Differences Between HashMap and ConcurrentHashMap in Java

What is the difference between HashMap and ConcurrentHashMap? Please compare and interpret from the perspective of source code

HashMap and ConcurrentHashMap are both part of the Java Collections Framework, but they serve different purposes and have different characteristics, especially in terms of concurrency. Below, I will compare and interpret their differences from the perspective of source code and functionality.

1. Thread Safety

• HashMap:

  • HashMap is not synchronized, which means it is not thread-safe. If multiple threads access a HashMap concurrently and at least one of the threads modifies the map structurally (adding or removing elements), it must be externally synchronized. This can lead to issues like data inconsistency or infinite loops.

• ConcurrentHashMap:

  • ConcurrentHashMap is designed for concurrent access. It allows multiple threads to read and write without locking the entire map. Instead, it uses a technique called "lock striping," which divides the map into segments, allowing multiple threads to operate on different segments concurrently.

2. Performance

• HashMap:

  • Since HashMap is not synchronized, it generally performs better in single-threaded scenarios or when external synchronization is managed properly.

• ConcurrentHashMap:

  • ConcurrentHashMap is optimized for concurrent access, which can lead to better performance in multi-threaded environments. It allows for higher throughput by reducing contention among threads.

3. Null Keys and Values

• HashMap:

  • HashMap allows one null key and multiple null values.

• ConcurrentHashMap:

  • ConcurrentHashMap does not allow null keys or null values. This is to avoid ambiguity in concurrent operations, as a null value could be interpreted as a marker for "no value" or "not present."

4. Iteration

• HashMap:

  • Iterators for HashMap are fail-fast. If the map is modified after the iterator is created (except through the iterator's own remove method), it will throw a ConcurrentModificationException.

• ConcurrentHashMap:

  • Iterators for ConcurrentHashMap are weakly consistent. They reflect the state of the map at the time the iterator was created, but they may not reflect subsequent modifications. This allows for safe iteration even when the map is being modified concurrently.

5. Source Code Perspective

From a source code perspective, the implementation of these two classes reflects their differences:

• HashMap:
  • The core structure of HashMap is based on an array of buckets, where each bucket is a linked list (or a tree in case of high collision). The methods for adding, removing, and accessing elements are straightforward and do not involve any locking mechanisms.

public class HashMap<K,V> {
    // Internal structure
    Node<K,V>[] table;
    // Methods for put, get, remove, etc.
}

• ConcurrentHashMap:
  • The implementation of ConcurrentHashMap is more complex. It uses an array of segments (or buckets) and employs locking at the segment level. This means that when one thread is modifying a segment, other threads can still access other segments without being blocked.

public class ConcurrentHashMap<K,V> {
    // Internal structure
    Segment<K,V>[] segments;
    // Methods for put, get, remove, etc., with locking mechanisms
}

Conclusion

In summary, the choice between HashMap and ConcurrentHashMap depends on the specific use case:

• Use HashMap when you are working in a single-threaded environment or when you can manage synchronization externally.
• Use ConcurrentHashMap when you need a thread-safe implementation that allows concurrent access without significant performance penalties.

Understanding these differences is crucial for writing efficient and safe concurrent applications in Java.