以正确的顺序迭代ConcurrentHashMap

Question

I use a ConcurrentHashMap to store English words and their occurrences within a text.

private Map<String, Integer> _vocab = new ConcurrentHashMap<String,Integer>();

I need the map to be sorted by the value in descending order, so I did:

   public void sort(){
        this._vocab = this._vocab.entrySet()
                .stream()
                .sorted(Collections.reverseOrder(Map.Entry.comparingByValue()))
                .collect(
                        toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e2,
                                LinkedHashMap::new));
    }

Which seems to be working. However, now I want to limit the hashmap to for example 1000 elements, since I'm only interested in the top 1000 entries:

public void limitToNElements(int n){
    ConcurrentHashMap<String, Integer> res = new ConcurrentHashMap<String, Integer>();

    Iterator it = this._vocab.entrySet().iterator();
    int i = 0;
    while (it.hasNext() && i < n) {
        Map.Entry pair = (Map.Entry)it.next();
        String word = (String)pair.getKey();
        res.put((String)pair.getKey(), (Integer)pair.getValue());

        i += 1;
    }

    this._vocab = res;
}

The problem is, that the iterator is not iterating in the correct order, but rather randomly.

So I read that only LinkedHashMaps work this way, but since there is no Concurrent version and I'm afraid to do the synchronization myself, I'm wondering if there is any workaround to achieve what I want?

Edit: I used a HashMap because insertion/occurrence counting was very simple:

for(String word: input){
  this._vocab.put(word, this._vocab.getOrDefault(word, 0)+1);
}

Answer 1

You could just apply limit on the stream:

public void sort() {
    this._vocab = this._vocab.entrySet()
            .stream()
            .sorted(Collections.reverseOrder(Map.Entry.comparingByValue()))
            .limit(1000) // Here
            .collect(
                    toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e2,
                            LinkedHashMap::new));
}