Using Java 8 Streams for List‑to‑Map, Grouping, Filtering, Summing, and Deduplication

This article demonstrates how to leverage Java 8 Stream API to efficiently transform a List of objects into a Map, perform grouping, filtering, aggregation, and deduplication operations.

Define the Apple class

public class Apple {
    private Integer id;
    private String name;
    private BigDecimal money;
    private Integer num;
    public Apple(Integer id, String name, BigDecimal money, Integer num) {
        this.id = id;
        this.name = name;
        this.money = money;
        this.num = num;
    }
}

Create test data

List<Apple> appleList = new ArrayList<>();
Apple apple1 = new Apple(1, "苹果1", new BigDecimal("3.25"), 10);
Apple apple12 = new Apple(1, "苹果2", new BigDecimal("1.35"), 20);
Apple apple2 = new Apple(2, "香蕉", new BigDecimal("2.89"), 30);
Apple apple3 = new Apple(3, "荔枝", new BigDecimal("9.99"), 40);
appleList.add(apple1);
appleList.add(apple12);
appleList.add(apple2);
appleList.add(apple3);

1. Grouping by id

Map<Integer, List<Apple>> groupBy = appleList.stream()
    .collect(Collectors.groupingBy(Apple::getId));
System.err.println("groupBy:" + groupBy);

2. Convert List to Map (id as key, Apple as value)

Map<Integer, Apple> appleMap = appleList.stream()
    .collect(Collectors.toMap(Apple::getId, a -> a, (k1, k2) -> k1));
System.err.println("appleMap:" + appleMap);

3. Filter elements (e.g., name equals "香蕉")

List<Apple> filterList = appleList.stream()
    .filter(a -> a.getName().equals("香蕉"))
    .collect(Collectors.toList());
System.err.println("filterList:" + filterList);

4. Sum a numeric field (total money)

BigDecimal totalMoney = appleList.stream()
    .map(Apple::getMoney)
    .reduce(BigDecimal.ZERO, BigDecimal::add);
System.err.println("totalMoney:" + totalMoney); // totalMoney:17.48

5. Find maximum and minimum values (example with Dish )

Optional<Dish> maxDish = Dish.menu.stream()
    .collect(Collectors.maxBy(Comparator.comparing(Dish::getCalories)));
maxDish.ifPresent(System.out::println);

Optional<Dish> minDish = Dish.menu.stream()
    .collect(Collectors.minBy(Comparator.comparing(Dish::getCalories)));
minDish.ifPresent(System.out::println);

6. Remove duplicates based on id

List<Apple> unique = appleList.stream()
    .collect(Collectors.collectingAndThen(
        Collectors.toCollection(() -> new TreeSet<>(Comparator.comparingLong(Apple::getId))),
        ArrayList::new));

The article also includes a table of static factory methods of Collectors, an illustrative image, and a link to the original source containing additional interview questions.