设计一个方法，找出任意指定单词在一本书中的出现频率。
你的实现应该支持如下操作：
- WordsFrequency(book)构造函数，参数为字符串数组构成的一本书
- get(word)查询指定单词在书中出现的频率
示例：
WordsFrequency wordsFrequency = new WordsFrequency({"i", "have", "an", "apple", "he", "have", "a", "pen"});
wordsFrequency.get("you"); //返回0，"you"没有出现过
wordsFrequency.get("have"); //返回2，"have"出现2次
wordsFrequency.get("an"); //返回1
wordsFrequency.get("apple"); //返回1
wordsFrequency.get("pen"); //返回1
代码：
class WordsFrequency {
public Map<String, Integer> map = new HashMap<>();
public WordsFrequency(String[] book) {
for(String word: book){
map.put( word, map.getOrDefault(word,0)+1 );
}
}
public int get(String word) {
return map.getOrDefault(word, 0);
}
}
/**
* Your WordsFrequency object will be instantiated and called as such:
* WordsFrequency obj = new WordsFrequency(book);
* int param_1 = obj.get(word);
*/ 
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