ConcurrentHashMap in java

Prerequisites: Need of ConcurrentMap
ConcurrentHashMap ConcurrentHashMap class is introduced in JDK 1.5, which implements ConcurrentMap as well as to the Serializable interface also. ConcureentHashMap is the enhancement of HashMap as we know that while dealing with Threads in our application HashMap is not a good choice because performance-wise HashMap is not up to the mark.

Key points of ConcurrentHashMap:

• The underlined data structure for ConcurrentHashMap is Hashtable.
• ConcurrentHashMap class is thread-safe i.e. multiple threads can operate on a single object without any complications.
• At a time any number of threads are applicable for reading operation without locking the ConcurrentHashMap object which is not there in HashMap.
• In ConcurrentHashMap, the Object is divided into the number of segments according to the concurrency level.
• The default concurrency-level of ConcurrentHashMap is 16.
• In ConcurrentHashMap, at a time any number of threads can perform retrieval operation but for updated in an object, a thread must lock the particular segment in which thread wants to operate. This type of locking mechanism is known as Segment locking or bucket locking. Hence at a time, 16 updation operations can be performed by threads.
• null insertion is not possible in ConcurrentHashMap as key or value.

Constructors of ConcurrentHashMap:

1. ConcurrentHashMap m=new ConcurrentHashMap();:Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16).
2. ConcurrentHashMap m=new ConcurrentHashMap(int initialCapacity);:Creates a new, empty map with the specified initial capacity, and with default load factor (0.75) and concurrencyLevel (16).
3. ConcurrentHashMap m=new ConcurrentHashMap(int initialCapacity, float load factor);:
   Creates a new, empty map with the specified initial capacity and load factor and with the default concurrency level (16).
4. ConcurrentHashMap m=new ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel);:Creates a new, empty map with the specified initial capacity, load factor and concurrency level.
5. ConcurrentHashMap m=new ConcurrentHashMap(Map m);: Creates a new map with the same mappings as the given map.

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Output:

{100=Hello, 102=Geeks, 103=Hello}

Methods in ConcurrentHashMap:

