我的Java程序中出现NullPointerExecption

Question

我刚刚开始学习Java，我试图计算一个单词在我传递给它的字符串中出现的次数。我正在使用HashMap类，我传递的单词是键，它们的计数是值。我不知道为什么我总是在计数器中出现NPE异常，for循环在我看来正确，我是否正确使用了split函数？

import java.util.HashMap;

public class WordCount {

    private HashMap<String,Integer> map;

    public WordCount() {
        map = new HashMap<String,Integer>();
    }

    public WordCount(String words) {
        String[] list = words.split(",");

        for(int i = 0; i < list.length; i++) {
            if(!map.containsKey(list[i]))  {
                map.put(list[i],1);
            } else {
                map.put(list[i],map.get(list[i])+1);
            }
        }
    }

    public void addWord(String toAdd) {
        if(!map.containsKey(toAdd)) {
            map.put(toAdd,1);
        } else {
            map.put(toAdd,map.get(toAdd)+1);
        }
    }

    public void startOver() {
        if(map.isEmpty()) {
            return;
        } else {
            for(String s: map.keySet()) {
                map.remove(s);
        }
    }
}

public int countWord(String word) {
    return map.get(word);
}


}

/ ******************************************* /

public class Main
{
    public static void main(String[] args)
    {
        String s = "hello,bye,how,are,you,ok";
        WordCount wordC = new WordCount(s);

        System.out.println("Number of Words: " + wordC.countWord("how"));
    }

}