CODEWITHSUNDEEP
javalistcollectionssetjava-6
Jacques René Mesrine
Jacques René Mesrine

Reputation: 47805

Most concise way to convert a Set<T> to a List<T>

For example, I am currently doing this:

Set<String> setOfTopicAuthors = ....

List<String> list = Arrays.asList( 
    setOfTopicAuthors.toArray( new String[0] ) );

Can you beat this ?

Upvotes: 212

Views: 233127

Answers (6)

akhil_mittal
akhil_mittal

Reputation: 24157

Considering that we have Set<String> stringSet we can use following:

Plain Java

List<String> strList = new ArrayList<>(stringSet);

Guava

List<String> strList = Lists.newArrayList(stringSet);

Apache Commons

List<String> strList = new ArrayList<>();
CollectionUtils.addAll(strList, stringSet);

Java 10 (Unmodifiable List)

List<String> strList = List.copyOf(stringSet);
List<String> strList = stringSet.stream().collect(Collectors.toUnmodifiableList());

Java 8 (Modifiable Lists)

import static java.util.stream.Collectors.*;
List<String> stringList1 = stringSet.stream().collect(toList());

As per the doc for the method toList()

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).

So if we need a specific implementation e.g. ArrayList we can get it this way:

List<String> stringList2 = stringSet.stream().
                              collect(toCollection(ArrayList::new));

Java 8 (Unmodifiable Lists)

We can make use of Collections::unmodifiableList method and wrap the list returned in previous examples. We can also write our own custom method as:

class ImmutableCollector {
    public static <T> Collector<T, List<T>, List<T>> toImmutableList(Supplier<List<T>> supplier) {
            return Collector.of( supplier, List::add, (left, right) -> {
                        left.addAll(right);
                        return left;
                    }, Collections::unmodifiableList);
        }
}

And then use it as:

List<String> stringList3 = stringSet.stream()
             .collect(ImmutableCollector.toImmutableList(ArrayList::new));

Another possibility is to make use of collectingAndThen method which allows some final transformation to be done before returning result:

    List<String> stringList4 = stringSet.stream().collect(collectingAndThen(
      toCollection(ArrayList::new),Collections::unmodifiableList));

One point to note is that the method Collections::unmodifiableList returns an unmodifiable view of the specified list, as per doc. An unmodifiable view collection is a collection that is unmodifiable and is also a view onto a backing collection. Note that changes to the backing collection might still be possible, and if they occur, they are visible through the unmodifiable view. But the collector method Collectors.unmodifiableList returns truly immutable list in Java 10.

Upvotes: 25

Prabhakar Manthena
Prabhakar Manthena

Reputation: 2303

Try this for Set:

Set<String> listOfTopicAuthors = .....
List<String> setList = new ArrayList<String>(listOfTopicAuthors);

Try this for Map:

Map<String, String> listOfTopicAuthors = .....
// List of values:
List<String> mapValueList = new ArrayList<String>(listOfTopicAuthors.values());
// List of keys:
List<String> mapKeyList = new ArrayList<String>(listOfTopicAuthors.KeySet());

Upvotes: 14

jp093121
jp093121

Reputation: 1762

not really sure what you're doing exactly via the context of your code but...

why make the listOfTopicAuthors variable at all?

List<String> list = Arrays.asList((....).toArray( new String[0] ) );

the "...." represents however your set came into play, whether it's new or came from another location.

Upvotes: 0

Andrejs
Andrejs

Reputation: 27677

If you are using Guava, you statically import newArrayList method from Lists class:

List<String> l = newArrayList(setOfAuthors);

Upvotes: 2

Adamski
Adamski

Reputation: 54695

List<String> l = new ArrayList<String>(listOfTopicAuthors);

Upvotes: 94

Schildmeijer
Schildmeijer

Reputation: 20946

List<String> list = new ArrayList<String>(listOfTopicAuthors);

Upvotes: 454

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