Main differences between HashMap and ConcurrentHashMap are:

1. Synchronization: A HashMap is not synchronized. But a

ConcurrentHashMap is a synchronized object.

2. Null Key: A HashMap can have one null key and any

number of null values. A ConcurrentHashMap cannot have

null keys or null values.

3. Multi-threading: A ConcurrentHashMap works well in a

multi-threading environment.

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