Differences of Java 16’s Stream.toList() and Stream.collect(Collectors.toList())?

One difference is that Stream.toList() provides a List implementation that is immutable (type ImmutableCollections.ListN that cannot be added to or sorted) similar to that provided by List.of() and in contrast to the mutable (can be changed and sorted) ArrayList provided by Stream.collect(Collectors.toList()).

Demo:

import java.util.stream.Stream;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<String> list = Stream.of("Hello").toList();
        System.out.println(list);
        list.add("Hi");
    }
}

Output:

[Hello]
Exception in thread "main" java.lang.UnsupportedOperationException
    at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:142)
    at java.base/java.util.ImmutableCollections$AbstractImmutableCollection.add(ImmutableCollections.java:147)
    at Main.main(Main.java:8)

Please check this article for more details.

Update:

Interestingly, Stream.toList() returns a nulls-containing list successfully.

import java.util.stream.Stream;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<Object> list = Stream.of(null, null).toList();
        System.out.println(list);
    }
}

Output:

[null, null]

On the other hand, List.of(null, null) throws NullPointerException.

import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<Object> list = List.of(null, null);
    }
}

Output:

Exception in thread "main" java.lang.NullPointerException
    at java.base/java.util.Objects.requireNonNull(Objects.java:208)
    at java.base/java.util.ImmutableCollections$List12.<init>(ImmutableCollections.java:453)
    at java.base/java.util.List.of(List.java:827)
    at Main.main(Main.java:5)

Note: I’ve used openjdk-16-ea+34_osx-x64 to compile and execute the Java SE 16 code.

Useful resources:

1. JDK Bug#JDK-8180352
2. Calling Java varargs method with single null argument?