How to solve: I don't get why my equality operator implementation dosen't work here and returns true

Question Asked by Be h on December 31, 2021 (source).

class SomeObject {
final List list;
final int integer;
SomeObject({required this.integer, required this.list});
@override
bool operator == (Object other) {
if (other is SomeObject && integer == other.integer) {
if (other.list.length != list.length) {
return false;
}
for (int i = 0; i < list.length; i++) {
if (list[i] == other.list[i]) {
return true;
}
}
return false;
}
return false;
}

@override
// TODO: implement hashCode
int get hashCode => Object.hashAll(list);
}

void main(){
var b = SomeObject(integer: 4, list: [5, 6, 7]);
var c = SomeObject(integer: 4, list: [5, 6, 8]);
print(b == c);
// Here equality operator returns true while it should return false.
// I think there is some problem in comparing individual list elements.
}

Consider using != and return false in this part:

for (int i = 0; i < list.length; i++) {
if (list[i] == other.list[i]) {
return true;
}
}

If we look carefully why it is failing:

var b = SomeObject(integer: 4, list: [5, 6, 7]);
var c = SomeObject(integer: 4, list: [5, 6, 8]);

First loop: i = 0

if (list == other.list) { // 5 == 5
return true; // it stops and immediately returns true
}

If we use != and return false, it will break the loop as soon as the elements at index i are not equal.

for (int i = 0; i < list.length; i++) {
if (list[i] != other.list[i]) {
return false;
}
}

Last loop: i = 2

if (list != other.list) { // 7 == 8
return false;
}

But if you want a shorter way to do this:

import 'package:flutter/foundation.dart';

class SomeObject {
final List list;
final int integer;

SomeObject({
required this.list,
required this.integer,
});

@override
bool operator ==(Object other) {
if (identical(this, other)) return true;

return other is SomeObject &&
listEquals(other.list, list) &&
other.integer == integer;
}

@override
int get hashCode => list.hashCode ^ integer.hashCode;
}

SHARE: