Here are the key points about ConcurrentHashMap and HashMap in Java:

Concurrency:

• ConcurrentHashMap: Designed for concurrent use by multiple threads. It provides thread safety without synchronizing the whole map.
• HashMap: Not thread-safe; if used in a multi-threaded environment without proper synchronization, it can lead to data corruption.

Performance:

• ConcurrentHashMap: Generally provides better performance in multi-threaded scenarios because it uses a segment-level locking mechanism.
• HashMap: Offers better raw performance in single-threaded scenarios since there’s no overhead of locking mechanisms.

Null Keys and Values:

• ConcurrentHashMap: This does not allow null keys or values.
• HashMap: Allows one null key and multiple null values.

Iterators:

• ConcurrentHashMap: Uses a weakly consistent iterator, which means it might not reflect all changes made to the map after the iterator is created but won’t throw a ConcurrentModificationException.
• HashMap: Provides a fail-fast iterator, meaning if the map is structurally modified after the iterator’s creation, in any way except through the iterator’s own remove method, it will throw a ConcurrentModificationException.

Internal Structure:

• ConcurrentHashMap: Divides the map into segments and only locks a particular segment during write operations, which allows concurrent read operations and writes to different segments.
• HashMap: Uses a single array and linked lists (or trees for densely populated buckets in newer versions of Java).

Scalability:

• ConcurrentHashMap: Scales better in a multi-threaded environment due to segment-level locking.
• HashMap: If not synchronized externally, it’s not suitable for multi-threaded environments.

Concurrent Operations:

• ConcurrentHashMap: Provides several atomic operations (like putIfAbsent, remove, and replace) which are not available in the standard HashMap API.

Default Initial Capacity and Load Factor:

• While both have customization options, by default, HashMap and ConcurrentHashMap typically use an initial capacity of 16 and a load factor of 0.75.

Difference Between ConcurrentHashMap and HashMap in Java

Criteria	ConcurrentHashMap	HashMap
Synchronization	Segment-level locking; allows concurrent read without locking and partially locked writes	Not synchronized
Performance	Generally better performance in multi-threaded scenarios due to finer-grained locking	Faster in single-threaded scenarios since no locking is involved
Null Keys and Values	Does not allow null keys or values	Allows one null key and multiple null values
Fail-Fast	Iterator is weakly consistent and does not throw `ConcurrentModificationException`	Iterator is fail-fast and can throw `ConcurrentModificationException`
Use-case	When multiple threads need to modify a map concurrently	When thread-safety is not required or is managed externally
Thread Safety	Yes	No
Concurrency Level	Allows specifying the concurrency level to divide the map into segments, increasing throughput	Not applicable as it’s unsynchronized

Example: Difference Between ConcurrentHashMap and HashMap in Java

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentHashMapVsHashMapDemo {

    public static void main(String[] args) {

        // Create a HashMap and add elements
        Map<String, String> hashMap = new HashMap<>();
        hashMap.put("India", "Delhi");
        hashMap.put("Japan", "Tokyo");
        hashMap.put("USA", "Washington D.C.");

        System.out.println("HashMap:");
        hashMap.forEach((key, value) -> System.out.println(key + " -> " + value));

        // Create a ConcurrentHashMap and add elements
        Map<String, String> concurrentHashMap = new ConcurrentHashMap<>();
        concurrentHashMap.put("India", "Delhi");
        concurrentHashMap.put("Japan", "Tokyo");
        concurrentHashMap.put("USA", "Washington D.C.");

        System.out.println("\nConcurrentHashMap:");
        concurrentHashMap.forEach((key, value) -> System.out.println(key + " -> " + value));
    }
}

Output:

HashMap:
India -> Delhi
Japan -> Tokyo
USA -> Washington D.C.

ConcurrentHashMap:
India -> Delhi
Japan -> Tokyo
USA -> Washington D.C.

Explanation:

1. HashMap: A HashMap is an unsynchronized, key-value based map which allows null keys and values. It doesn't guarantee any specific order of entries. It's not thread-safe. When accessed or modified by multiple threads simultaneously, it can produce inconsistent results or even throw ConcurrentModificationException.

2. ConcurrentHashMap: A ConcurrentHashMap is a thread-safe variant of HashMap. It's designed to handle higher levels of concurrency. Instead of locking the entire collection, it divides the map into segments and locks only the necessary segments. This ensures better performance in multi-threaded scenarios. It does not allow null keys or values.

3. In the provided example, both the HashMap and ConcurrentHashMap have the same entries added, and when printed, they display the entries in the order they were added.

4. If you need a map for single-threaded applications or scenarios where map modifications are rare, HashMap is a good choice. However, if you're dealing with concurrent modifications in multi-threaded applications, ConcurrentHashMap should be preferred.