Inheritance Equality Footgun

Here is something that I only learned about recently. If you are writing an equals method in f.e. Java manually it's super easy to make a certain mistake.

Take this pseudo-code for example:

class Line {
  int width

  Line(int width) { this.width = width }

  equals(Object other) {
    if(!(other instanceof Line line)) return false
    return width == line.width
  }
}

class Rect extends Line {
  int height

  Rect(int width, int height) { this.width = width; this.height = height }

  equals(Object other) {
    if(!(other instanceof Rect rect)) return false
    return width == rect.width && height == rect.height
  }
}

The issue isn't obvious at first, but take this snippet will make it obvious:

var line = new Line(5)
var rect = new Rect(5, 7)

line.equals(rect) // true
rect.equals(line) // false

To make it right the dynamic type needs to be checked:

equals(Object other) {
    if (other == null || getClass() != other.getClass()) return false
    Line line = (Line) other
    return width == line.width
}

equals(Object other) {
    if (other == null || getClass() != other.getClass()) return false
    Rect rect = (Rect) other
    return width == rect.width && height == rect.height
}

I wonder how many times I've made such an error without knowing 🤔