Java Map — Advanced Operations

import java.util.*;
import java.util.stream.Collectors;

public class MapOps {
    public static void main(String[] args) {
        // computeIfAbsent — lazy init
        Map<String, List<String>> multiMap = new HashMap<>();
        multiMap.computeIfAbsent("fruits", k -> new ArrayList<>()).add("apple");
        multiMap.computeIfAbsent("fruits", k -> new ArrayList<>()).add("banana");
        multiMap.computeIfAbsent("vegs", k -> new ArrayList<>()).add("carrot");
        System.out.println(multiMap); // {fruits=[apple, banana], vegs=[carrot]}

        // merge — combine values
        Map<String, Integer> scores = new HashMap<>();
        scores.merge("alice", 10, Integer::sum);
        scores.merge("alice", 20, Integer::sum);
        scores.merge("bob", 15, Integer::sum);
        System.out.println(scores); // {alice=30, bob=15}

        // compute — transform in-place
        scores.compute("alice", (k, v) -> v == null ? 1 : v * 2);

        // getOrDefault
        int bobScore = scores.getOrDefault("charlie", 0);

        // putIfAbsent — only set if missing
        scores.putIfAbsent("alice", 999); // no-op, alice exists

        // Iterate entries
        scores.forEach((name, score) ->
            System.out.printf("%s: %d%n", name, score));

        // Stream groupingBy
        record Product(String name, String category, double price) {}
        var products = List.of(
            new Product("iPhone", "Electronics", 999),
            new Product("MacBook", "Electronics", 1999),
            new Product("Shirt", "Clothing", 49),
            new Product("Jeans", "Clothing", 79)
        );

        Map<String, DoubleSummaryStatistics> stats = products.stream()
            .collect(Collectors.groupingBy(
                Product::category,
                Collectors.summarizingDouble(Product::price)
            ));
        stats.forEach((cat, s) ->
            System.out.printf("%s: avg=$%.0f, count=%d%n", cat, s.getAverage(), s.getCount()));

        // Invert map (swap keys and values)
        Map<String, Integer> original = Map.of("a", 1, "b", 2, "c", 3);
        Map<Integer, String> inverted = original.entrySet().stream()
            .collect(Collectors.toMap(Map.Entry::getValue, Map.Entry::getKey));

        // Unmodifiable copy
        Map<String, Integer> frozen = Map.copyOf(scores);
    }
}