Step 1 - Old object unchanged after copyWith

class CounterState {
  final int value;

  CounterState(this.value);

  CounterState copyWith({int? value}) {
    return CounterState(value ?? this.value);
  }
}

void main() {
  CounterState before = CounterState(1);
  CounterState after = before.copyWith(value: 2);
  assert(before.value == 1);
  assert(after.value == 2);
  assert(!identical(before, after));
  print('ok');
}

Step 2 - Value equality for User

class User {
  final String id;
  final String name;

  User({required this.id, required this.name});

  User copyWith({String? id, String? name}) {
    return User(id: id ?? this.id, name: name ?? this.name);
  }

  @override
  bool operator ==(Object other) {
    return other is User && other.id == id && other.name == name;
  }

  @override
  int get hashCode => Object.hash(id, name);
}

void main() {
  User a = User(id: '1', name: 'Asha');
  User b = User(id: '1', name: 'Asha');
  User c = a.copyWith(name: 'Rafi');
  assert(a == b);
  assert(a != c);
  final set = {a, b};
  assert(set.length == 1);
  print('ok');
}

Step 3 - Run with checks enabled

• Use dart run (asserts enabled in VM mode by default for dart run on script).
• If an assert fails, fix ==/hashCode or copyWith until invariants hold.

Practice tasks

• Add ==/hashCode to Product (sku enough for equality, or include name and price if you want full value semantics).
• Write asserts showing two different copyWith chains from the same OrderItem produce distinct objects but predictable lineTotal.
• Build a Map<User, String> with two User keys that are value-equal; second insert should overwrite, map length 1.