In Java, ConcurrenHashMap was introduced in 1.5v to resolve the problems on HashMap in a multithreaded environment. While HashMap is not thread-safe, ConcurrentHashMap is thread-safe.
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Properties of ConcurrentHashMap in Java
- HashTable is underline data structure.
- Concurrent read and thread safe update operations are allowed.
- For read operation thread could not require any lock but for update operation thread requires a lock but it is the lock of only a particular part of map.
- Instead of whole map concurrent update is acheived by dividing the map into smaller portion, which is defined by concurrency level.
- The default concurrency level is 16 that is ConcurrentHashMap allows 16 read operations and simultaneously 16 write(update) operations.
- Null value is not allowed for both key and value.
HashMap Constructor
ConcurrentHashMap()
ConcurrentHashMap concurrentHashMap = new ConcurrentHashMap(); This constructor creats an empty ConcurrentHashMap with default initial capacity 16 and default load factor/fill ratio 0.75 and default concurrency level 16.
ConcurrentHashMap( int initialCapacity )
ConcurrentHashMap concurrentHashMap = new ConcurrentHashMap(int initialCapacity); ConcurrentHashMap( int initialCapacity,float loadFactor )
ConcurrentHashMap concurrentHashMap = new ConcurrentHashMap(int initialCapacity, float loadFactor ); ConcurrentHashMap( int initialCapacity,float loadFactor,int concurrencyLevel )
ConcurrentHashMap concurrentHashMap = new ConcurrentHashMap(int initialCapacity, float loadFactor , int concurrencyLevel); ConcurrentHashMap Java Implementation
import java.util.concurrent.ConcurrentHashMap;
public class ConcurrentHashMapImplementation {
public static void main(String[] args) {
ConcurrentHashMap concurrentHashMap = new ConcurrentHashMap();
concurrentHashMap.put(1,"The");
concurrentHashMap.put(2,"Code");
concurrentHashMap.put(3,"Data");
concurrentHashMap.put(4,"Java");
System.out.println(concurrentHashMap);
concurrentHashMap.replace(4, "Java, Python, C, C++, Go");
System.out.println(concurrentHashMap);
}
}
Different Between HashMap and ConcurrentHashMap
| HashMap | ConcurrentHashMap |
|---|---|
| HashMap is not thread-safe. | ConcurrentHashMap is thread-safe. |
| Relatively performance is high | Relatively performance is low |
Iterator of HashMap is fail-fast. If the HashMap is modified while is an iterator in use, it will throw ConcurrentModificationException. | Iterator of ConcurrentHashMap is fail-safe. It will not throw ConcurrentModificationException even if the map is modified during iteration. |
| Null is allowed for both key and value | Null is not allowed for both key and value. |
| Introduced in Java 1.2v | Introduced in Java 1.5v |