Java8Features/StreamJava8Part4GroupingBy at main · simi14/Java8Features · GitHub

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//Use of groupingBy method in various ways
//Can help solve complex queries without using SQL
//reference https://www.baeldung.com/java-groupingby-collector

import java.util.*;
import java.util.stream.*;
import java.util.stream.Collectors;
import java.util.Comparator;
public class MyClass {

    public static void main(String args[]) {
        List<BlogPost> list = initializeValues();
        groupBySingleColumn(list);
        ComplexGrouping(list);
        groupByMultipleFields(list);
        groupByTypeGetAverageLikes(list);
        groupByTypeGetSumOfLikes(list);
        groupByTypeMaxMin(list);
    }
    public static List<BlogPost> initializeValues(){
        List<BlogPost> list=Arrays.asList(new BlogPost("Shoe Dog","Phil Knight",BlogPostType.BUSINESS,30),
        new BlogPost("Made In America","Sam Walton",BlogPostType.BUSINESS,25),
        new BlogPost("Life's Amazing Secrets","Gaur Gopal Das",BlogPostType.INSPIRATIONAL,20),
        new BlogPost("The Way Of the Monk","Gaur Gopal Das",BlogPostType.INSPIRATIONAL,15),
        new BlogPost("Attitude is Everything","Keller Jeff",BlogPostType.INSPIRATIONAL,25),
        new BlogPost("Fault In Our Stars","John Green",BlogPostType.ROMANCE,30));
        return list;

    }
    //Grouping by a Single Column
    public static void groupBySingleColumn(List<BlogPost> list){
        //----------------------------grouping by Author------------------------------------------
        Map<String,List<BlogPost>> mapGroupByAuthor=list.stream().collect(Collectors.groupingBy(BlogPost::getAuthor));
        System.out.println(mapGroupByAuthor);
        /*Output :
        {Phil Knight=[title : Shoe Dog, author : Phil Knight, type : BUSINESS, likes : 30], Sam Walton=[title : Made In America, author : Sam Walton, type : BUSINESS, likes : 25], John Green=[title : Fault In Our Stars, author : John Green, type : ROMANCE, likes : 30], Gaur Gopal Das=[title : Life's Amazing Secrets, author : Gaur Gopal Das, type : INSPIRATIONAL, likes : 20, title : The Way Of the Monk, author : Gaur Gopal Das, type : INSPIRATIONAL, likes : 15], Keller Jeff=[title : Attitude is Everything, author : Keller Jeff, type : INSPIRATIONAL, likes : 25]}
        */

        //-----------------------------grouping by type (BlogPostType)----------------------------
        Map<BlogPostType,List<BlogPost>> mapGroupByType=list.stream().collect(Collectors.groupingBy(BlogPost::getType));
        System.out.println(mapGroupByType);
        /*Output :
        {ROMANCE=[title : Fault In Our Stars, author : John Green, type : ROMANCE, likes : 30], INSPIRATIONAL=[title : Life's Amazing Secrets, author : Gaur Gopal Das, type : INSPIRATIONAL, likes : 20, title : The Way Of the Monk, author : Gaur Gopal Das, type : INSPIRATIONAL, likes : 15, title : Attitude is Everything, author : Keller Jeff, type : INSPIRATIONAL, likes : 25], BUSINESS=[title : Shoe Dog, author : Phil Knight, type : BUSINESS, likes : 30, title : Made In America, author : Sam Walton, type : BUSINESS, likes : 25]}
        */
    }
    //Complex Grouping-> Don't use it
    public static void ComplexGrouping(List<BlogPost> list){

      Map<Tuple, List<BlogPost>> postsPerTypeAndAuthor = list.stream()
  .collect(Collectors.groupingBy(post -> new Tuple(post.getType(), post.getAuthor())));
        //Map<Tuple, List<BlogPost>> postsPerTypeAndAuthor = list.stream().collect(Collectors.groupingBy(i->i.groupBy(i.getType(),i.getAuthor())));
        System.out.println(postsPerTypeAndAuthor);

