Convert Set to List without creating new List

Question

I am using this code to convert a Set to a List:

Map> mainMap = new HashMap<>();

for (int i         


        

Answer 1

Java 8 provides the option of using streams and you can get a list from Set setString as:

List stringList = setString.stream().collect(Collectors.toList());

Though the internal implementation as of now provides an instance of ArrayList:

public static 
    Collector> toList() {
        return new CollectorImpl<>((Supplier>) ArrayList::new, List::add,
                                   (left, right) -> { left.addAll(right); return left; },
                                   CH_ID);
    }

but JDK does not guarantee it. As mentioned here:

There are no guarantees on the type, mutability, serializability, or thread-safety of the List returned; if more control over the returned List is required, use toCollection(Supplier).

In case you want to be sure always then you can request for an instance specifically as:

List stringArrayList = setString.stream()
                     .collect(Collectors.toCollection(ArrayList::new));