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());
}
