core19CollectionsDemo

1package pkg1core;2 3/*4 * core19CollectionsDemo.java5 * --------------------6 * The Collections Framework: List, Set, Map, Queue, Deque, and when to use each.7 *8 * EXPLANATION:9 *  - List: ordered, indexed, allows duplicates (ArrayList / LinkedList).10 *  - Set: no duplicates (HashSet=unordered, LinkedHashSet=insertion, TreeSet=sorted).11 *  - Map: key->value (HashMap, LinkedHashMap, TreeMap).12 *  - Queue/Deque: FIFO/LIFO (ArrayDeque), PriorityQueue=min-heap.13 */14import java.util.*;15 16public class core19CollectionsDemo {17    public static void main(String[] args) {18        // ---- List ----19        List<String> list = new ArrayList<>(List.of("b", "a", "c", "a"));20        list.add("d");21        Collections.sort(list);22        System.out.println("List (sorted, dups kept): " + list + " get(0)=" + list.get(0));23 24        // ---- Set (uniqueness) ----25        Set<String> hash = new HashSet<>(list);26        Set<String> tree = new TreeSet<>(list);                 // sorted, unique27        System.out.println("HashSet (unordered, unique): " + hash);28        System.out.println("TreeSet (sorted, unique): " + tree);29 30        // ---- Map ----31        Map<String, Integer> counts = new HashMap<>();32        for (String s : list) counts.merge(s, 1, Integer::sum); // frequency count idiom33        System.out.println("Frequency map: " + counts);34        System.out.println("getOrDefault('z',0): " + counts.getOrDefault("z", 0));35        counts.computeIfAbsent("e", k -> 0);36        System.out.println("after computeIfAbsent: " + counts);37 38        // TreeMap keeps keys sorted39        TreeMap<String, Integer> sortedMap = new TreeMap<>(counts);40        System.out.println("TreeMap firstKey=" + sortedMap.firstKey() + " lastKey=" + sortedMap.lastKey());41 42        // ---- Queue (FIFO) ----43        Queue<Integer> q = new ArrayDeque<>();44        q.offer(1); q.offer(2); q.offer(3);45        System.out.println("Queue poll order: " + q.poll() + ", " + q.poll());46 47        // ---- Deque as Stack (LIFO) ----48        Deque<Integer> stack = new ArrayDeque<>();49        stack.push(1); stack.push(2); stack.push(3);50        System.out.println("Stack pop order: " + stack.pop() + ", " + stack.pop());51 52        // ---- PriorityQueue (min-heap) ----53        PriorityQueue<Integer> pq = new PriorityQueue<>(List.of(5, 1, 3, 2, 4));54        StringBuilder order = new StringBuilder();55        while (!pq.isEmpty()) order.append(pq.poll()).append(' ');56        System.out.println("PriorityQueue ascending: " + order.toString().trim());57 58        // Iteration safety: remove while iterating via Iterator59        List<Integer> nums = new ArrayList<>(List.of(1, 2, 3, 4, 5, 6));60        Iterator<Integer> it = nums.iterator();61        while (it.hasNext()) if (it.next() % 2 == 0) it.remove();62        System.out.println("After removing evens: " + nums);63    }64}