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

Since it hasn't been mentioned so far, as of Java 10 you can use the new copyOf factory method:

List.copyOf(set);

From the Javadoc:

Returns an unmodifiable List containing the elements of the given Collection, in its iteration order.

Note that this creates a new list (ImmutableCollections$ListN to be precise) under the hood by

1. calling Collection#toArray() on the given set and then
2. putting these objects into a new array.

So the newly allocated memory should be negligible.

Answer 2

I would do :

Map<String, Collection> mainMap = new HashMap<String, Collection>();

for(int i=0; i<something.size(); i++){
  Set set = getSet(...); //return different result each time
  mainMap.put(differentKeyName,set);
}

Answer 3

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

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

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

public static <T>
    Collector<T, ?, List<T>> toList() {
        return new CollectorImpl<>((Supplier<List<T>>) 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<String> stringArrayList = setString.stream()
                     .collect(Collectors.toCollection(ArrayList::new));

Answer 4

You can use the List.addAll() method. It accepts a Collection as an argument, and your set is a Collection.

List<String> mainList = new ArrayList<String>();
mainList.addAll(set);

EDIT: as respond to the edit of the question.
It is easy to see that if you want to have a Map with Lists as values, in order to have k different values, you need to create k different lists.
Thus: You cannot avoid creating these lists at all, the lists will have to be created.

Possible work around:
Declare your Map as a Map<String,Set> or Map<String,Collection> instead, and just insert your set.

Answer 5

We can use following one liner in Java 8:

List<String> list = set.stream().collect(Collectors.toList());

Here is one small example:

public static void main(String[] args) {
        Set<String> set = new TreeSet<>();
        set.add("A");
        set.add("B");
        set.add("C");
        List<String> list = set.stream().collect(Collectors.toList());
}

Answer 6

Use constructor to convert it:

List<?> list = new ArrayList<?>(set);