I Built My Own Fail-Fast HashMap — Here's Why a Boolean Flag Wasn't Enough
If you've done LeetCode's Design HashMap , you've implemented put , get , and remove . What that exercise usually skips is the part that actually breaks in production: what happens when someone mutates the map while another piece of code is iterating over it. I ran into this directly while building MyHashMap , a from-scratch single-threaded HashMap (separate chaining, resize on load factor).…
When building MyHashMap, a single-threaded HashMap with separate chaining and dynamic resizing, the primary challenge was handling concurrent mutations while iterators were in use. While implementing put, get, and remove functions, ensuring iterators correctly detected concurrent changes proved to be the tougher 80% of the task. The standard issue arises when an iterator's next() method is invoked mid-iteration, potentially leading to undefined behavior if not properly guarded against.
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