并发集合：容器安全不代表跨调用业务自动原子

import java.util.*;
import java.util.concurrent.*;

public class Main {
    record Hit(String word,long count) {}
    static List<Hit> top(List<String> input,int k,int workers) throws Exception {
        if(k<0 || workers<=0) throw new IllegalArgumentException();
        List<String> tokens=List.copyOf(input); // 固定输入并拒绝null词元。
        if(k==0 || tokens.isEmpty()) return List.of();
        ConcurrentHashMap<String,Long> counts=new ConcurrentHashMap<>();
        int parallel=Math.min(workers,tokens.size());
        ExecutorService executor=Executors.newFixedThreadPool(parallel);
        List<Future<?>> jobs=new ArrayList<>();
        try {
            for(int t=0;t<parallel;t++) {
                int first=t;
                jobs.add(executor.submit(() -> {
                    for(long i=first;i<tokens.size();i+=parallel)
                        counts.merge(tokens.get((int)i),1L,Math::addExact); // 按键原子累加。
                }));
            }
            for(Future<?> job:jobs) job.get(5,TimeUnit.SECONDS); // 写入全部结束后再排名。
        } finally {
            for(Future<?> job:jobs) if(!job.isDone()) job.cancel(true);
            executor.shutdownNow();
        }
        Comparator<Hit> worst=Comparator.comparingLong(Hit::count)
                .thenComparing(Hit::word,Comparator.reverseOrder());
        PriorityQueue<Hit> heap=new PriorityQueue<>(worst);
        for(var entry:counts.entrySet()) {
            Hit hit=new Hit(entry.getKey(),entry.getValue());
            if(heap.size()<k) heap.add(hit);
            else if(worst.compare(hit,heap.peek())>0) { heap.poll(); heap.add(hit); }
            // 堆顶始终是保留集合中最差者，较好候选才能替换。
        }
        List<Hit> result=new ArrayList<>(heap);
        result.sort(Comparator.comparingLong(Hit::count).reversed().thenComparing(Hit::word));
        return List.copyOf(result);
    }
    static void check(boolean ok) { if(!ok) throw new AssertionError(); }
    public static void main(String[] args) throws Exception {
        check(top(List.of("b","a","b","a","c"),2,3).equals(List.of(new Hit("a",2),new Hit("b",2))));
        check(top(List.of("java","ai","java"),9,2).equals(List.of(new Hit("java",2),new Hit("ai",1))));
        check(top(List.of(),2,1).isEmpty());
        check(top(List.of("a"),0,1).isEmpty());
        check(top(Collections.nCopies(1000,"hot"),1,4).equals(List.of(new Hit("hot",1000))));
        check(top(List.of("z","b","a"),1,2).equals(List.of(new Hit("a",1))));
        try { top(Arrays.asList("a",null),1,1); throw new AssertionError(); }
        catch(NullPointerException expected) { }
        System.out.println("PASS 7");
    }
}