List 集合按照某个字段或者属性分组的两种方式

2022-11-13
浙江
本文字数：1329 字
阅读完需：约 4 分钟

Java8 之前的分组

代码如下：

    public static List<User> getUserList(){        List<User> userList = new ArrayList<>();        userList.add(new User("小明",11,"北京",66));        userList.add(new User("小红",12,"上海",99));        userList.add(new User("小詹",14,"杭州",77));        userList.add(new User("小龙",16,"伦敦",55));        userList.add(new User("小斯",9,"杭州",33));        userList.add(new User("小詹",9,"上海",33));        userList.add(new User("小布",12,"伦敦",88));        userList.add(new User("小布",12,"上海",55));        return userList;    }
    /**     * java8之前对象集合根据某个字段分组     */    @Test    public void sortUserListByAge(){        List<User> userList = getUserList();        Map<String, List<User>> groupByUserCityMap = new HashMap<>();        for (User user : userList) {            List<User> tmpList = groupByUserCityMap.get(user.getCity());            if (tmpList == null) {                tmpList = new ArrayList<>();                tmpList.add(user);                groupByUserCityMap.put(user.getCity(), tmpList);            } else {                tmpList.add(user);            }        }        System.out.println("按照城市分组后结果："+groupByUserCityMap);    }

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输出如下：

按照城市分组后结果：{上海=[User(name=小红, age=12, city=上海, score=99), User(name=小詹, age=9, city=上海, score=33), User(name=小布, age=12, city=上海, score=55)], 伦敦=[User(name=小龙, age=16, city=伦敦, score=55), User(name=小布, age=12, city=伦敦, score=88)], 杭州=[User(name=小詹, age=14, city=杭州, score=77), User(name=小斯, age=9, city=杭州, score=33)], 北京=[User(name=小明, age=11, city=北京, score=66)]}

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Java8 的分组

代码如下：

    /**     * java8根据某个字段分组     */    @Test    public void java8GroupUserList(){        List<User> userList = getUserList();        Map<String, List<User>> groupByUserNameMap = userList.stream().collect(Collectors.groupingBy(User::getName));        System.out.println("按照姓名分组后结果："+groupByUserNameMap);    }

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输出如下：

按照姓名分组后结果：{小龙=[User(name=小龙, age=16, city=伦敦, score=55)], 小詹=[User(name=小詹, age=14, city=杭州, score=77), User(name=小詹, age=9, city=上海, score=33)], 小明=[User(name=小明, age=11, city=北京, score=66)], 小红=[User(name=小红, age=12, city=上海, score=99)], 小斯=[User(name=小斯, age=9, city=杭州, score=33)], 小布=[User(name=小布, age=12, city=伦敦, score=88), User(name=小布, age=12, city=上海, score=55)]}

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