What you are doing may be the simplest way, provided your stream stays sequential—otherwise you will have to put a call to sequential() before forEach.
The reason the call to sequential() is necessary is that the code as it stands (forEach(targetLongList::add)) would be racy if the stream was parallel. Even then, it will not achieve the effect intended, as forEach is explicitly nondeterministic—even in a sequential stream the order of element processing is not guaranteed. You would have to use forEachOrdered to ensure correct ordering. The intention of the Stream API designers is that you will use collector in this situation, as below:
targetLongList = sourceLongList.stream()
.filter(l -> l > 100)
.collect(Collectors.toList());
What you are doing may be the simplest way, provided your stream stays sequential—otherwise you will have to put a call to sequential() before forEach.
The reason the call to sequential() is necessary is that the code as it stands (forEach(targetLongList::add)) would be racy if the stream was parallel. Even then, it will not achieve the effect intended, as forEach is explicitly nondeterministic—even in a sequential stream the order of element processing is not guaranteed. You would have to use forEachOrdered to ensure correct ordering. The intention of the Stream API designers is that you will use collector in this situation, as below:
targetLongList = sourceLongList.stream()
.filter(l -> l > 100)
.collect(Collectors.toList());
One approach is to use Collectors.toList to collect the stream into a list:
targetLongList =
sourceLongList.stream().
filter(l -> l > 100).
collect(Collectors.toList());
If a specific List implementation is desired, Collectors.toCollection can be used instead:
targetLongList =
sourceLongList.stream().
filter(l -> l > 100).
collect(Collectors.toCollection(ArrayList::new));
Why would the JDK Stream.toList() instantiate multiple lists?
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Stream::toList() is weird
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From JDK 17:
default List<T> toList() {
return (List<T>) Collections.unmodifiableList(new ArrayList<>(Arrays.asList(this.toArray())));
}
public static <T> List<T> asList(T... a) {
return new ArrayList<>(a);
} What would be the reason for it to create a list, then another list, then another list? It could go from array to unmodifiable list (if it were supported) and save some memory, no?
An old discussion thread on r/javahelp about that topic:
https://www.reddit.com/r/javahelp/comments/t9qf3s/if_both_are_wrapped_in/
The "new ArrayList" in Arrays.asList() is a private internal version of ArrayList inside Arrays.java, which is backed by the given array, and a completely different type than the java.util.ArrayList being created in Stream.java. You're only making one copy - in Stream.toList().
Take another look at Arrays.java, and you'll see the private class right after it.