1. clear? (): This method removes all of the mappings from this map.
2. compute? (K key, BiFunction<K, V> remapping function): This method attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
3. Is the computer absent?(K key, Function<K, V> mapping function): This method computes if the specified key is not already associated with a value, attempts to compute its value using the given mapping function and enters it into this map unless null.
4. computer present?(K key, BiFunction<K,V> remapping function): This method computes if the value for the specified key is present, attempts to compute a new mapping given the key and its current mapped value.
5. contains?(Object value): This method tests if some keymaps into the specified value in this table.
6. containsKey?(Object key): This method tests if the specified object is a key in this table.
7. contains value?(Object value): This method returns true if this map maps one or more keys to the specified value.
8. elements?(): This method returns an enumeration of the values in this table.
9. entrySet?(): This method returns a Set view of the mappings contained in this map.
10. equals?(Object o): This method Compares the specified object with this map for equality.
11. forEach(long parallelismThreshold, BiConsumer<K, V> action): This method performs the given action for each (key, value).
12. forEach?(long parallelismThreshold, BiFunction<K,V> transformer, Consumer<U> action): This method performs the given action for each non-null transformation of each (key, value).
13. forEach?(BiConsumer<K, V> action): This method performs the given action for each entry in this map until all entries have been processed or the action throws an exception.
14. for each entry?(long parallelismThreshold, Consumer<Map.Entry<K,V>> action): This method performs the given action for each entry.
15. for each entry?(long parallelismThreshold, Function<Map.Entry<K,V>> transformer, Consumer<U> action): This method performs the given action for each non-null transformation of each entry.
16. forEachKey?(long parallelismThreshold, Consumer<K> action): This method performs the given action for each key.
17. forEachKey?(long parallelismThreshold, Function<K> transformer, Consumer<U> action): This method performs the given action for each non-null transformation of each key.
18. forEachValue?(long parallelismThreshold, Consumer<V> action): This method performs the given action for each value.
19. forEachValue?(long parallelismThreshold, Function<V, U> transformer, Consumer<U> action): This method performs the given action for each non-null transformation of each value.
20. get?(Object key): This method returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
21. getOrDefault?(Object key, V defaultValue): This method returns the value to which the specified key is mapped, or the given default value if this map contains no mapping for the key.
22. hashCode?(): This method returns the hash code value for this Map, i.e., the sum of, for each key-value pair in the map, key.hashCode() ^ value.hashCode().
23. isEmpty?(): This method returns true if this map contains no key-value mappings.
24. keys? (): This method returns an enumeration of the keys in this table.
25. keySet?(): This method returns a Set view of the keys contained in this map.
26. keySet? (V mapped value): This method returns a Set view of the keys in this map, using the given common mapped value for any additions (i.e., Collection.add(E) and Collection.addAll(Collection)).
27. mappingCount?(): This method returns the number of mappings.
28. merge? (K key, V value, BiFunction<K, V> remapping function): This method If the specified key is not already associated with a (non-null) value, associates it with the given value.
29. newKeySet?(): This method Creates a new Set backed by a ConcurrentHashMap from the given type to Boolean.TrUE.
30. newKeySet?(int initialCapacity): This method Creates a new Set backed by a ConcurrentHashMap from the given type to Boolean.TrUE.
31. put? (K key, V value): This method Maps the specified key to the specified value in this table.
32. putAll?(Map<K, V> m): This method Copies all of the mappings from the specified map to this one.
33. putIfAbsent?(K key, V value): This method If the specified key is not already associated with a value, associates it with the given value.
34. reduce? (long parallelismThreshold, BiFunction<K, V> transformer, BiFunction<U, V> reducer): This method returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, or null if none.
35. reduceEntries?(long parallelismThreshold, BiFunction<Map.Entry<K, V>> reducer): This method returns the result of accumulating all entries using the given reducer to combine values, or null if none.
36. reduceEntries?(long parallelismThreshold, Function<Map.Entry<K, V>> transformer, BiFunction<U, V> reducer): This method returns the result of accumulating the given transformation of all entries using the given reducer to combine values, or null if none.
37. reduceEntriesToDouble?(long parallelismThreshold, ToDoubleFunction<Map.Entry<K, V>> transformer, double basis, DoubleBinaryOperator reducer): This method returns the result of accumulating the given transformation of all entries using the given reducer to combine values and the given basis as an identity value.
38. reduceEntriesToInt?(long parallelismThreshold, ToIntFunction<Map.Entry<K, V>> transformer, int basis, IntBinaryOperator reducer): This method returns the result of accumulating the given transformation of all entries using the given reducer to combine values and the given basis as an identity value.
39. reduceEntriesToLong?(long parallelismThreshold, ToLongFunction<Map.Entry<K, V>> transformer, long basis, LongBinaryOperator reducer): This method returns the result of accumulating the given transformation of all entries using the given reducer to combine values and the given basis as an identity value.
40. reduceKeys?(long parallelismThreshold, BiFunction<K> reducer): This method returns the result of accumulating all keys using the given reducer to combine values, or null if none.
41. reduceKeys?(long parallelismThreshold, Function<K> transformer, BiFunction<U> reducer): This method returns the result of accumulating the given transformation of all keys using the given reducer to combine values, or null if none.
42. reduceKeysToDouble?(long parallelismThreshold, ToDoubleFunction<K> transformer, double basis, DoubleBinaryOperator reducer): This method returns the result of accumulating the given transformation of all keys using the given reducer to combine values and the given basis as an identity value.
43. reduceKeysToInt?(long parallelismThreshold, ToIntFunction<K> transformer, int basis, IntBinaryOperator reducer): This method returns the result of accumulating the given transformation of all keys using the given reducer to combine values and the given basis as an identity value.
44. reduceKeysToLong?(long parallelismThreshold, ToLongFunction<K> transformer, long basis, LongBinaryOperator reducer): This method returns the result of accumulating the given transformation of all keys using the given reducer to combine values and the given basis as an identity value.
45. reduceToDouble?(long parallelismThreshold, ToDoubleBiFunction<K, V> transformer, double basis, DoubleBinaryOperator reducer): This method returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values and the given basis as an identity value.
46. reduceToInt?(long parallelismThreshold, ToIntBiFunction<K, V> transformer, int basis, IntBinaryOperator reducer): This method returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.
47. reduceToLong?(long parallelismThreshold, ToLongBiFunction<K, V> transformer, long basis, LongBinaryOperator reducer): This method returns the result of accumulating the given transformation of all (key, value) pairs using the given reducer to combine values, and the given basis as an identity value.
48. reduced values?(long parallelismThreshold, BiFunction<V> reducer): This method returns the result of accumulating all values using the given reducer to combine values, or null if none.
49. reduced values?(long parallelismThreshold, Function<V, U> transformer, BiFunction<U> reducer): This method returns the result of accumulating the given transformation of all values using the given reducer to combine values, or null if none.
50. reduceValuesToDouble?(long parallelismThreshold, ToDoubleFunction<V> transformer, double basis, DoubleBinaryOperator reducer): This method returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.
51. reduceValuesToInt?(long parallelismThreshold, ToIntFunction<V> transformer, int basis, IntBinaryOperator reducer): This method returns the result of accumulating the given transformation of all values using the given reducer to combine values, and the given basis as an identity value.
52. reduceValuesToLong?(long parallelismThreshold, ToLongFunction<V> transformer, long basis, LongBinaryOperator reducer): This method returns the result of accumulating the given transformation of all values using the given reducer to combine values and the given basis as an identity value.
53. remove?(Object key): This method removes the key (and its corresponding value) from this map.
54. remove?(Object key, Object value): This method removes the entry for a key only if currently mapped to a given value.
55. replace?(K key, V value): This method replaces the entry for a key only if currently mapped to some value.
56. replace?(K key, V oldValue, V newValue): This method replaces the entry for a key only if currently mapped to a given value.
57. replaceAll?(BiFunction<K, V> function): This method replaces each entry’s value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.
58. Search? (long parallelismThreshold, BiFunction<K, V, U> search function): This method returns a non-null result from applying the given search function on each (key, value), or null if none.
59. Searchmetrics? (long parallelismThreshold, Function<Map.Entry<K, V>, U> search function): This method returns a non-null result from applying the given search function on each entry, or null if none.
60. searches? (long parallelismThreshold, Function<K, U> search function): This method returns a non-null result from applying the given search function on each key, or null if none.
61. search values?(long parallelismThreshold, Function<V, U> search function): This method returns a non-null result from applying the given search function on each value, or null if none.
62. size?(): This method returns the number of key-value mappings in this map.
63. toString?(): This method returns a string representation of this map.
64. values?(): This method returns a Collection view of the values contained in this map.

Reference: https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ConcurrentHashMap.html