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Why Java HashMap Allows Only One Null Key

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Created January 8, 2026

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Video Transcript

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Ever wondered why Java's HashMap allows exactly one null key? Let's dive into

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the hash bucket. In Java, a null key doesn't have a hash code.

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So, HashMap always maps it to index zero. Since maps require

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unique keys, any new value for a null key simply overwrites the old one.

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This keeps the map consistent. One key, one location, even if that

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key is literally nothing. Understanding this helps you master Java collections.

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Stay curious and keep coding!

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