How can you ensure atomic addition of data in HashMap under multithreading conditions?

In multithreading conditions, the standard HashMap in Java is not thread-safe, so for atomic data addition, special approaches should be used:

1. Use ConcurrentHashMap — a thread-safe implementation of Map that allows safe addition and modification of data from multiple threads.

2. Method putIfAbsent — atomically adds a value if the key is absent:

ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.putIfAbsent("key", 1);

3. Use methods compute, computeIfAbsent for atomic update operations:

map.compute("key", (k, v) -> (v == null) ? 1 : v + 1);

If a regular HashMap is used, access must be synchronized manually, for example, via Collections.synchronizedMap or synchronized blocks, but this is less efficient.

In summary, for atomic data addition in a multithreaded environment, it is better to use ConcurrentHashMap with its atomic methods.