core29HashMapDemo

1package pkg1core;2 3/*4 * core29HashMapDemo.java5 * ---------------------6 * java.util.HashMap as a teaching focus: put/get/update, and why map keys7 * need stable equals() and hashCode().8 *9 * EXPLANATION:10 *  - Keys are unique; put with an existing key replaces the value.11 *  - Lookup uses hashCode (bucket) then equals (match within the bucket).12 *  - Mutating a key after put can make the entry unfindable.13 *14 * See also: core19CollectionsDemo (Map among other collections),15 *           pkg3datastructures/datastructures6HashTableImpl (under the hood).16 */17import java.util.HashMap;18import java.util.Map;19import java.util.Objects;20 21public class core29HashMapDemo {22 23    /** Mutable on purpose — shows why map keys must stay stable. */24    static class MutableId {25        int id;26        MutableId(int id) { this.id = id; }27 28        @Override29        public boolean equals(Object o) {30            return o instanceof MutableId other && id == other.id;31        }32 33        @Override34        public int hashCode() {35            return Objects.hash(id);36        }37 38        @Override39        public String toString() {40            return "id=" + id;41        }42    }43 44    public static void main(String[] args) {45        Map<String, Integer> ages = new HashMap<>();46        ages.put("Ana", 20);47        ages.put("Bob", 19);48        ages.put("Ana", 21);                                    // update existing key49        System.out.println("ages=" + ages + " get(Ana)=" + ages.get("Ana"));50        System.out.println("getOrDefault(Zed)=" + ages.getOrDefault("Zed", 0));51        ages.merge("Bob", 1, Integer::sum);52        System.out.println("after merge Bob=" + ages.get("Bob"));53 54        // One null key is allowed (special case — prefer clear keys in new code)55        Map<String, String> labels = new HashMap<>();56        labels.put(null, "missing-key");57        System.out.println("null key value=" + labels.get(null));58 59        // Key contract: equal keys share a slot; mutating hash fields breaks lookup60        Map<MutableId, String> byId = new HashMap<>();61        MutableId key = new MutableId(7);62        byId.put(key, "task-7");63        System.out.println("before mutate: get=" + byId.get(key) + " containsKey=" + byId.containsKey(key));64 65        key.id = 99;                                            // changes equals/hashCode66        System.out.println("after mutate:  get=" + byId.get(key) + " containsKey=" + byId.containsKey(key));67        System.out.println("map still holds one entry: " + byId); // entry is effectively lost to lookup68 69        MutableId sameAsOriginal = new MutableId(7);70        System.out.println("lookup with id=7 again: " + byId.get(sameAsOriginal)); // usually null now71    }72}