Which collection matches the invariant?

Goal & mental model verified

Select the collection by its contract: List preserves a sequence, Set models uniqueness, Map associates keys and values. Hash-based containers rely on equality and hashing; HashMap does not promise iteration order.

Worked example · design exercise synthesis

A broker list preserves display order; a Set tracks unique coverage codes; a Map indexes quotes by stable QuoteId. Use a sorted map when sorted traversal is part of the requirement.

Engineering decision synthesis

Prefer interface-typed variables and explicit generic parameters. Choose ArrayList for ordinary indexed sequences; choose a deque for queue/stack operations instead of treating one structure as universal.

Pitfall & diagnosis synthesis

Changing fields used by a key’s equality/hash after insertion can make lookup fail. Concurrent access needs a suitable concurrency strategy, not merely a different collection name.

Improve & validate synthesis

Write down ordering, duplicate and mutation requirements before selection. Measure realistic access patterns before swapping structures for supposed performance gains.