Collections Framework overview

Iterable
 └─ Collection
     ├─ List      → ArrayList, LinkedList            ordered, index, duplicates OK
     ├─ Set       → HashSet, LinkedHashSet           no duplicates
     │   └─ SortedSet → TreeSet                      sorted, no duplicates
     └─ Queue     → PriorityQueue                    process in order of priority
         └─ Deque → ArrayDeque, LinkedList           add/remove at both ends (queue or stack)

Map (NOT a Collection)  → HashMap, LinkedHashMap      key → value, keys unique
 └─ SortedMap → TreeMap                              keys sorted

import java.util.*;

public class Main {
    public static void main(String[] args) {
        String[] visits = {"Riya", "Kabir", "Riya", "Om", "Riya"};

        List<String> log = new ArrayList<>(Arrays.asList(visits));    // keeps everything
        Set<String> unique = new HashSet<>(log);                     // drops repeats
        Set<String> sorted = new TreeSet<>(log);                     // drops repeats, sorts
        Queue<String> line = new ArrayDeque<>(log);                  // first in, first out
        Map<String, Integer> count = new TreeMap<>();
        for (String v : log) count.put(v, count.getOrDefault(v, 0) + 1);

        System.out.println("List : " + log);
        System.out.println("Set  : " + unique.size() + " unique");
        System.out.println("Tree : " + sorted);
        System.out.println("Queue: serve " + line.poll() + ", next " + line.peek());
        System.out.println("Map  : " + count);
    }
}

import java.util.*;

public class Main {
    public static void main(String[] args) {
        List<Integer> fixed = Arrays.asList(1, 2, 3);   // fixed size: set OK, add not
        fixed.set(0, 9);
        System.out.println(fixed);

        List<Integer> frozen = List.of(1, 2, 3);        // Java 9+: fully immutable
        try {
            frozen.add(4);
        } catch (UnsupportedOperationException e) {
            System.out.println("List.of can't change");
        }
        System.out.println(Collections.max(frozen) + " " + Collections.frequency(List.of("a", "b", "a"), "a"));
    }
}

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Map<String, Integer> price = new HashMap<>();
        price.put("chai", 15);
        price.put("dosa", 60);
        price.put("vada", 25);

        List<String> orders = new ArrayList<>();
        Set<String> customers = new HashSet<>();
        Queue<String> tokens = new ArrayDeque<>();

        String[][] today = {{"Riya", "chai"}, {"Kabir", "dosa"}, {"Riya", "vada"}};
        for (String[] t : today) {
            orders.add(t[1]);
            customers.add(t[0]);
            tokens.offer(t[0] + ":" + t[1]);
        }

        int total = 0;
        for (String dish : orders) total += price.get(dish);

        System.out.println("Orders: " + orders);
        System.out.println("Customers: " + customers.size());
        System.out.println("Serve first: " + tokens.poll());
        System.out.println("Takings: ₹" + total);
    }
}