Categorize products

Main.java

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Main {
    public static void main(String[] args) {

        // Assumptions:
        // - list contains unique products
        // - multiple products can share the same category
        // - categories are ordered alphabetically
        // - products keep the order the appear in the list
        // - list contains only lowercase english letters
        // - list entry format is <product>:<category>

        String[] productsList = {
            "apple:fruit",
            "coca:drink",
            "orange:fruit",
            "pepsi:drink",
            "banana:fruit"
        };

        groupByCategory(productsList);
    }

    private static void groupByCategory(String[] productsList) {
        Map<String, List<String>> productsPerCategory = new HashMap<>();

        for(String prodEntry : productsList) {
            String[] parts = prodEntry.split(":");
            String prod = parts[0];
            String cat = parts[1];

            if(!productsPerCategory.containsKey(cat)) {
                productsPerCategory.put(cat, new ArrayList<>());
            }

            productsPerCategory.get(cat).add(prod);
        }

        List<String> categories = new ArrayList<>(productsPerCategory.keySet());
        Collections.sort(categories);

        for(String cat : categories) {
            System.out.println(cat + ":");

            for(String prod : productsPerCategory.get(cat)) {
                System.out.println("\t" + prod);
            }
        }
    }
}

simplify Map creation:

Map<String, List<String>> productsPerCategory = Arrays.asList(productsList).stream().map(product -> product.split(":")) .collect(Collectors.groupingBy(produit -> produit[1], Collectors.mapping(produit -> produit[0], Collectors.toList())));