HashMap null check in Merge Operation

The behavior is mandated by the Map.merge contract:

Throws:

…

NullPointerException - if the specified key is null and this map does not support null keys or the value or remappingFunction is null

Note that using Map.merge for Collectors.toMap without a merge function is an implementation detail; it not only disallows null values, it does not provide the desired behavior for reporting duplicate keys, the Java 8 implementation wrongly reports one of the two values as key when there are duplicate keys.

In Java 9, the implementation has been completely rewritten, it does not use Map.merge anymore. But the new implementation is behavioral compatible, now having code explicitly throwing when the value is null. So the behavior of Collectors.toMap not accepting null values has been fixed in the code and is not an artifact of using Map.merge anymore. (Still speaking of the toMap collector without a merge function only.)

Unfortunately, the documentation does not tell.

Because internally for Collectors.toMap, Map#merge is used - you can't really do anything about it. Using the static Collectors.toMap is not an option (which by the way is documented to throw a NullPointerException).

But spinning a custom collector to be able to do what you want (which you have not shown) is not that complicated, here is an example:

 Map<Integer, Integer> result = Arrays.asList(null, 1, 2, 3)
            .stream()
            .collect(
                    HashMap::new,
                    (map, i) -> {
                        map.put(i, i);
                    },
                    HashMap::putAll);