For what reasons does the HashMap data structure not allow the use of null as a key?

In Java, the HashMap data structure allows the use of null as a key, but with limitations. However, if we consider the general reasons why some hash table implementations do not allow null as a key, they are related to:

• Ambiguity in behavior when computing the hash code: null does not have a hashCode() method, so the hash cannot be obtained directly.
• Possible errors when comparing keys: the equals() method is not called for null, which complicates the search logic.
• To simplify implementation and increase reliability, some data structures prohibit null to avoid NullPointerException.

In standard Java HashMap, null as a key is allowed (one null key), but in other implementations, such as ConcurrentHashMap, null keys are prohibited for thread safety and predictable behavior reasons.