Real World Example

A Flipkart-like order system uses several collections together: a Map for the product catalogue, a List for cart lines, a Set for applied coupons (no duplicates), and a Queue of orders waiting to be packed.

Example

import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Queue;
import java.util.Set;
import java.util.TreeMap;

public class Main {
    public static void main(String[] args) {
        Map<String, Integer> catalogue = new LinkedHashMap<>();
        catalogue.put("P1", 999);
        catalogue.put("P2", 2499);
        catalogue.put("P3", 349);

        List<String> cart = new ArrayList<>(List.of("P1", "P3", "P3", "P2"));
        Set<String> coupons = new LinkedHashSet<>();
        coupons.add("NEW50");
        coupons.add("NEW50");                     // ignored: already applied

        Map<String, Integer> qty = new TreeMap<>();
        for (String id : cart) qty.merge(id, 1, Integer::sum);

        int total = 0;
        for (var e : qty.entrySet()) {
            int line = catalogue.get(e.getKey()) * e.getValue();
            total += line;
            System.out.println(e.getKey() + " x" + e.getValue() + " = " + line);
        }
        if (coupons.contains("NEW50")) total -= 50;
        System.out.println("Coupons " + coupons + " -> payable " + total);

        Queue<String> packing = new ArrayDeque<>(List.of("ORD-101", "ORD-102"));
        packing.offer("ORD-103");
        System.out.println("Packing next: " + packing.poll() + ", waiting: " + packing);
    }
}

Output

P1 x1 = 999
P2 x1 = 2499
P3 x2 = 698
Coupons [NEW50] -> payable 4146
Packing next: ORD-101, waiting: [ORD-102, ORD-103]

Order Processing in an E-commerce App

Example