null, who cares if it was explicitly put there or not? there's simply no value for the supplied key, the map does not contain such key, simple as that. Null is null, there's no nullnull or null^2
  3. Existing libraries make extensive use of map as a means to pass properties to them, many of them optional, this makes Map.of() useless as a construct for such structures.
  4. Kotlin enforces nullability at compile time and allows for maps with null values with no known issues.
  5. The reason behind not allowing null values is probably just due to implementation detail and convenience of the creator.
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