    }
    //Grouping by Multiple Fields
    public static void groupByMultipleFields(List<BlogPost> list){
        Map<BlogPostType,Map<String,List<BlogPost>>> map=list.stream().collect(Collectors.groupingBy(BlogPost::getType,Collectors.groupingBy(BlogPost::getAuthor)));
        map.forEach((K,V)->{
            System.out.print(K+ " ");
            V.forEach((k,v)->{
                System.out.println(k+" "+v+"\t");
            });
            System.out.println();
        });
        /*Output:
        ROMANCE John Green [title : Fault In Our Stars, author : John Green, type : ROMANCE, likes : 30]

        INSPIRATIONAL Gaur Gopal Das [title : Life's Amazing Secrets, author : Gaur Gopal Das, type : INSPIRATIONAL, likes : 20, title : The Way Of the Monk, author : Gaur Gopal Das, type : INSPIRATIONAL, likes : 15]
        Keller Jeff [title : Attitude is Everything, author : Keller Jeff, type : INSPIRATIONAL, likes : 25]

        BUSINESS Phil Knight [title : Shoe Dog, author : Phil Knight, type : BUSINESS, likes : 30]
        Sam Walton [title : Made In America, author : Sam Walton, type : BUSINESS, likes : 25]
        */
    }
    //Getting the Average from Grouped Results
    public static void groupByTypeGetAverageLikes(List<BlogPost> list){
        Map<BlogPostType,Double> map=list.stream().collect(Collectors.groupingBy(BlogPost::getType,Collectors.averagingInt(BlogPost::getLikes)));
        map.forEach((K,V)->{System.out.println(K+" "+V);});
        /*Output
            ROMANCE 30.0
            INSPIRATIONAL 20.0
            BUSINESS 27.5
        */
    }
     //Getting the Sum from Grouped Results
    public static void groupByTypeGetSumOfLikes(List<BlogPost> list){
        Map<BlogPostType,Integer> map=list.stream().collect(Collectors.groupingBy(BlogPost::getType,Collectors.summingInt(BlogPost::getLikes)));
        map.forEach((K,V)->{System.out.println(K+" "+V);});
        /*Output
            ROMANCE 30
            INSPIRATIONAL 60
            BUSINESS 55
        */
    }
    //Getting the Maximum or Minimum from Grouped Results
    public static void groupByTypeMaxMin(List<BlogPost> list){
        //--------------Optional is used to prevent code to return null pointer exception-------------
        Map<BlogPostType,Optional<BlogPost>> mapMax=list.stream().collect(Collectors.groupingBy(BlogPost::getType,Collectors.maxBy(Comparator.comparingInt(BlogPost::getLikes))));
        mapMax.forEach((K,V)->{if(V.isPresent()){System.out.println(K+" "+V.get());}});
        /*Output
            ROMANCE title : Fault In Our Stars, author : John Green, type : ROMANCE, likes : 30
            INSPIRATIONAL title : Attitude is Everything, author : Keller Jeff, type : INSPIRATIONAL, likes : 25
            BUSINESS title : Shoe Dog, author : Phil Knight, type : BUSINESS, likes : 30
        */

    }
}
class BlogPost {
    String title;
    String author;
    BlogPostType type;
    int likes;

    BlogPost(String title,String author,BlogPostType type,int likes){
        this.title=title;
        this.author=author;
        this.type=type;
        this.likes=likes;
    }

    public String getAuthor(){return author;}
    public String getTitle(){return title;}
    public BlogPostType getType(){return type;}
    public int getLikes(){return likes;}
    public String toString(){
        return "title : "+title+", author : "+author+", type : "+type+", likes : "+likes;
    }
    public Tuple groupBy(BlogPostType type,String author){
        if(this.type==type && this.author==author){
            return new Tuple(type,author);
        }
        return new Tuple(type,author);
    }
}

enum BlogPostType {
    INSPIRATIONAL,
    BUSINESS,
    ROMANCE
}
class Tuple {
    BlogPostType type;
    String author;
    Tuple(BlogPostType type,String author)
    {
        this.type=type;
        this.author=author;
    }
    public BlogPostType getType(){return type;}
    public String getAuthor(){return author;}
    public String toString(){
        return "Type : "+type+", Author : "+author;
    }
}