collection.frequency方法，可以统计出某个对象在collection中出现的次数
比如：
frequency(Collection<?> c, Object o)
则在collection中，找出o的次数，比如用在统计中就很有用了，比如代码：
@SuppressWarnings("unchecked")
public static void main(String[] args) {
String text = "a r b k c d se f g a d f s s f d s ft gh f ws w f v x s g h d h j j k f sd j e wed a d f";
List<String> list = new ArrayList<String>();
list.addAll(Arrays.asList(text.split(" ")));
Set<String> uniqueWords = new HashSet<String>(list);
for (String word : uniqueWords) {
System.out.println(word + ": " + Collections.frequency(list, word));
}
}