Collectors

import java.text.DecimalFormat;

import java.util.List;

import java.util.Map;

import java.util.TreeMap;

import java.util.Arrays;

import java.util.stream.Stream;

import java.util.stream.Collectors;

class Main {

private final static DecimalFormat formatter = new DecimalFormat("0.00");

public static void main(String[] args) {

List<Item> items = Arrays.asList(

new Item("Laptop", "Electronics", 1200.0),

new Item("Mouse", "Electronics", 25.0),

new Item("Shirt", "Clothing", 30.0)

);

Map<String, List<String>> itemNamesByCategories = getItemNamesByCategories(items);

System.out.println(itemNamesByCategories);

Map<String, Double> itemTotalByCategories = getTotalByCategories(items);

System.out.println(itemTotalByCategories.entrySet().stream()

.collect(Collectors.toMap(entry -> entry.getKey(), entry -> formatter.format(entry.getValue()))));

Map<String, Map<String, Object>> itemStatisticsByCategories = getStatisticsByCategories(items);

System.out.println(itemStatisticsByCategories);

}

/**

* Exercise #1

*

* Map<String, List<String>> where key is category, value is list of uppercase item names.

*

*/

private static Map<String, List<String>> getItemNamesByCategories(List<Item> items) {

return items.stream()

.collect(Collectors.groupingBy(Item::getCategory, TreeMap::new, Collectors.mapping(item -> item.getName().toUpperCase(), Collectors.toList())));

}

/**

* Exercise #2

*

* Write code to get: Map<String, Double> where key is category, value is rounded total price (2 decimals).

*

*/

private static Map<String, Double> getTotalByCategories(List<Item> items) {

return items.stream()

.collect(Collectors.groupingBy(Item::getCategory, TreeMap::new, Collectors.collectingAndThen(Collectors.summingDouble(item -> item.getPrice()),

value -> Math.round(value * 100.0) / 100.0

)));

}

/**

* Exercise #3

*

* Get: Map<String, Map<String, Object>> with:

* - Key: Category

* - Value: Map with keys "count", "total", "average"

*

* Expected structure:

*

* {

* Electronics: {count=2, total=1225.00, avg=612.50},

* Clothing: {count=1, total=30.00, avg=30.00}

* }

*/

private static Map<String, Map<String, Object>> getStatisticsByCategories(List<Item> items) {

return items.stream()

.collect(Collectors.groupingBy(Item::getCategory, TreeMap::new, Collectors.collectingAndThen(

Collectors.toList(),

list -> {

Map<String, Object> stats = new TreeMap<>();

stats.put("count", list.size());

stats.put("total", formatter.format(list.stream().mapToDouble(Item::getPrice).sum()));

stats.put("avg", formatter.format(list.stream().mapToDouble(Item::getPrice).average().orElse(0.0)));

return stats;

}

)));

}

}

class Item {

private String name;

private String category;

private double price;

public Item(String name, String category, double price) {

this.name = name;

this.category = category;

this.price = price;

}

public String getName() { return name; }

public String getCategory() { return category; }

public double getPrice() { return price; }

}