在字典中找出三个最常见的词

Question

问题是找出字典中最常见的三个单词。我想出了下面的代码，但由于某种原因它不起作用（我的意思是当我尝试在 eclipse 中运行它时，它直接引导我进入调试页面，尽管我在编译器上没有收到任何错误screen)，调试后找不到原因。你能帮我找出问题所在吗？

Exception in thread "main" java.lang.NullPointerException at java.util.PriorityQueue.offer
(Unknown Source) at java.util.PriorityQueue.add
(Unknown Source) at generalquestions.MostCommonWords.mostCommonStringFinder
(MostCommonWords.java:41) at generalquestions.MostCommonWords.main
(MostCommonWords.java:61)

  public static Queue<Integer> mostCommonStringFinder (String document, int k){

    if (document == null){
        throw new IllegalArgumentException();
    }
    if (document.isEmpty()){
        throw new IllegalArgumentException("Document is empty");
    }   

    String [] wordHolder = document.split(" ");     

    HashMap<String, Integer> map = new HashMap<String, Integer>();


    for (String s : wordHolder){

        if (!map.containsKey(s)){
            map.put(s, 1);

        }
        else{
            int value = map.get(s);
            value++;
            map.put(s, value);
        }           
    }

    Queue<Integer> minHeap = new PriorityQueue<>();

    for ( int i = 0 ; i < k ; i++){

        minHeap.add(map.get(i));            
    }       

    for(int j = k ; j < map.size() ; j++){

        if(map.get(j) > minHeap.peek()){
            minHeap.poll();
            minHeap.add(map.get(j));
        }
    }

    return minHeap;

}

Answer 1

map 是一个 HashMap<String, Integer>，因此键是字符串，即文本中的单词。

map.get(i) 将始终 return null，因为映射中没有 Integer 键。

因为你 return 正在 Queue<Integer> 我假设期望是最高 k 字数，所以用 Queue<Integer> minHeap 替换所有内容：

List<Integer> counts = new ArrayList<>(map.values());
Collections.sort(counts, Collections.reverseOrder());
return counts.subList(0, k);

并将 return 类型更改为 List<Integer>。

测试

String text = "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod " +
              "tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, " +
              "quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo " +
              "consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse " +
              "cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat " +
              "non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.";
System.out.println(mostCommonStringFinder(text, 3));

输出

[3, 2, 2]

在字典中找出三个最常见的词

Find the three most common words in a dictionary

java

algorithm

heap

dictionary

data-structures