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What is the purpose of CollectionUtils in Apache Commons Collections?

CollectionUtils is a static helper class packed with null-safe operations for working with any java.util.Collection, so callers don't have to hand-roll the same boilerplate checks repeatedly.

isEmpty(coll) / isNotEmpty(coll) - null-safe emptiness checks
union(), intersection(), subtract(), disjunction() - set-style operations across two collections
filter() / select() - keep elements matching a Predicate
collect() - transform elements with a Transformer
forAllDo() - run a Closure against every element

These methods accept plain Collection references, so they work uniformly across Lists, Sets, and Bags without extra casting.

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CollectionUtils.isEmpty(coll) is mainly valued for being: null-safe, so it won't throw if coll itself is null

✓ Correct! Well done.

faster than coll.isEmpty()

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only usable on Lists

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Which CollectionUtils method applies a Closure to every element? collect()

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forAllDo()

✓ Correct! Well done.

filter()

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