Converting a list of objects to a list of optional objects in Java

Question

The following code converts a list of objects into a list of optional objects. Is there an elegant way to do this using Java 8's streams?

List<Object> originalList = Arrays.asList(new Object(), new Object());
List<Optional<Object>> convertedList = new ArrayList<>();
for (Object object : originalList) {
    convertedList.add(Optional.of(object));
}

I tried the following piece of code.

List<Optional<Object>> convertedList = originalList.stream().map((o) -> Optional.of(o));

However this gives the following compile error:

of(T) in Optional cannot be applied to (java.lang.Object)

This is using the java.util.Optional class rather than the Guava one.

Answer 1

You forgot to collect the elements of the Stream in a list. It should be:

List<Optional<Object>> convertedList  = originalList.stream()
                                                    .map((o) -> Optional.of(o))
                                                    .collect(Collectors.toList()); 
                                               //or .collect(Collectors.toCollection(LinkedList::new)); if you want a specific List implementation

Note that you could avoid the creation of a lamdba and use a method reference in this case with .map(Optional::of).

I don't see why you would get a List of Optional if you know in advance that the elements in the list are non-null. Maybe it was just to get in practice with Java 8, but as @Eran said you should use Optional::ofNullable instead.

Answer 2

It looks like you forgot to collect the Stream to a list :

List<Optional<Object>> convertedList = 
    originalList.stream()
                .map((o) -> Optional.ofNullable(o))
                .collect(Collectors.toList());

You should use ofNullable though, since of required a non null object (it will throw a NullPointerException when passed a null reference).