[FIXED] Differences of Java 16's Stream.toList() and Stream.collect(Collectors.toList())?

Issue

JDK 16 now includes a toList() method directly on Stream instances. In previous Java versions, you always had to use the collect method and provide a Collector instance.

The new method is obviously fewer characters to type. Are both methods interchangeable or are there subtle differences one should be aware of?

var newList = someCollection.stream()
    .map(x -> mapX(x))
    .filter(x -> filterX(x))
    .toList();

// vs.

var oldList = someCollection.stream()
    .map(x -> mapX(x))
    .filter(x -> filterX(x))
    .collect(Collectors.toList());

(This question is similar to Would Stream.toList() perform better than Collectors.toList(), but focused on behavior and not (only) on performance.)

Solution

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?

Answered By - Arvind Kumar Avinash
Answer Checked By - David Goodson (JavaFixing Volunteer)