Great follow-up! Let’s walk through this slowly — you’re thinking logically, but there’s a subtle trap when multiple threads are involved. Let me break it down step by step, and I promise this will click by the end.

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🔥 First, what does null mean in a Map?

In any Map, null can mean two different things:

Case	Meaning
1️⃣ Key is missing	The key does not exist in the map (so get() returns null).
2️⃣ Key exists, value is null	The key exists, but its associated value is null.

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🔥 In a single-threaded environment, this is manageable:

You have control, so you know how your code uses null values.

Example:

map.put("key1", null);  // Key exists, value is null
map.remove("key2");     // Key does not exist

System.out.println(map.get("key1")); // null — value is null
System.out.println(map.get("key2")); // null — key is missing

It’s your choice whether null means “nothing here” or “this key is deliberately set to null.” You can control the meaning.

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🔥 Now, enter multi-threading…

• Thread 1 writes:

map.put("key1", null);

Thread 2 reads at exactly the same moment:

String value = map.get("key1");

Now, when value is null, Thread 2 has no reliable way to know why. Two possibilities exist:

Case	Explanation
1️⃣ Key was removed a millisecond ago	null means “key does not exist anymore.”
2️⃣ Key exists, but someone set value to null	null means “key exists, value is deliberately null.”

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🔥 The core problem: The meaning of null changes depending on timing.

• Between the time Thread 2 called get() and processed the result, Thread 1 could have removed the key or set its value to null.
• This leads to race conditions — the meaning of null is no longer clear or reliable when multiple threads are modifying the map concurrently.

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💡 Example – Race Condition in Action

map.put("user123", "ACTIVE");

// Thread 1: Deletes user
map.remove("user123");

// Thread 2: Checks if user is active
String status = map.get("user123");  // returns null
if (status == null) {
    System.out.println("User does not exist");
}

// Now what happens if Thread 1 does this instead?
map.put("user123", null);  // User exists, but is marked as "no state"

// In this case, `get()` will also return null — now `Thread 2` has no idea:
   // Was the user removed?
   // Or does the user exist, but with null state?

🔥 Why does ConcurrentHashMap just ban null?

This ambiguity does not exist if null is simply forbidden. It forces you to write unambiguous logic.

✅ Instead of:

map.put("user123", null);  // ambiguous

You do this:

map.remove("user123");     // user123 is gone, clear meaning

Or, if you want to keep the key but show “no value”:

map.put("user123", "NO_STATE");   // explicit placeholder

This way:

• null always means “key not present.”
• A value (even “NO_STATE”) means “key exists, and this is the current value.”

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🔔 This is the whole reason:

If null is allowed	If null is forbidden
null could mean missing key OR null value	null always means key is missing
You have to guess why null appeared	No guessing — always clear
Race conditions when two threads compete	No ambiguity in race conditions

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💡 In short: null is ambiguous in concurrent code. ConcurrentHashMap forces you to write clear, race-free logic.

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🔨 Short example of correct code using ConcurrentHashMap:

ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();

// Thread 1
map.put("user123", "ACTIVE");

// Thread 2 - safely remove key if needed
map.remove("user123");

// Thread 3 - safely check
String value = map.get("user123");
if (value == null) {
    System.out.println("User does not exist");
} else {
    System.out.println("User state: " + value);
}

✅ Final takeaway:

In ConcurrentHashMap, null means key is not present — always.
This removes a whole class of subtle race conditions.