Java: Add to Guava ImmutableList if optional value is present

Question

Looking for an ideal way to add values optionally to list. Final list must be immutable.

Example-

Optional<Item> optionalItem = getOptionalItemFromSomewhereElse();

List<Item> list = ImmutableList.builder()
                      .add(item1) 
                      .add(item2)
                      .optionallyAdd(optionalItem)
                  .build();

Answer 1

I would add the optional item at the end, if it's present:

ImmutableList.Builder<Item> builder = ImmutableList.<Item>builder()
    .add(item1)
    .add(item2);
optionalItem.ifPresent(builder::add);

After that, I'd build the list:

ImmutableList<Item> list = builder.build();

Answer 2

Assuming you're using Guava, here's a simple one-liner:

List<Item> list = Stream.concat(Stream.of(item1, item2), Streams.stream(optionalItem))
        .collect(ImmutableList.toImmutableList());

Note: This requires at minimum Java 8 and Guava 21.