Java Stream sorted() to generic List

Question

I have a List called "catalog" of Articles (generic Type).

An Article has following Methods:

public int getUnitsInStore()
public long getUnitPrice()

Now i want to sort this list by the total value of a single Article (units * pricePerUnit) using Java Stream sorted().

I've tried:

catalog = catalog.stream()
    .map(a -> a.getUnitPrice() * a.getUnitsInStore())
    .sorted((a, b)->a.compareTo(b))
    .collect(Collectors.toCollection(List<Article>::new));

But its giving me following error:

Cannot instantiate the type List<Article>

What i'm doing wrong?

EDIT: I've also tried:

catalog = catalog.stream()
    .map(a -> a.getUnitPrice() * a.getUnitsInStore())
    .sorted((a, b)->b.compareTo(a)).collect(Collectors.toList());

It says:

Type mismatch: cannot convert from List<Long> to List<Article>

Answer 1

You can't do new List() , so you can't do List::new . It's an interface. It can't be instantiated.

If you were to change it to ArrayList<Article>::new , you wouldn't get that error.

However

.collect(Collectors.toCollection(ArrayList<Article>::new));

is basically just a more verbose way of using a type witness:

.collect(Collectors.<Article>toList());

All that said, Java should also be able to infer the type from the assignment. If the stream is Stream<ArticleParent> and you are trying to assign to List<Article> , it should be able to infer that. You've omitted the declaration of the field, so I'll assume that you're right that the compiler cannot infer it correctly for some reason.

Answer 2

Try this.

catalog = catalog.stream()
    .sorted(Comparator.comparing(a -> a.getUnitPrice() * a.getUnitsInStore()))
    .collect(Collectors.toList());

You can also sort in reverse order.

catalog = catalog.stream()
    .sorted(Comparator.comparing((Article a) -> a.getUnitPrice() * a.getUnitsInStore()).reversed())
    .collect(Collectors.toList());