Stream的Collectors.groupingBy支持key为null进行分组

场景

在使用Collectors.groupingBy分组时,如果分组的字段中有值为null,会抛出空指针异常
例如:

// An highlighted block
calss User{
	private Integer age;
	private String sex;
}

public static void main(String[] args){
	User user1 = new User(18, "女");
	User user2 = new User(19, "女");
	User user3 = new User(20, "男");
	//没有性别的人需要用null表示
	User user4 = new User(18, null);
	groupBy(Arrays.aslist(user1,user2,user3,user4))
	
}
//对用户按照年纪分组
public void groupBy(List<User> userList){
    //此时这里会抛出空指针异常
	Map<String, List<User>> userMap = userList.stream.collect(Collectors.groupingBy(x -> x.getSex()));
}

解决方案

public void groupBy(List<User> userList){
    //使用option包装一层
	Map<Option<String>, List<User>> userMap = userList.stream.collect(Collectors.groupingBy(x -> Optional.ofNullable(x.getSex())));
}