Streams: collectingAndThen finishes with a transformation

collectingAndThen applies a final function to a collector's result - the standard way to return an unmodifiable collection from a pipeline.

Code

var raw = List.of("b", "a", "c", "a");

List<String> sortedUnmodifiable = raw.stream().distinct().sorted()
        .collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList));
System.out.println(sortedUnmodifiable);

try {
    sortedUnmodifiable.add("d");
} catch (UnsupportedOperationException e) {
    System.out.println("result is unmodifiable");
}

Output

[a, b, c]
result is unmodifiable

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Published 2026-07-30