Difference Between HashMap and ConcurrentHashMap in Java

1. Introduction

Both HashMap and ConcurrentHashMap are used to store key-value pairs, but they differ mainly in:

• Thread safety
• Performance in multi-threaded environments
• Internal working mechanism

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2. HashMap

Explanation

• HashMap is not thread-safe.

• Multiple threads accessing it simultaneously may lead to:

  • Data inconsistency
  • Infinite loops (in older Java versions during resizing)

• It is best suited for single-threaded environments.

Example

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1, "Java");
        map.put(2, "Python");

        System.out.println(map);
    }
}

Explanation of Code

• Simple key-value storage
• No synchronization → faster but unsafe in multi-threading

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3. ConcurrentHashMap

Explanation

• ConcurrentHashMap is thread-safe.
• Designed for high concurrency.

• Uses advanced techniques like:

  • Segment locking (Java 7)
  • CAS (Compare-And-Swap) & synchronized blocks (Java 8+)

• Allows multiple threads to read/write simultaneously.

Example

import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentMapExample {
    public static void main(String[] args) {
        ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>();
        map.put(1, "Java");
        map.put(2, "Python");

        System.out.println(map);
    }
}

Explanation of Code

• Thread-safe operations without locking entire map
• Better performance than Hashtable in multi-threading

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4. Key Differences Table

Feature	HashMap	ConcurrentHashMap
Thread Safety	Not thread-safe	Thread-safe
Performance	Faster (single thread)	Optimized for multi-thread
Locking Mechanism	No locking	Partial locking / CAS
Null Keys/Values	Allows 1 null key	Does NOT allow null key/value
Fail-fast / Fail-safe	Fail-fast iterator	Fail-safe iterator
Use Case	Single-threaded apps	Multi-threaded apps

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5. Internal Working Difference

HashMap

• Uses array + linked list / tree
• No synchronization
• Entire structure is unsafe in concurrent access

ConcurrentHashMap

• Divides map into segments (Java 7) OR
• Uses bucket-level locking (Java 8+)
• Only a portion of the map is locked during updates

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6. Multi-threading Example

HashMap (Problem)

import java.util.HashMap;

public class HashMapThreadIssue {
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();

        Runnable task = () -> {
            for (int i = 0; i < 1000; i++) {
                map.put(i, "Value");
            }
        };

        new Thread(task).start();
        new Thread(task).start();
    }
}

Explanation

• Multiple threads modify map simultaneously
• May lead to inconsistent data

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ConcurrentHashMap (Solution)

import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentHashMapSafe {
    public static void main(String[] args) {
        ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>();

        Runnable task = () -> {
            for (int i = 0; i < 1000; i++) {
                map.put(i, "Value");
            }
        };

        new Thread(task).start();
        new Thread(task).start();
    }
}

Explanation

• Safe concurrent updates
• No data corruption

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7. When to Use What?

• Use HashMap when:

  • Application is single-threaded
  • No concurrent modifications

• Use ConcurrentHashMap when:

  • Multiple threads access/modify data
  • High performance is required

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8. Summary

• HashMap → Fast but not thread-safe
• ConcurrentHashMap → Thread-safe with high performance
• Choose based on concurrency needs

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