GroupingBy Exercise

import java.util.List;

import java.util.Map;

import java.util.TreeMap;

import java.util.Arrays;

import java.util.stream.Collectors;

class Main {

public static void main(String[] args) {

List<Item> items = Arrays.asList(

new Item("Mouse", 15.0),

new Item("Keyboard", 25.0),

new Item("Mouse", 30.0),

new Item("Keyboard", 120.0),

new Item("Mouse", 10.0)

);

Map<String, Integer> itemsCountByName = items.stream()

.map(Item::getName)

.collect(Collectors.groupingBy(name -> name, TreeMap::new, Collectors.summingInt(n -> 1)));

itemsCountByName.forEach((k,v) -> System.out.println(k + ": " + v));

Map<String, Double> averagePricePerProduct = items.stream()

.collect(Collectors.groupingBy(Item::getName, TreeMap::new, Collectors.averagingDouble(Item::getPrice)));

averagePricePerProduct.forEach((k,v) -> System.out.println(k + ": " + String.format("%.2f", v)));

Map<String, Double> totalPerCategory = items.stream()

.collect(Collectors.groupingBy(item -> getCategory(item.getPrice()), Collectors.summingDouble(Item::getPrice)));

final String[] categories = {"Cheap", "Moderate", "Expensive"};

for(String category : categories) {

System.out.println(category + ": " + String.format("%.2f", totalPerCategory.get(category)));

}

}

private static String getCategory(double price) {

if (price < 20) {

return "Cheap";

} else if (price >= 20 && price <= 100) {

return "Moderate";

}

return "Expensive";

}

}