Javamedium

Streams vs collections: what is the difference and a common pitfall?

Answer

A collection stores data; a stream describes a pipeline of operations (filter/map/reduce) to produce a result. Streams are typically single-use, and a common pitfall is side effects (mutating external state) inside `map/forEach`, which makes code harder to reason about.

List<String> activeNames = users.stream()
  .filter(User::isActive)
  .map(User::getName)
  .toList();

Advanced answer

Deep dive

Expanding on the short answer — what usually matters in practice:

Context (tags): streams, collections, side-effects, functional
JVM: memory (heap/stack), GC, and what drives latency.
Contracts: equals/hashCode/toString, mutability and consequences.
Performance: boxing, allocations, collections, inlining.
Explain the "why", not just the "what" (intuition + consequences).
Trade-offs: what you gain/lose (time, memory, complexity, risk).
Edge cases: empty inputs, large inputs, invalid inputs, concurrency.

Examples

Here’s an additional example (building on the short answer):

List<String> activeNames = users.stream()
  .filter(User::isActive)
  .map(User::getName)
  .toList();

Common pitfalls

Too generic: no concrete trade-offs or examples.
Mixing average-case and worst-case (e.g., complexity).
Ignoring constraints: memory, concurrency, network/disk costs.

Interview follow-ups

When would you choose an alternative and why?
What production issues show up and how do you diagnose them?