Скачать или смотреть 🚀 Understanding HashMap: Null Keys, Null Values & Iteration Magic!

Today, we’re diving into the behavior of Java’s HashMap when it comes to null keys and null values, plus how iteration works using keySet(). If you want to master Java Maps and avoid common pitfalls, this is for you!

🔹 Why Use HashMap--String, String--?
Maps string keys to string values (e.g., username → userID).

Great for configurations, user data, and quick lookups.

Focus here: handling of null keys and null values.

🔹 What Happens When You Put a Null Key?
HashMap allows only ONE null key.

Think of it as a special locker with no label.

Adding a new null key overwrites the old null key’s value.

No errors or exceptions thrown here!

🔹 What About Null Values?
You can have multiple entries with null values.

These are like labeled lockers that are empty—totally allowed!

Example: "Key1" → null, "Key2" → null, no problem.

🔹 Normal Key-Value Entries?
Regular entries with both non-null keys and values work normally.

Example: "Key3" → "Value3" stored as expected.

🔹 Iteration Using keySet()
You get all keys, including null, via keySet().

For each key, use get(key) to fetch its value.

Prints all pairs: null keys and values included!

Example output:

Key: null → Value for null key

Key: Key1 → null

Key: Key2 → null

Key: Key3 → Value3

🧪 Quick Data Recap
Null key with non-null value stored successfully.

Multiple keys with null values stored fine.

One regular key-value pair as usual.

💡 Key Takeaways
✅ Exactly one null key allowed in HashMap.

✅ Multiple null values allowed with different keys.

⚠️ Adding a null key again replaces previous null key’s value.

✅ Iteration over keys works perfectly, including nulls.

Why Should You Care?
Avoid surprises with null keys and values in your code!

Improve debugging and testing for HashMaps.

Write cleaner, bug-free Java Map logic.

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