Step 1 - Default == is identity

class Box {
  final int v;
  Box(this.v);
}

void main() {
  Box a = Box(1);
  Box b = Box(1);
  print(a == b);
  print(identical(a, b));
}

• Without overrides, == is reference equality: same object only.

Step 2 - Override == and hashCode together

class Box {
  final int v;
  Box(this.v);

  @override
  bool operator ==(Object other) {
    return other is Box && other.v == v;
  }

  @override
  int get hashCode => Object.hash(v);
}

void main() {
  Box a = Box(1);
  Box b = Box(1);
  print(a == b);
  final set = {a, b};
  print(set.length);
}

• If two values are equal, they must have the same hashCode. Break that rule and Set/Map behave badly.

Step 3 - Why collections care

• Set and Map use hashCode for buckets and == for collisions.
• Value objects used as keys should be immutable (or at least not change fields that affect hashCode after insertion).

Good habit

• Use Object.hash(field1, field2, ...) for combining fields (from dart:core).

Practice tasks

• Add ==/hashCode to User with name and age (both part of equality).
• Build a Set of three User instances where two are value-equal; length should be 2.
• Explain in one sentence why mutating a field that affects hashCode after inserting into a Set is dangerous.