Vfrdtyky

Currently I got this code:

@GetMapping("/all/")

    public Iterable<Article> getAllArticle(){

        ArrayList<ArticleEntity> articleEntities = Lists.newArrayList(articleProviderComponent.getAllArticle());

        ArrayList<Article> articles = Lists.newArrayList();

        for(ArticleEntity articleEntity : articleEntities){

            articles.add(ArticleMapper.articleEntity2Article(articleEntity));

        }

        return articles;

    }

The repository returns an Iterable, which I want to convert to a ArrayList. Beside that I want to convert each Entity to a POJO.

I tried using something like
list.foreach(ArticleMapper::ArticleMapper.articleEntity2Article) which works fine, but does not return a new list.

A simple map will do the job:

List<Article> articles = articleEntities.stream()

                                        .map(ArticleMapper::articleEntity2Article)

                                        .collect(Collectors.toList());

map converts each element to something else using the method given.

Stream the List, map it to your target type and collect to a List:

List<Article> articles = articleEntities.stream().map(ArticleMapper::articleEntity2Article).collect(Collectors.toList());

Basically the other answers are showing the right direction. But if you want to keep the same semantics of your code something additional has to be taken into account.

According to JavaDoc Collecotrs.toList

Returns a Collector that accumulates the input elements into a new
List. 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).

The returned articles are of type ArrayList<ArticleEntity>. Thus this list is e.g. mutable.

The return type of the method getAllArticle() is Iterable<Article>. Thus we can call

Iterator<Article> iterator = getAllArticle().iterator(); 

But if you need to call iterator.remove() "there are no guarantees on the [...] mutability" of the underlying collection. Hence this could result in an (JavaDoc)

UnsupportedOperationException - if the remove operation is not
supported by this iterator

To keep the behaviour of the original code use Collectors.toCollection:

List<Article> articles = articleEntities.stream()

        .map(ArticleMapper::articleEntity2Article)

        .collect(Collectors.toCollection(Lists::newArrayList));

This will return articles built by Lists.newArrayList().

As Iterable is made to be as general as possible, it does not provide a capability to stream it's content, but with the help of StreamSupport you can generate a stream from the Iterable instance and reduce the amount of operations to one:

final List<Article> articlesList = 

 StreamSupport.stream(articleProviderComponent.getAllArticle().spliterator(), false)            

 .map(ArticleMapper::articleEntity2Article)

 .collect(Collectors.toList());