I had some extra time this week so went through a bunch of Sonar findings. One was interesting – in Java 17 you can use .toList() instead of .collect(Collectors.toList()) on a stream.

[Yes, I know this was introduced in Java 16. I live in a world where only LTS releases matter]

Cool. I can fix a lot of these without thinking. It’s a search and replace on the project level after all. I then ran the JUnit regression tests and got failures. That was puzzling to me because I’ve been using .toList() in code I write for a good while without incident.

After looking into it, I found the problem. .toList() guarantees the returned List is immutable. However, Collectors.toList() makes no promises about immutability. The result might be immutable. Or you can change it freely. Surprise?

That’s according to the spec. On the JDK I’m using (and Jenkins is using), Collectors.toList() was returning an ArrayList. So people were treating the returned List as mutable and it was working. I added a bunch of “let’s make this explicitly mutable” and then I was able to commit.

Here’s an example that illustrates the diference

import java.util.*;
import java.util.stream.*;

public class PlayTest {

	public static void main(String[] args) {

		var list = List.of("a", "b", "c");
		var collectorReturned = collector(list);
		var toListReturned = toList(list);
		
		System.out.println(collectorReturned.getClass());  // ArrayList (but doesn't have to be)
		System.out.println(toListReturned.getClass());  // class java.util.ImmutableCollections$ListN
		
		collectorReturned.add("x");
		System.out.println(collectorReturned);  // [bb, cc, x]
		toListReturned.add("x");  // throws UnsupportedOperationException

	}

	private static List<String> toList(List<String> list) {
		return list.stream()
				.filter(s -> ! s.equals("a"))
				.map(s -> s + s)
				.toList();
	}

	private static List<String> collector(List<String> list) {
		return list.stream()
				.filter(s -> ! s.equals("a"))
				.map(s -> s + s)
				.collect(Collectors.toList());
				
	}

Collectors.toList() also makes no promises about serializablity or thread safety but I wasn’t expecting it to.