How to use a Java 8 Lambda expression to convert a List of one type to a List of its subtype

Question

I can only seem to find how to do this in C# not Java.

I have a List<TypeX> but I know that every single element in that list is actually a subclass of TypeX Called TypeY.

How can I write a Lambda expression that accepts List<TypeX> and returns List<TypeY>?

Answer 1

For my example, I will use the following classes:

class TypeX {}
class TypeY extends TypeX {}

Then I have a List<TypeX>:

final List<TypeX> xList = ...

All you need to do is use the a method reference to TypeY.class.cast:

final List<TypeY> yList = xList.stream()
                               .map(TypeY.class::cast)
                               .collect(toList());

You can also filter() to exclude items that will cause an error:

final List<TypeY> yList = xList.stream()
                               .filter(TypeY.class::isInstance)
                               .map(TypeY.class::cast)
                               .collect(toList());

---

Examples use:

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

Answer 2

Projecting in Java is done using the map method:

List<TypeY> res = listTypeX
    .stream()
    .map((x) -> (TypeY)x)
    .collect(Collectors.toList());