I have a really simple Java code which I try to re...

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[Marcus Fihlon]

I have a really simple Java code which I try to rewrite using Kotlin. The Java code looks like this:

import com.google.common.collect.ImmutableList;

import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

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

public class SortCollectJava {

    private final Map<Long, Question> questions = new ConcurrentHashMap<>();

    List<Question> readAll() {
        return ImmutableList.copyOf(
                questions.values().stream()
                        .sorted(comparingLong(Question::getQuestionId))
                        .collect(toList()));
    }
}

My Kotlin code looks like this:

import com.google.common.collect.ImmutableList
import java.util.concurrent.ConcurrentHashMap

import java.util.Comparator.comparingLong
import java.util.stream.Collectors.toList

class SortCollectKotlin {

    private val questions = ConcurrentHashMap<Long, Question>()

    internal fun readAll(): List<Question> {
        return ImmutableList.copyOf(
                questions.values.stream()
                        .sorted(comparingLong<Question>({ it.questionId }))
                        .collect<List<Question>, Any>(toList<Question>()))
    }
}

I have a compile error at the toList call:

Type mismatch: inferred type is Collector<Question!, *, (Mutable)List<Question!>!>! but Collector<in Question!, Any!, List<Question>!>! was expected

I’m new to Kotlin but as far as I understand this code should work. Maybe I do not understand the meaning of “!”. How should this code look like?