Java 8 stream api how to collect List to Object

Question

I have two simple class ImageEntity and ImageList

how to collect result list ImageEntity to ImageList ?

List<File> files = listFiles();
        ImageList imageList = files.stream().map(file -> {
            return new ImageEntity(
                                   file.getName(), 
                                   file.lastModified(), 
                                   rootWebPath + "/" + file.getName());
        }).collect(toCollection(???));

class

public class ImageEntity {
private String name;
private Long lastModified;
private String url;
 ...
}

and

public class ImageList {
 private List<ImageEntity> list;

 public ImageList() {
    list = new ArrayList<>();
 }

 public ImageList(List<ImageEntity> list) {
    this.list = list;
 }
 public boolean add(ImageEntity entity) {
    return list.add(entity);
 }
 public void addAll(List<ImageEntity> list) {
     list.addAll(entity);
 }

}

It's not an elegant solution

ImageList imgList = files.stream().
  .map(file -> { return new ImageEntity(file.getName(), file.lastModified(), rootWebPath + "/" + file.getName()) })
  .collect(ImageList::new, (c, e) -> c.add(e), (c1, c2) -> c1.addAll(c2));

It can be a solution through collectingAndThen ?

what else have any ideas?

Misha · Accepted Answer · 2016-04-26 05:24:36Z

18

Since ImageList can be constructed from a List<ImageEntity>, you can use Collectors.collectingAndThen:

import static java.util.stream.Collectors.toList;
import static java.util.stream.Collectors.collectingAndThen;

ImageList imgList = files.stream()
    .map(...)
    .collect(collectingAndThen(toList(), ImageList::new));

On a separate note, you don't have to use the curly braces in your lambda expression. You can use file -> new ImageEntity(file.getName(), file.lastModified(), rootWebPath + "/" + file.getName())

edited Apr 26, 2016 at 5:24

answered Apr 26, 2016 at 5:18

Misha

28.3k7 gold badges66 silver badges83 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Atum Over a year ago

Good ! very elegant!Thank you very much!

Shirish Bari · Accepted Answer · 2016-04-26 08:47:40Z

1

you can try below also

ImageList imgList = new ImageList (files.stream().
  .map(file -> { return new ImageEntity(file.getName(), file.lastModified(), rootWebPath + "/" + file.getName()) })
  .collect(Collectors.toList()));

answered Apr 26, 2016 at 8:47

Shirish Bari

2,7321 gold badge30 silver badges39 bronze badges

2 Comments

hzitoun Over a year ago

That's really weird, how Collectors.toList() returns Object instead of list of objects. Can you explain please ?

Shirish Bari Over a year ago

Its an error on my part , I considered the ImageList as List<Images> . We should have something like .collect(converttoImageList());

Hank D · Accepted Answer · 2016-04-27 05:50:49Z

The collectingAndThen approach has the downside of creating a list and then copying it.

If you'd like something more reusable than your initial collect example, and that, like your example, doesn't end doing an extra copy in the collectingAndThen collector, you can take the three parameters of the collect and make a function similar to Collectors.toList() that collects straight to your ImageList, like this:

public static Collector<ImageEntity,?,ImageList> toImageList() {
    return Collector.of(ImageList::new, (c, e) -> c.add(e), (c1, c2) -> c1.addAll(c2));
}

You could then use it like this:

ImageList imgList = files.stream().
    .map(file -> new ImageEntity(file.getName(), file.lastModified(), rootWebPath + "/" + file.getName()))
    .collect(toImageList());

Collectives™ on Stack Overflow

Java 8 stream api how to collect List to Object

3 Answers 3

1 Comment

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related