java8->list转map判空，分组

    /**
     * list转map/分组
     * @Author: Mr.li
     * @Date: 2020/9/8
     */
    @Test
    public void demo7() {
        List<User> userList = new ArrayList<User>() {
            {
                add(new User("付1", "男1", 20));
                add(new User("付1", "男", 20));
                add(new User("付2", "男", 21));
                add(new User("付3", "男", 22));
                add(new User("付4", "男", 23));
                add(new User("付5", "男", 24));
                add(new User(null, "男", 25));
            }
        };
        User user6 = new User("zhangsan", "男", 24);
        userList.add(user6);
        /** list 转map
         * 注意：要是key重复的话 会报错Duplicate key ....
         * key name  都是1
         * 可以用 (k1,k2)->k1 来设置，如果有重复的key,则保留key1,舍弃key2
         */
        Map<String, Object> compMap = userList.stream().filter(e->e.getName()!=null).collect(Collectors.toMap(User::getName, a -> a.getAge() + "-->" + a.getName(), (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);
        }*/
        compMap.forEach((k,v)->{
            System.out.println(k+":"+v);
        });


        /**
         * list中以某个属性分组，比如用name分组
         */
        Map<String, List<User>> map = userList.stream().filter(e->e.getName()!=null).collect(Collectors.groupingBy(User::getName));
        System.out.println("Map: "+map);
        System.out.println("User: "+ getUser(user6));
    }
    /**
     * 得到user
     * @Author: Mr.li
     * @Date: 2020/9/8
     */
    public User getUser(User user) {
        return Optional.ofNullable(user)
                .filter(u -> "zhangsan".equals(u.getName()))
                .orElseGet(() -> {
                    User user1 = new User();
                    user1.setName("zhangsan");
                    return user1;
                });
    }

result：