[FIXED] Java8 - How does explicit type matches one variant - not other type?

Issue

I have a simple snippet as below. I referred this

List<Document> list = new LinkedList<Document>();
FindIterable<Document> itr = collection.find(findQuery)
                       .forEach((Document doc) -> list.add(doc));
return list;

It compiles without any issues.

1. I guess that we are telling compiler that doc is of type Document. Why is it needed?

But If I do the below, it throws ambiguous error. I referred this But couldn't relate and understand exactly.

collection.find(findQuery).forEach(list::add);

2. Could anyone please explain why second statement is not working?

3. is there any better way of writing the first one [working one]?

Java version: 1.8.0_231

import statements:

import java.util.List;
import java.util.Optional;
import com.mongodb.client.FindIterable;
import org.bson.Document;

Solution

FindIterable inherits two forEach methods:

• com.mongodb.client.MongoIterable.forEach(Block<? super TResult>)

• java.lang.Iterable.forEach(Consumer<? super T>)

You could rewrite your paramter with either

Consumer<Document> consumer = documents::add;
Block<Document> block = list::add;

And either will work. These too will work:

.forEach((Consumer<Document>) doc -> list.add(doc))
.forEach((Consumer<Document>) list::add);

However, when you call forEach(list::add) or forEach(doc -> list.add(doc)), the compiler is unable to pick which overload will determine the method reference's target type (because the expression is compatible with both in that context).

Now, I'm not 100% sure why .forEach((Document doc) -> list.add(doc)) successfully selects/links the signature of Consumer<Document> instead of the one with Block<? super Document>, I'm surmizing it has to do with the generic bounds (but I'm still reading on this).

The choice for you should be easy because the Block<? super Document> version is deprecated.

Answered By - ernest_k