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What is a PredicatedCollection in Apache Commons Collections?

A PredicatedCollection is a decorator that wraps an existing Collection and validates every element against a supplied Predicate before allowing it to be added.

Predicate<String> nonEmpty = s -> s != null && !s.isEmpty();
Collection<String> validated =
    PredicatedCollection.predicatedCollection(new ArrayList<>(), nonEmpty);

validated.add("ok");     // succeeds
validated.add("");       // throws IllegalArgumentException immediately

Because the check runs at add-time rather than being scattered across every call-site that inserts data, the collection's contents are guaranteed valid the instant they enter it, with a clear, fail-fast error if something doesn't qualify.

Related decorators - PredicatedList, PredicatedSet, PredicatedMap, and PredicatedBag - apply the exact same idea to their respective collection types.

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A PredicatedCollection validates elements: only when the collection is iterated

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at the moment they are added

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once per day on a schedule

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If an element fails the predicate, PredicatedCollection.add(): silently ignores the element

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throws an IllegalArgumentException

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converts it to null before storing

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