LeetCode692题，对堆的比较器代码有所不解

题目如下：
Given an array of strings words and an integer k, return the k most frequent strings.

Return the answer sorted by the frequency from highest to lowest. Sort the words with the same frequency by their lexicographical order.

```java

![img](https://img-mid.csdnimg.cn/release/static/image/mid/ask/62286606113611.png "=600 #left")
String[] words = {"i","love","leetcode","i","love","coding"};
    int k = 2;
    List<String> res = topKFrequent(words, k);
    System.out.println(res);
}
public static List<String> topKFrequent(String[] words, int k) {
    HashMap<String, Integer> mapping = new HashMap<>();
    for(String word: words){
        if(!mapping.containsKey(word)) {
            mapping.put(word, 0);
        }
        int count = mapping.get(word) + 1;
        mapping.put(word,count);
    }
    PriorityQueue<String> pq = new PriorityQueue<>((w1, w2) -> mapping.get(w1).equals(mapping.get(w2)) ?                                                               w2.compareTo(w1): mapping.get(w1) - mapping.get(w2));
    for(String word: mapping.keySet()){
        pq.add(word);
        if(pq.size() > k){
            pq.poll();
        }
    }
    List<String> res = new ArrayList<>();
    while(!pq.isEmpty()){
        res.add(pq.poll());
    }
    Collections.reverse(res);
    return res;

我想问这一行代码
  PriorityQueue<String> pq = new PriorityQueue<>((w1, w2) -> mapping.get(w1).equals(mapping.get(w2)) ?                                                               w2.compareTo(w1): mapping.get(w1) - mapping.get(w2));
是什么意思？

PriorityQueue<>这个类有一个需要实现的抽象方法，有2个参数。->前后是个lambda表达式的意思，也就是对这个方法的实现