【Stream笔记】根据指定条件使用Stream查询List当中重复的实体，并且返回实体

通过groupingBy 与Function做筛选

// 方法
public static <E, R> List<E> getDuplicateElements(List<E> list, Function<E, R> function) {		
        Map<R, List<E>> collect = list.stream().collect(Collectors.groupingBy(function));
        return collect.entrySet().stream().filter(entry -> entry.getValue().size() > 1).flatMap(entry -> entry.getValue().stream()).collect(Collectors.toList());

    }

// 调用
    void contextLoads() {
        class User {
            public Integer id;
            public String name;

            public User(Integer id, String name) {
                this.id = id;
                this.name = name;
            }

            @Override
            public String toString() {
                return "User{" +
                        "id=" + id +
                        ", name='" + name + '\'' +
                        '}';
            }
        }
        List<User> list = new ArrayList<>();
        list.add(new User(1, "1"));
        list.add(new User(1, "2"));
        list.add(new User(2, "3"));
        list.add(new User(3, "3"));


        List<User> duplicateElements = getDuplicateElements(list, user -> user.id);
        System.out.println(duplicateElements);
    }