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Java / Map and its implementations

Why null is not allowed in ConcurrentHashmap?

The reason is that if map.get(key) returns null, you can't detect whether the key explicitly maps to null vs the key isn't mapped. In a non-concurrent map, you can check this via map.contains(key), but in a concurrent one, the map might have changed between calls.

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More Related questions...

List the differences between HashTable and HashMap in Java collections. Similarities between HashMap and HashTable in Java. What is a resource bundle in Java? What is the Big-O notation for operations in a Hashmap? Is HashMap faster compared to ArrayList in terms of search? Difference between HashMap and ConcurrentHashMap. Is ConcurrentHashmap good for frequently updating concurrent data structure? What are the commonly used implementations of Map in Java? Difference between Collections.synchronizedMap(Map) and ConcurrentHashMap. Difference between Collections.synchronizedMap and Hashtable. Why is ConcurrentHashMap faster than Hashtable in Java? What are the different Reference types available in Java? Which methods you need to override to use an object as key in HashMap ? Why Map does not extend the Collection interface in Java Collections Framework? How Remove method works internally in Java HashMap? Explain NavigableMap in Java collection. Explain rehashing process in Java HashMap. How null key is handled in HashMap? What is the data-structure behind the Java HashMap implementation? Why are immutable objects in HashMap so effective? Why null is not allowed in ConcurrentHashmap? Does ConcurrentHashMap allow null keys and null values ? Difference between remove() and clear() methods in HashMap. Is hashcode method invoked internally when calling get and put method in Java? How hash collision is handled in HashMap? What are the interfaces that TreeMap implements? What interfaces that HashMap implements? Big O notation for TreeMap in Java. Is TreeMap synchronized in Java? Does TreeMap allow null key? Key object constraints on a TreeMap. Does IdentityHashMap use hashcode method? What design pattern that Collections.synchronizedMap implement? Does EnumMap allow null key? Difference between HashMap and IdentityHashMap. What is a ConcurrentSkipListMap? How do you sort a TreeMap by comparing its value? Default initial table size of ConcurrentHashMap. Does Map implement Iterable?
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Set and its implementations

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