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What is the purpose of ListUtils in Apache Commons Collections?

ListUtils focuses specifically on List-oriented operations that the JDK's Collections class doesn't cover directly.

union(), intersection(), subtract() - combine two Lists while preserving duplicate counts, unlike Set-based operations
isEqualList() - compares two Lists element-by-element, tolerating differing List implementations
predicatedList() / transformedList() - wrap a List with validation or auto-transformation
defaultIfNull() - substitute an empty List when the source is null

Because these operations account for List semantics like ordering and duplicate counts, they behave differently from the Set-based versions found in CollectionUtils when duplicates are involved.

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How does ListUtils.union() treat duplicate elements compared to a Set union? it preserves duplicate counts, since Lists allow duplicates

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it always removes duplicates

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it throws an exception on duplicates

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ListUtils.defaultIfNull() is used to: sort a List in natural order

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substitute a default empty List when the source is null

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remove null elements from a List

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