Collectors.toMap and Duplicate Keys

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

public class ListToMap {
    record Product(String sku, String name) { }

    public static void main(String[] args) {
        List<Product> products = List.of(
                new Product("A1", "Pen"), new Product("B2", "Book"), new Product("A1", "Pencil"));

        try {
            products.stream().collect(Collectors.toMap(Product::sku, Product::name));
        } catch (IllegalStateException e) {
            System.out.println(e.getMessage());       // Duplicate key A1 (attempted merging values Pen and Pencil)
        }

        // 1. Keep the first value. The fourth argument keeps the insertion order.
        Map<String, String> keepFirst = products.stream()
                .collect(Collectors.toMap(Product::sku, Product::name, (first, second) -> first, LinkedHashMap::new));
        System.out.println(keepFirst);                // {A1=Pen, B2=Book}

        // 2. Combine the values with a merge function.
        Map<String, String> joined = products.stream()
                .collect(Collectors.toMap(Product::sku, Product::name, (a, b) -> a + " / " + b, LinkedHashMap::new));
        System.out.println(joined);                   // {A1=Pen / Pencil, B2=Book}

        // 3. Group into lists, so nothing is lost.
        Map<String, List<String>> grouped = products.stream()
                .collect(Collectors.groupingBy(Product::sku, LinkedHashMap::new,
                        Collectors.mapping(Product::name, Collectors.toList())));
        System.out.println(grouped);                  // {A1=[Pen, Pencil], B2=[Book]}
    }
}