Null collections converted to empty

By default, immutables.io will create empty collections if none are supplied.

@Value.Immutable
public abstract class MyTestPojo {
    public abstract List<String> myList();
}

The following will create the object with an empty collection:

MyTestPojo pojo = ImmutableMyTestPojo.builder()
        .build();

However, if the value is explicitly set to null, immutables will throw NPE.

MyTestPojo pojo2 = ImmutableMyTestPojo.builder()
        .myList(null)
        .build();

This could be avoided by allowing nulls with @Nullable. This would result in the collection being null. I would like this case to gracefully handle null and convert it to an empty collection.

I suggest you look into the normalization feature, which makes it possible to perform some modifications before the final instance is returned:

@Value.Immutable
public abstract class MyTestPojo {
    @Nullable
    public abstract List<String> myList();

    @Value.Check
    protected MyTestPojo check() {
        if ( myList() == null ) {
            return ImmutableMyTestPojo.copyOf(this).withMyList();
        }

        return this;
    }
}

Note that the myList property is annotated with @Nullable. The check will override the null value with an empty list during instantiation.