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())); 

2 Answers

entrySet() returns a set of Pair<K, V>.

forEach()'s lambda therefore takes a single parameter of that type; not two integer parameters.

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()); }