java
9 minJava equals and hashCode: Fix Broken HashMap and HashSet Lookups
Diagnose collection bugs caused by inconsistent equality, mutable keys, inheritance, and accidental identity comparisons.
Search the library
8 matching resultsArticles and interview answers
Q&A
Q&AWhy can a HashMap fail to find a key that was inserted?
Mutable key fields or inconsistent equals/hashCode behavior can make lookup search a different bucket.
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HashMap uses hashCode to choose a bucket and equals to identify a key within it. Equal values must share a hash, and hash-relevant state must not change while the object is used as a key. Check equality in both directions, inspect hash values, and prefer immutable key objects.
Q&A
Q&AWhat happens when two Java HashMap keys have the same hash code?
The keys share a bucket, then equality distinguishes entries within that bucket.
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A hash helps select a bucket but does not establish key identity. HashMap checks equality among entries in the bucket; collisions can be stored in a list and, in sufficiently large bins, treeified under implementation conditions. Correct equals/hashCode behavior is essential, and resizing changes bucket placement without making hashes unique.
Q&A
Q&AWhy does a timestamp cursor need a unique tie-breaker?
Equal timestamps otherwise make continuation skip or repeat peers.
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Carry the full ordering tuple, such as created_at and ID, and match its direction in the continuation predicate. Stable continuation still does not freeze membership across requests.
distributed systems
12 minKafka Consumer Groups: Architecture & Common Pitfalls
Consumer groups are the mechanism that turns Kafka partitions into scalable, failure-tolerant work units. Learn how rebalances, offsets, and lag interact under load before you ship your first serious pipeline.
Q&A
Q&AHow do you find the third-highest distinct salary in SQL?
Use DENSE_RANK for distinct values and state whether you want salary values or employee rows.
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Filter out NULL salaries, compute DENSE_RANK() OVER (ORDER BY salary DESC), then select rank 3. DENSE_RANK treats equal salaries as the same rank. PostgreSQL, MySQL 8+, and Oracle support this window-function approach; if you instead mean the third row, use ROW_NUMBER with a deterministic tie-breaker.
Q&A
Q&AWhy can a Python list not be an element of a set?
Set members must be hashable, and a mutable list does not have a stable hash.
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Sets and dictionary keys rely on a stable hash and equality relationship while a value is stored. A list is mutable, so changing it could invalidate the collection’s lookup structure; Python therefore makes lists unhashable. Use a tuple if its members are also hashable and the value should be immutable.
python
8 minPython Collections: Lists, Tuples, Sets, and Map/Reduce Trade-offs
Choose Python collection types by invariants and access patterns, then use map, comprehensions, and reductions without obscuring intent.