java8--List转为Map、分组、过滤、求和等操作

public static void main(String[] args) {
    List<User> userList = new ArrayList<User>();
    User user0 = new User("付萌朝1", "男1", 20);
    User user1 = new User("付萌朝1", "男", 20);
    User user2 = new User("付萌朝2", "男", 21);
    User user3 = new User("付萌朝3", "男", 22);
    User user4 = new User("付萌朝4", "男", 23);
    User user5 = new User("付萌朝5", "男", 24);
    userList.add(user0);
    userList.add(user1);
    userList.add(user2);
    userList.add(user3);
    userList.add(user4);
    userList.add(user5);
    /**
     * list 转map
     * 注意：要是key重复的话 会报错Duplicate key ....
     * key name  都是付萌朝1
     * 可以用 (k1,k2)->k1 来设置，如果有重复的key,则保留key1,舍弃key2
     * result
     *  付萌朝4=23===男
     付萌朝3=22===男
     付萌朝2=21===男
     付萌朝1=20===男1
     付萌朝5=24===男
     */
    Map<String,Object> compMap= userList.stream().collect(Collectors.toMap(User::getName, a -> a.getAge() + "===" + a.getSex(), (k1, k2) -> k1));
    for(Object obj : compMap.keySet()) {
        String key = (String) obj;//取到每一个key值
        String value = (String) compMap.get(key);
        System.out.println(key + "=" + value);
    }

    /**
     * list中以某个属性分组，比如用name分组
     */
    Map<String,List<User>> map= userList.stream().collect(Collectors.groupingBy(User::getName));
    System.out.println(map);

}