使用Java8从对象值映射中获取最大值键

Question

I have a below map with Integer as key and Entity object as values as below

Map<Integer, Entity> skuMap;

Entity.java:

private StatisticsEntity statistics;

StatisticsEntity.java:

private Integer count;

I want to get the key where the maximum count of StatisticsEntity from the skuMap.

I got the result with below codesnippet of Java8

Integer sku = skuMap.entrySet().stream().max((s1, s2) -> Integer.compare(s1.getValue().getStatistics().getCount(), s2.getValue().getStatistics().getCount())).orElse(null).getKey();

But I want to refactor the above with Comparator.comparingInt , any help would be appreciated.

Answer 1

Optional<Integer> sku = skuMap.entrySet().stream()
    .max(
        Comparator.comparingInt(
            entry -> entry.getValue().getStatistics().getCount()
        )
    )
    .map(Map.Entry::getKey);

Answer 2

I would've prefer one of those 2 approaches to avoid NullPointerException(in case of empty map or no statistics):

int max =  skuMap.values().stream().mapToInt(e -> e.getStatistics().getCount()).max().orElse(0);


Integer max2 = skuMap.values().stream().map(e -> e.getStatistics().getCount()).max(Integer::compareTo).orElse(null);

If you prefer with Comparator.comparingInt , it have a ToIntFunction as parameter. Input is the element you want to compare and output should be an integer - in your case count. Behind the scene,it will return exactly what you've written above (s1, s2) -> Integer.compare(s1.getValue().getStatistics().getCount(), s2.getValue().getStatistics().getCount()) :

Comparator.comparingInt(entry -> entry.getValue().getStatistics().getCount())

Also I think all IDE suggest the replacement. In IntelliJ, I've just press Alt+Enter on your version and it suggests me the replacement with Comparator.comparingInt .