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.
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16 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&AWhy is a mutable default argument dangerous in Python?
Default expressions are evaluated once, so the same mutable object is reused across calls.
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A default list or dictionary is created when the function is defined, not every time it is called. Mutations therefore leak between calls. Use None as a sentinel and construct a fresh object inside, or use default_factory for dataclasses and similar models.
Q&A
Q&AWhy promote the same immutable artifact rather than rebuild for production?
A rebuild is a new artifact, even at the same source commit. Promotion preserves the tested output while environment configuration remains deliberate and observable.
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A rebuild is a new artifact, even at the same source commit. Promotion preserves the tested output while environment configuration remains deliberate and observable.
Q&A
Q&AWhy is one global SQLAlchemy Session unsafe in an async FastAPI service?
A Session carries mutable identity-map and transaction state and is not safe for concurrent requests.
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Concurrent coroutines can interleave operations on the same Session, mixing transaction state and identity-map contents. Create a session per request or task, scope its transaction clearly, and size the connection pool across all worker processes and replicas. A global Engine/pool is different from a shared mutable Session.
devops
17 minRelease Safety Advanced: State, Schema Compatibility, and Artifact Trust
Connect immutable infrastructure, safe schema evolution, IaC lock recovery, artifact provenance, and CI isolation into one release discipline.
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.
Q&A
Q&AWhen should you choose a Python list, tuple, or set?
Base the choice on ordering, mutation, duplicates, and membership operations.
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A list is ordered, mutable, and permits duplicates. A tuple is an immutable sequence suited to fixed records, though named structures can improve readability. A set stores unique hashable values and is useful for membership; do not use it when output order or duplicates are part of the contract.