AR7 AR7 - 6 months ago 21
Java Question

How to avoid "incompatible parameter types in lambda expression" when adding to an ArrayList?

I have the following code

public static List<Integer> topKFrequent(int[] nums, int k) {
List<Integer> myList = new ArrayList<>();
HashMap<Integer, Integer> map = new HashMap<>();

for (int n : nums) {
if (!map.containsKey(n)) map.put(n, 1);
else map.put(n, map.get(n) + 1);
}

map.entrySet().stream()
.sorted(Map.Entry.<Integer, Integer>comparingByValue().reversed())
.limit(k)
.forEach((key, value) -> myList.add(key));

return myList;
}


The
forEach
throws the error

Error:(20, 16) java: incompatible types: incompatible parameter types in lambda expression


How can I fix/avoid this error? I'm not quite sure how to apply the answer here that explains the problem: Lambda Expression and generic method

Edit:

Given the answer, the correction is to replace the lambda inside the forEach with

.forEach((entry) -> myList.add(entry.getKey()));

Answer

You are going about it in a java7-ish way. Modifying external data structures from inside forEach is not how Streams API was meant to be used. Streams API documentation specifically warns against such use in the Side-Effects section of java.util.stream package summary

Instead of appending to list or map from inside forEach, use collect:

import static java.util.Comparator.reverseOrder;
import static java.util.Map.Entry.comparingByValue;
import static java.util.stream.Collectors.counting;
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.toList;


public static List<Integer> topKFrequent(int[] nums, int k) {
    Map<Integer, Long> freq = Arrays.stream(nums).boxed()
            .collect(groupingBy(x->x, counting()));

    return freq.entrySet()
            .stream()
            .sorted(comparingByValue(reverseOrder()))
            .limit(k)
            .map(Map.Entry::getKey)
            .collect(toList());
}