why null not allowed in concurrenthashmap?
317674
The main reason that nulls aren't allowed in ConcurrentMaps (ConcurrentHashMaps, ConcurrentSkipListMaps) is that ambiguities that may be just barely tolerable in non-concurrent maps can't be accommodated. ... contains(key) , but in a concurrent one, the map might have changed between calls.
700778
NULL and nil are equal to each other, but nil is an object value while NULL is a generic pointer value ( (void*)0 , to be specific). [NSNull null] is an object that's meant to stand in for nil in situations where nil isn't allowed. For example, you can't have a nil value in an NSArray.
498477
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.
121506
The reason for allowing only one null key is because keys in a HashMap has to be unique. On the other hand Hashtable does not allow null keys. This is because the objects used as keys in a Hashtable implements the hashCode() and equals() methods for their storage and retrieval.
681289
null checking shows you whether there is an object generally. isEmpty checking shows you whether the content of existing String object is empty.
906981
['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.']
1072209
4 Answers. It is never OK to read something possibly modified in a different thread without synchronization. What level of synchronization is needed depends on what you are actually reading. For primitive types, you should have a look at atomic reads, e.g. in the form of std::atomic<bool> .
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6 Answers. Thread. join() waits for the thread to completely finish, whereas a synchronized block can be used to prevent two threads from executing the same piece of code at the same time.
839321
Operation: LinkedHashMap does a mapping of keys to values while LinkedHashSet simply stores a collection of things. Null Object: LinkedHashMap have elements in key value pair so have only one null key and multiple null value, while LinkedHashSet simply stores a collection of things with one null value.
375019
Using Iterator can be used on any collection and since it doesn't have to be thread safe there is no additional locking overhead. In the example of the micro-benchmark given, the reason the Enumeration is faster than Iterator is because it is tested first. ;)
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ViewData and TempData require typecasting and null checking whereas ViewBag doesn't need such checking.
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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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UPDATE test SET test1 = NVL(test1, 'hello'), test2 = NVL(test2, 'world') WHERE test2 IS NULL OR test1 IS NULL; Though it may fire your update triggers even for the rows that are effectively unchanged. COALESCE() works similarly to NVL() in this circumstance -- returning the first non-null value.
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It is, the EJB 3.1 specification, JSR-318, page 82 explains the reasons: "… The container serializes calls to each stateful and stateless session bean instance. ... Working with EJBs without any further configuration is thread-safe regardless whether you are invoking one method or multiple methods concurrently.
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Null has no bounds, it can be used for string, integer, date, etc. fields in a database. Empty string is just regarding a string; it's a string like 'asdfasdf' is, but is just has no length.
479852
Null Keys and Null Values TreeMap sorts elements in natural order and doesn't allow null keys because compareTo() method throws NullPointerException if compared with null. So if we try to run the next code: TreeMap<String, Integer> treeMap = new TreeMap<>(); treeMap. put(null, 5); treeMap.
461743
HashMap allows a single null key and multiple null values. TreeMap does not allow null keys but can have multiple null values. HashMap allows heterogeneous elements because it does not perform sorting on keys. TreeMap allows homogeneous values as a key because of sorting.