test.java

package com.vinoth.java8.interfaces;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;



public class Test1 {
		public static void main(String[] args){
 			Activity a1 = new Activity(null,"Vinoth");
 			Activity a2 = new Activity("1","Kumar");
 			Activity a3 = new Activity("2","Varsh");
 			Activity a4 = new Activity("3","Vinoth");
 			Activity a5 = new Activity("4","AAA");
 			Activity a6 = new Activity(null,"Varsh");
 			
 			List<Activity> activities = Arrays.asList(a1,a2,a3,a4,a5,a6);
 			List<Activity> filteredActivity = squashDTO(activities);
 			List<Activity> expandedActivity = expandDTO(filteredActivity);
 			
 			System.out.println(expandedActivity);
 			
		}
		
private static List<Activity> expandDTO(List<Activity> dtoList){
	List<Activity> newList = new ArrayList<Activity>();
	
	
	for(Activity a : dtoList){
		for(String activityId : a.getActivityIds()){
			Activity n = new Activity(activityId,a.getName());
			  newList.add(n);
		 }
	}
	return newList;
}
		

private static List<Activity> squashDTO(List<Activity> dtoList){
	List<Activity> filteredActivity = new ArrayList<>();
		Map<String, List<Activity>> activityMap = dtoList.stream().collect(Collectors.groupingBy(Activity::getName));
		
		
		activityMap.forEach(( key,value) ->{ 
			Activity a = new Activity(value.get(0).getName());
			List<String> activityIds = new ArrayList<>();
			for(Activity aa : value){
				
				activityIds.add(aa.getActivityId());
			}
			a.setActivityIds(activityIds);
			filteredActivity.add(a);
			
		});
		
		return filteredActivity;
}
}
class Activity{
	String name;
	
	String activityId;
	
	List<String> activityIds = new ArrayList<>();
	
	public Activity(String name){
		this.name = name;
		
	}
	
	public Activity(String id, String name){
		this.name = name;
		this.activityId = id;
	}

	public String getName() {
		return name;
	}
	
	public String getActivityId() {
		return activityId;
	}

	public List<String> getActivityIds() {
		return activityIds;
	}

	public void setActivityIds(List<String> activityIds) {
		this.activityIds = activityIds;
	}

	@Override
	public String toString() {
		return "Activity [name=" + name + ", activityId=" + activityId
				+ ", activityIds=" + activityIds + "]\n";
	}
}