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();

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