Streams: Stream.toList() returns an unmodifiable list

Java 16 added Stream.toList() - shorter than collect(Collectors.toList()), but the result is unmodifiable. Pick the one that matches what the caller may do with it.

Code

var mutable = Stream.of("a", "b").collect(Collectors.toList());
mutable.add("c");
System.out.println("collect(toList()) is mutable: " + mutable);

var fixed = Stream.of("a", "b").toList();
try {
    fixed.add("c");
} catch (UnsupportedOperationException e) {
    System.out.println("stream.toList() is unmodifiable: " + fixed);
}

Output

collect(toList()) is mutable: [a, b, c]
stream.toList() is unmodifiable: [a, b]

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Published 2026-07-30