hashCode & == Override

Part of the Object Oriented Programming section of Coddy's Dart journey — lesson 71 of 110.

By default, two objects in Dart are only equal if they're the exact same instance in memory. This means two objects with identical values are still considered different:

class Point {
  int x, y;
  Point(this.x, this.y);
}

void main() {
  var p1 = Point(3, 4);
  var p2 = Point(3, 4);
  print(p1 == p2);  // false - different instances!
}

To compare objects by their values, you need to override both the == operator and hashCode. These two must always be overridden together - if two objects are equal, they must have the same hash code:

class Point {
  final int x, y;
  Point(this.x, this.y);
  
  @override
  bool operator ==(Object other) {
    if (other is! Point) return false;
    return x == other.x && y == other.y;
  }
  
  @override
  int get hashCode => Object.hash(x, y);
}

void main() {
  var p1 = Point(3, 4);
  var p2 = Point(3, 4);
  print(p1 == p2);  // true - same values!
}

The hashCode is crucial for collections like Set and Map keys. Dart's Object.hash() helper combines multiple values into a single hash code. Without a proper hashCode, your objects won't work correctly in hash-based collections even if == is overridden.

Challenge

Easy

Let's build a coordinate system where points can be compared by their values rather than their memory addresses! You'll create a Coordinate class that properly implements value-based equality, making it work correctly with equality checks and hash-based collections like Set.

You'll organize your code into two files:

coordinate.dart: Create a Coordinate class representing a point in 2D space with final int x and final int y fields. Override both the == operator and hashCode so that two coordinates with the same x and y values are considered equal. Use Object.hash() to generate the hash code from both fields.
main.dart: Import your coordinate file and demonstrate how value-based equality changes object comparison behavior:
- Create two separate Coordinate objects, both with x: 5 and y: 10
- Print whether they are equal using ==
- Print whether they are identical (same instance) using identical()
- Create a Set<Coordinate> and add both coordinates to it
- Print the length of the set (it should be 1 since they're equal!)
- Create a third coordinate with x: 3 and y: 7, add it to the set
- Print the final length of the set

This challenge demonstrates why hashCode and == must always be overridden together. The Set uses hash codes to organize elements, so without a proper hashCode, even equal objects might be stored as duplicates!

Expected output:

true
false
1
2

Cheat sheet

By default, two objects in Dart are equal only if they're the same instance in memory, not if they have the same values:

var p1 = Point(3, 4);
var p2 = Point(3, 4);
print(p1 == p2);  // false - different instances!

To compare objects by their values, override both the == operator and hashCode. These must always be overridden together:

class Point {
  final int x, y;
  Point(this.x, this.y);
  
  @override
  bool operator ==(Object other) {
    if (other is! Point) return false;
    return x == other.x && y == other.y;
  }
  
  @override
  int get hashCode => Object.hash(x, y);
}

Use Object.hash() to combine multiple values into a single hash code. The hashCode is crucial for collections like Set and Map - without it, equal objects won't work correctly in hash-based collections.

To check if two objects are the same instance (not just equal values), use identical():

print(identical(p1, p2));  // false - different instances

Try it yourself

import 'coordinate.dart';

void main() {
  // TODO: Create two separate Coordinate objects, both with x: 5 and y: 10

  // TODO: Print whether they are equal using ==

  // TODO: Print whether they are identical (same instance) using identical()

  // TODO: Create a Set<Coordinate> and add both coordinates to it

  // TODO: Print the length of the set (should be 1 since they're equal!)

  // TODO: Create a third coordinate with x: 3 and y: 7, add it to the set

  // TODO: Print the final length of the set
}