core22StreamsDemo

1package pkg1core;2 3/*4 * core22StreamsDemo.java5 * ----------------6 * The Streams API: building pipelines of intermediate + terminal operations,7 * and Collectors for grouping/joining/summarizing.8 *9 * EXPLANATION:10 *  - Streams are LAZY: intermediate ops (filter/map/sorted) build a pipeline;11 *    a terminal op (collect/reduce/forEach/count) triggers execution.12 *  - Streams don't mutate the source; prefer pure, stateless functions.13 */14import java.util.*;15import java.util.stream.*;16 17public class core22StreamsDemo {18 19    record Person(String name, String city, int age) {}20 21    public static void main(String[] args) {22        List<Integer> nums = List.of(5, 2, 8, 1, 9, 3, 8, 2);23 24        // filter -> map -> sorted -> distinct -> collect25        List<Integer> result = nums.stream()26                .filter(n -> n > 2)27                .map(n -> n * 10)28                .distinct()29                .sorted()30                .toList();31        System.out.println("pipeline: " + result);32 33        // Numeric streams + statistics34        IntSummaryStatistics stats = nums.stream().mapToInt(Integer::intValue).summaryStatistics();35        System.out.printf("count=%d sum=%d min=%d max=%d avg=%.2f%n",36                stats.getCount(), stats.getSum(), stats.getMin(), stats.getMax(), stats.getAverage());37 38        // reduce39        int product = Stream.of(1, 2, 3, 4).reduce(1, (a, b) -> a * b);40        System.out.println("product 1..4 = " + product);41 42        // Collectors: grouping, partitioning, joining43        List<Person> people = List.of(44                new Person("Alice", "NYC", 30),45                new Person("Bob", "LA", 25),46                new Person("Cara", "NYC", 35),47                new Person("Dan", "LA", 40));48 49        Map<String, List<String>> byCity = people.stream()50                .collect(Collectors.groupingBy(Person::city,51                         Collectors.mapping(Person::name, Collectors.toList())));52        System.out.println("grouped by city: " + byCity);53 54        Map<Boolean, List<String>> partition = people.stream()55                .collect(Collectors.partitioningBy(p -> p.age() >= 30,56                         Collectors.mapping(Person::name, Collectors.toList())));57        System.out.println("age>=30 partition: " + partition);58 59        Map<String, Double> avgAgeByCity = people.stream()60                .collect(Collectors.groupingBy(Person::city, Collectors.averagingInt(Person::age)));61        System.out.println("avg age by city: " + avgAgeByCity);62 63        String names = people.stream().map(Person::name).collect(Collectors.joining(", ", "[", "]"));64        System.out.println("joined names: " + names);65 66        // Generate / iterate (infinite streams with limit)67        List<Integer> firstFiveSquares = Stream.iterate(1, x -> x + 1).map(x -> x * x).limit(5).toList();68        System.out.println("first 5 squares: " + firstFiveSquares);69 70        // flatMap: flatten nested71        List<List<Integer>> nested = List.of(List.of(1, 2), List.of(3, 4), List.of(5));72        List<Integer> flat = nested.stream().flatMap(List::stream).toList();73        System.out.println("flattened: " + flat);74 75        // anyMatch / allMatch / findFirst76        System.out.println("any > 8? " + nums.stream().anyMatch(n -> n > 8));77        System.out.println("all > 0? " + nums.stream().allMatch(n -> n > 0));78        System.out.println("first even: " + nums.stream().filter(n -> n % 2 == 0).findFirst().orElse(-1));79    }80}