如何像在核心Java中一样进行分组和排序

Question

编写一个按日期分组的程序

Reviews{
private String reviewId;
private String tags;
private String topics;
private String date;
}
List<Reviews> reviewsList= new ArrayList<>();
reviewsList=[
{
reviewId=1;
tags="[\"group1\",\"group2\"]";
topics="[\"air\",\"Speed\"]";
date="2021-01-01 05:32:44"
},
{
reviewId=2;
tags="[\"Group1\"]";
topics="[\"Air\",\"quality\"]";
date="2021-01-07 09:32:44"
},
{
reviewId=3;
tags="[\"Group3\"]";
topics="[\"air\",\"blades\"]";
date="2021-01-07 01:32:24"
}]

主题输出格式应为：[{“日期”：“2021-01-01”，“主题列表”：[{“主题名称”：“空气”，“主题计数”：3}，{“主题名称”：“速度”，“主题计数”：1}]，{}，{}]

Answer 1

//intializing List    
List<Reviews> reviewsList= new ArrayList<>();
    
            reviewsList.add(new Reviews("1", "[\"group1\",\"group2\"]", "[\"air\",\"Speed\"]", "2021-01-01 05:32:44"));
            reviewsList.add(new Reviews("2", "[\"Group1\"]", "[\"Air\",\"quality\"]", "2021-01-07 09:32:44"));
            reviewsList.add(new Reviews("3", "[\"Group3\"]", "[\"air\",\"blades\"]", "2021-01-07 01:32:24"));
    
    //Converting date with seconds to date only ex: 2021-01-01 05:32:44 to 2021-01-01 as if time will be there every element will be unique
         List<Reviews> newList=   reviewsList.stream().map((x)->{
                DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
                LocalDate dateTime = LocalDate.parse(x.getDate(), formatter);
                x.setDate(dateTime.toString());
                return x;
            }).collect(Collectors.toList());
    
            
    
    //Grouping by date
            Map<String, List<Reviews>> groupedReviewByDate = newList.stream()
                    .collect(groupingBy(Reviews::getDate));
    
            
            // converting string key to date format so sorting could be easy
            SimpleDateFormat dateFormat=new SimpleDateFormat("yyyy-MM-dd");
            Map<Date, List<Reviews>> newMap = groupedReviewByDate.entrySet().stream()
                    .collect(Collectors.toMap(e -> {
                        try {
                            return dateFormat.parse(e.getKey());
                        } catch (ParseException ex) {
                            ex.printStackTrace();
                        }
                        return null;
                    }, Map.Entry::getValue));
    
           
    //Treemap will be sorted automatically on insertion
            Map<Date, List<Reviews>> m1 = new TreeMap(newMap);
            m1.forEach((k,v)->{
                System.out.println(k+"--- >"+v);
            });

课堂将是这样的

class  Reviews{
    private String reviewId;
    private String tags;
    private String topics;
    private String date;

    public Reviews(String reviewId, String tags, String topics, String date) {
        this.reviewId = reviewId;
        this.tags = tags;
        this.topics = topics;
        this.date = date;
    }

    public String getReviewId() {
        return reviewId;
    }

    public void setReviewId(String reviewId) {
        this.reviewId = reviewId;
    }

    public String getTags() {
        return tags;
    }

    public void setTags(String tags) {
        this.tags = tags;
    }

    public String getTopics() {
        return topics;
    }

    public void setTopics(String topics) {
        this.topics = topics;
    }

    public String getDate() {
        return date;
    }

    public void setDate(String date) {
        this.date = date;
    }

    @Override
    public String toString() {
        return "Reviews{" +
                "reviewId='" + reviewId + '\'' +
                ", tags='" + tags + '\'' +
                ", topics='" + topics + '\'' +
                ", date='" + date + '\'' +
                '}';
    }
}