why null key not allowed in concurrenthashmap?
533828
The main reason that null is not allowed in ConcurrentMaps such as ConcurrentHashMaps, ConcurrentSkipListMaps is to avoid ambiguities. ... In a non-concurrent map, you may check this using map. contains(key), but in a concurrent environment, the map might have changed between calls.
1297942
2 Answers. UUID is immutable so it's potentially thread safe, but apparently there was some evil caching going on in some accessors that made it unsafe (that bug is fixed now). ... nextBytes , which is used by the UUID. randomUUID factory, which definitely is thread-safe.
654385
['1) Call equals() and equalsIgnoreCase() method on known String literal rather unknown object.', '2) Prefer valueOf() over toString() where both return same result.', '5) Use of annotation @NotNull and @Nullable.']
220386
This is because of another common cause (not related to idmapd) for files on an NFS mount to be unexpectedly owned by "nobody": The concept of "root_squash". By default, an NFS Server which gets a request from a client machine's root user will "squash" the request and treat it as if it came from user "nobody".
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A NullReferenceException happens when you try to access a reference variable that isn't referencing any object. If a reference variable isn't referencing an object, then it'll be treated as null . Reference types default to null to indicate that they are not referencing any object. ...
302900
Answer: No. We can't have a Primary Key column with a NULL value. The reason for the same is very simple, primary key purpose is to uniquely identify records. ... This is the reason, Primary Key can't have NULL values as they are not compared with any other value.
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Interface LoadingCache<K,V> A semi-persistent mapping from keys to values. Values are automatically loaded by the cache, and are stored in the cache until either evicted or manually invalidated. ... Implementations of this interface are expected to be thread-safe, and can be safely accessed by multiple concurrent threads.
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Nothing: The String reference itself equals Nothing. This is another term for null.
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['First, it gets the hash code of the key object, which is passed, and finds the bucket location.', 'If the correct bucket is found, it returns the value.', 'If no match is found, it returns null.']
1021860
HashMap doesn't allow duplicate keys but allows duplicate values. That means A single key can't contain more than 1 value but more than 1 key can contain a single value. HashMap allows null key also but only once and multiple null values.
1067023
Hence there can only be one null key in one hashmap object. If you pass null as map key, it will go to 0 bucket . All values of null key will go there. That is why it returns same value, cause all keys you are providing are null and are in the same bucket of your HashMap.
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The HashMap class is roughly equivalent to Hashtable , except that it is non synchronized and permits nulls. ( HashMap allows null values as key and value whereas Hashtable doesn't allow null s). HashMap does not guarantee that the order of the map will remain constant over time.
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Primary Key can't accept null values. Unique key can accept only one null value. By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index. ... In SQL Server, Unique key can be made foreign key into another table.
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['1) Set bean properties.', '2) Set Map values.', '3) Create a Scoped variable on-page, request, session, or application scope.', 'But <c:set> comes with its own caveat like <c:set> can remove attributes or variable if provided value is null or throw exceptions which we will here.']
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As @Mike is saying, creation of enum is guaranteed to be thread safe. However, methods that you add to an enum class do not carry any thread safety guarantee. ... If this method has side effects (changes the state of some variable) then you need to think about protecting it (i.e., make it synchronized ) or parts thereof.
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Both poll() and remove() method is used to remove head object of the Queue. The main difference lies when the Queue is empty(). If Queue is empty then poll() method will return null . While in similar case , remove() method will throw NoSuchElementException .
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Primary key will not accept NULL values whereas Unique key can accept one NULL value. A table can have only primary key whereas there can be multiple unique key on a table. A Clustered index automatically created when a primary key is defined whereas Unique key generates the non-clustered index.