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HashMap vs SynchronizedMap vs ConcurrentHashMap in Java: Complete Guide With Real-World Examples
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Introduction:
If you have worked with Java for some time, chances are you have used HashMap.
It is probably one of the first collections developers learn after understanding arrays and lists. We use it to store users, products, configurations, counters, cache data, API responses, and almost everything that can be represented as a key-value relationship.
For example, imagine a simple application where we need to find a user by their ID.
Map<Long, User> users = new HashMap<>();
users.put(101L, new User("Alice"));
users.put(102L, new User("Bob"));
User user = users.get(101L);Nothing complicated.
But now imagine the same application is running in production.
There are hundreds of requests arriving at the same time. Multiple threads are reading from the map, some are adding users, and others are removing or updating entries.
Suddenly, our simple HashMap becomes a potential problem.
This is where developers start hearing about Collections.synchronizedMap() and ConcurrentHashMap.
