Java.Collections.In what case can an element be lost in HashMap?

⚠️ 1. Poor equals() and hashCode() implementation

If you create your own key class and don’t override equals() and hashCode() properly, HashMap won’t be able to:

• Find existing entries correctly,
• Prevent duplicate keys,
• Replace values correctly.

🔥 Example:

class MyKey {
    int id;
    MyKey(int id) { this.id = id; }
    // Oops! No hashCode() or equals()
}

Map<MyKey, String> map = new HashMap<>();
map.put(new MyKey(1), "A");
map.put(new MyKey(1), "B");
System.out.println(map.size()); // Output: 2 (should be 1)

➡️ Both keys look logically equal but go to different buckets, so the first value becomes unreachable — effectively “lost”.

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⚠️ 2. Mutating a key after insertion

If a key’s fields that affect hashCode() or equals() are modified after the key is put into the map, the map can no longer find it.

🔥 Example:

MyKey key = new MyKey(1);
map.put(key, "A");

key.id = 2; // Changes hashCode()
map.get(key); // Returns null ❌

➡️ The key has moved logically, but the map still looks in the old bucket → can’t find it.

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⚠️ 3. Concurrency issues (without synchronization)

If multiple threads modify a HashMap without proper synchronization, weird things can happen:

• Data loss,
• Infinite loops,
• Even internal corruption (in Java 7 and earlier — concurrent resizing could cause circular references).

💡 Always use ConcurrentHashMap or proper synchronization for thread-safe access.

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⚠️ 4. Collision overwriting

If multiple keys hash to the same bucket and equals() incorrectly treats them as equal, a new put() may overwrite an existing one, losing the previous value.

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✅ Safe practices to avoid “lost” elements:

• Always override both hashCode() and equals() in key classes.
• Make key fields immutable.
• Use proper thread-safe collections in multithreaded code.
• Avoid relying on default implementations of hashCode() and equals() from Object.