java - Arrays.sort 引发编译时错误 'Anonymous class derived from Comparator..'

Question

错误:

Anonymous class derived from Comparator must either be declared Abstract or implement abstract method compare(T,T).

我想根据 map 中的频率对数组进行排序。

HashMap<Integer,Integer> freq = new HashMap<>();
        for(int a:arr)
        {
            freq.putIfAbsent(a,freq.getOrDefault(a,0)+1);
        }
Arrays.sort(arr,new Comparator<>(){
            public int compare(int a,int b)
            {
                if(freq.get(b)!=freq.get(a))
                    return freq.get(b)-freq.get(a);
                else
                    return a-b;
            }
        });

我在这里缺少什么？

Answer 1

假设您有一个Integer[]，例如

Integer[] arr = { 10, 2, 30, 4 };

您可以按如下方式进行:

Arrays.sort(arr, new Comparator<Integer>() {
    @Override
    public int compare(Integer a, Integer b) {
        if (freq.get(b) != freq.get(a))
            return Integer.compare(freq.get(b), freq.get(a));
        else
            return a.compareTo(b);
    }
});

如果您有一个int []，例如

int[] arr = { 10, 2, 30, 4 };

将其转换为Integer[]为

Integer[] arr1 = Arrays.stream(arr).boxed().toArray(Integer[]::new);

然后将arr1排序为

Arrays.sort(arr1, new Comparator<Integer>() {
    @Override
    public int compare(Integer a, Integer b) {
        if (freq.get(b) != freq.get(a))
            return Integer.compare(freq.get(b), freq.get(a));
        else
            return a.compareTo(b);
    }
});