Stream: Performance & Pitfalls

Java Streams, Lambda & Functional ProgrammingTopic 18 of 305 questions

Q1Stream.peek(): debugging only, may be optimized away, side effects unreliable

Stream.peek is a debugging-only intermediate operation that may be optimized away by the JVM, making it unreliable for business logic or side effects.

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Q2When should you use toArray() vs collect(toList())?

Default to `collect(toList())` for flexibility; use `toArray()` only for array-demanding APIs, primitives, or varargs.

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Q3Why is using a HashSet for the lookup list faster than filter(list::contains) for intersection queries?

`list::contains` is O(n*m) for intersection; `set::contains` is O(n+m) because HashSet lookup is O(1) average.

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Q4Why is list.stream().filter(...).count() sometimes more efficient than collect(toList()).size()?

`count()` tallies without building a list; `collect(toList()).size()` wastes memory materializing a list just to read its size.

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Q5Explain why peek() should NOT be used for side effects in production code

The pipeline may legally skip `peek`, so never put real work in it; use `map`, `forEach`, or a collector instead.

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