The java8 collection object has three attributes: String, Integer, and Double. Now please group the collection according to the first attribute, find the sum of the other two attributes, and encapsulate the result into a new collection object.

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

public class Main {
    
    
    public static void main(String[] args) {
    
    
        List<Item> items = new ArrayList<>();
        items.add(new Item("Group1", 10, 2.5));
        items.add(new Item("Group1", 20, 3.5));
        items.add(new Item("Group2", 15, 4.0));
        items.add(new Item("Group2", 25, 5.0));

        List<Summary> summaries = items.stream()
                .collect(Collectors.groupingBy(Item::getProperty1))
                .entrySet()
                .stream()
                .map(entry -> {
    
    
                    String property1 = entry.getKey();
                    int sumOfProperty2 = entry.getValue().stream()
                            .mapToInt(Item::getProperty2)
                            .sum();
                    double sumOfProperty3 = entry.getValue().stream()
                            .mapToDouble(Item::getProperty3)
                            .sum();
                    return new Summary(property1, sumOfProperty2, sumOfProperty3);
                })
                .collect(Collectors.toList());

        summaries.forEach(System.out::println);
    }

    static class Item {
    
    
        private String property1;
        private int property2;
        private double property3;

        public Item(String property1, int property2, double property3) {
    
    
            this.property1 = property1;
            this.property2 = property2;
            this.property3 = property3;
        }

        public String getProperty1() {
    
    
            return property1;
        }

        public int getProperty2() {
    
    
            return property2;
        }

        public double getProperty3() {
    
    
            return property3;
        }
    }

    static class Summary {
    
    
        private String property1;
        private int sumOfProperty2;
        private double sumOfProperty3;

        public Summary(String property1, int sumOfProperty2, double sumOfProperty3) {
    
    
            this.property1 = property1;
            this.sumOfProperty2 = sumOfProperty2;
            this.sumOfProperty3 = sumOfProperty3;
        }

        public String getProperty1() {
    
    
            return property1;
        }

        public int getSumOfProperty2() {
    
    
            return sumOfProperty2;
        }

        public double getSumOfProperty3() {
    
    
            return sumOfProperty3;
        }

        @Override
        public String toString() {
    
    
            return "Summary{" +
                    "property1='" + property1 + '\'' +
                    ", sumOfProperty2=" + sumOfProperty2 +
                    ", sumOfProperty3=" + sumOfProperty3 +
                    '}';
        }
    }
}