Why Java Stream.toList() Differs from Collectors.toList() and How to Use Them

In a previous post I showed how to debug Java Stream operations. A reader asked why the stream can call toList() directly without collect(). The answer is that Stream.toList() was introduced in Java 16 and returns an unmodifiable list.

For developers still on Java 8, the equivalent is to use collect(Collectors.toList()), which creates a mutable list.

public class StreamTest {

    @Test
    void test() {
        List<String> list = List.of("blog.didispace.com", "spring4all.com", "openwrite.cn", "www.didispace.com");

        List<String> result = list.stream()
                .filter(e -> e.contains("didispace.com"))
                .filter(e -> e.length() > 17)
                .toList();

        System.out.println(result);
    }
}

Java 8 equivalent:

List<String> result = list.stream()
    .filter(e -> e.contains("didispace.com"))
    .filter(e -> e.length() > 17)
    .collect(Collectors.toList());

Stream.toList() and Collectors.toList()

Although both produce a List, they differ: Stream.toList() returns an unmodifiable list, while collect(Collectors.toList()) returns a mutable list that can be modified.

Source of Stream.toList() (Java 16):

default List<T> toList() {
    return (List<T>) Collections.unmodifiableList(new ArrayList<>(Arrays.asList(this.toArray())));
}

To obtain an unmodifiable list in earlier Java versions, use Collectors.toUnmodifiableList(), which is available since Java 10:

List<String> result = list.stream()
    .filter(e -> e.contains("didispace.com"))
    .filter(e -> e.length() > 17)
    .collect(Collectors.toUnmodifiableList());

Note that Collectors.toUnmodifiableList() is not available in Java 8; it requires at least Java 10.