Java Collection Tips

https://stackoverflow.com/questions/51872623/why-guava-standardtable-put-method-refuse-null-value

null in general seen as key or value in any Collection or Map is seen as a bad decision. The jdk itself has some examples, ConcurrentHashMap does not allow null keys or values, so does ConcurrentSkipListMap and many others.

So does static factory methods added in java-9, like Map::of. Even HashMap that does allow a null key and value, so these would both not fail:

HashMap<String, String> map = new HashMap<>();
map.put("", null);
map.put(null, "");

Using any of the new java-8 method of a HashMap will throw a NullPointerException:

 // value trying to be null
 chm.merge("", null, (k, v) -> k);

And the last point is that all guava Collections are not null-tolerant, which to me is great decision.

https://www.techiedelight.com/remove-null-values-map-java/

https://stackoverflow.com/questions/45210398/why-does-map-of-not-allow-null-keys-and-values

As others pointed out, the Map contract allows rejecting nulls...

[S]ome implementations prohibit null keys and values [...]. Attempting to insert an ineligible key or value throws an unchecked exception, typically NullPointerException or ClassCastException.

... and the collection factories (not just on maps) make use of that.

They disallow null keys and values. Attempts to create them with null keys or values result in NullPointerException.

But why?

Allowing null in collections is by now seen as a design error. This has a variety of reasons. A good one is usability, where the most prominent trouble maker is Map::get. If it returns null, it is unclear whether the key is missing or the value was null. Generally speaking, collections that are guaranteed null free are easier to use. Implementation-wise, they also require less special casing, making the code easier to maintain and more performant.

You can listen to Stuart Marks explain it in this talk but JEP 269 (the one that introduced the factory methods) summarizes it as well:

Null elements, keys, and values will be disallowed. (No recently introduced collections have supported nulls.) In addition, prohibiting nulls offers opportunities for a more compact internal representation, faster access, and fewer special cases.

Since HashMap was already out in the wild when this was slowly discovered, it was too late to change it without breaking existing code but most recent implementations of those interfaces (e.g. ConcurrentHashMap) do not allow null anymore and the new collections for the factory methods are no exception.

(I thought another reason was that explicitly using null values was seen as a likely implementation error but I got that wrong. That was about to duplicate keys, which are illegal as well.)

So disallowing null had some technical reason but it was also done to improve the robustness of the code using the created collections.