awesome lambda

join集合元素的某一个字段
如果有个 List list 想对name来join, 就需要每个都取出来一次.
用该方法则为:
join(list, Person::getName, ",");

public static <T> String join(List<T> list, Function<T, String> keyExtractor, String separator){
		String [] arr = new String[list.size()];
		for (int i=0;i<list.size();i++){
			arr[i] = keyExtractor.apply(list.get(i));
		}
		return join(arr, separator);
	}