ConcurrentHashMap (Java Platform SE 7 )

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Parameters: value - a value to search for. It does not support the add or addAll operations. Map Overrides: clear in class java. Search and transformation functions provided as arguments should similarly return null to indicate the lack of any result in which case it is not used.

The value can be at most 32 since ints are used as bitsets over segments. Like but unlike , this class does not allow null to be used as a key or value. Yes it is possible that two threads can simultaneously write on the ConcurrentHashMap. So what does the thread-safe Map means?

Difference between HashMap and ConcurrentHashMap

The table is a map from a key to a type T. The traits type HashCompare defines how to hash a key and how to compare two keys. Typically, when accessing a container element, you are interested in either updating it or reading it. An accessor represents update write access. As long as it points to an element, all other attempts to look up that key in the table block until the accessor is done. This feature can greatly improve concurrency in situations where elements are frequently read and infrequently updated. concurrent hash map To do so, declare it in the innermost block possible. To release access even sooner than the end of the block, use method release. It implicitly requests write access. Therefore before removing the key, it waits on any other extant accesses on key.

ConcurrentHashMap achieves this by a simple tweak. This is the same policy as ConcurrentReaderHashMap. As I understand from the above one that each shard will serve one thread. Object value Removes the key, value pair from this table. ConcurrentHashMap Example ConcurrentHashMap is the class that is similar to HashMap but works fine when you try to modify your map at runtime. So every modification is performed on Map is locked on Map object. Updates This class supports a hard-wired preset concurrency level of 32. Type Parameters: K - the element type of the returned set Parameters: initialCapacity - The implementation performs internal sizing to accommodate this many elements. If the level of concurrency required is not specified then it is takes 16 as the default value. The concurrency properties of bulk operations follow from those of ConcurrentHashMap: Any non-null result returned from get key and related access methods bears a happens-before relation with the associated insertion or update. I have 2 threads trying to update the first shard elements same time. U long parallelismThreshold, transformer, reducer Returns the result of accumulating the given transformation of all keys using the given reducer to combine values, or null if none.