java8一些常用方法

List users=new ArrayList();
Map> listMap = users.stream().collect(Collectors.groupingBy(h -> h.getUserName()));
for (String userName: listMap.keySet()) {
        List uList=listMap.get(userName);
}

 List userFilterList= users.stream().filter(s ->
                        A.getName().equals(s.getUserName())).collect(Collectors.toList());

List list= users.stream().map(User::getUserName).collect(Collectors.toList());