I am just starting to study Dart and a question arose, a clear answer to which I could not find or formulate for myself.
I’m trying to figure out the difference between == and identity(). Using the example of this code snippet:
void main() {
var a = [1, 2, 3];
var b = a;
var c = [1, 2, 3];
print(identical(a, b)); // true
print(identical(a, c)); // false
print(a == c); // false
}
I have a little experience in python and there this code gives me understandable results:
>>> a = [1, 2, 3]
>>> b = [1, 2, 3]
>>> a == b
True
>>> a is b
False
And in Dart it is not clear for me. If identity and == on basic data types give the same results, then why are they different 😅 ? Is there a difference between them if I’m not familiar with the classes in Dart?
I will be grateful for any clarifications that will help me figure out the issue.
From the Dart documentation on List ==:
Lists are, by default, only equal to themselves. Even if other is also a list, the equality comparison does not compare the elements of the two lists.
If you want to compare the two lists by comparing their elements, you’ll want to use something like listEquals which also maps to the equals method of a list:
print(a.equals(c));
The example below might help to illustrate the usage of the operator == and the function identical.
-
By default,
==andidenticalreturn the same result. See first example, where two objects of typeAare compared. ThehashCodecan be thought of as an object id.
Note: Classes may override the==operator and the getterhashCode. For this reason it makes sense to have the functionidentical
which is the equivalent of Python’sis. -
The second example is slightly different since both variables
s1ands2are assigned a literal, in this case aStringliteral. (To achieve a similar behaviour with classA, we could define aconstconstructor and use that constructor when assigning the values ofa1anda2.) -
The third example is similar to the second one. In this case,
l1andl2are both assigned the same constant list object. -
The forth example shows that (non-constant) collections (lists, sets, maps, etc) are only equal
==if they are also identical, even if their elements are the same.
class A{}
void main() {
print('nClass A');
final a1 = A();
final a2 = A();
print(a1 == a2); // false
print(identical(a1, a2)); // false
print(a1.hashCode);
print(a2.hashCode);
print('nString');
final s1 = 'hi';
final s2 = 'hi';
print(s1 == s2); // true
print(identical(s1, s2)); // true
print(s1.hashCode);
print(s2.hashCode);
print('nConst list');
final l1 = const [0, 1, 2];
final l2 = const [0, 1, 2];
print(l1 == l2); // true
print(identical(l1, l2)); // true
print(l1.hashCode);
print(l2.hashCode);
print('nList');
final l3 = [0, 1, 2];
final l4 = [0, 1, 2];
print(l3 == l4); // false
print(identical(l3, l4)); // false
print(l3.hashCode);
print(l4.hashCode);
}
The console output is shown below:
Class A
false
false
945090685
653872300
String
true
true
793667904
793667904
Const list
true
true
996765025
996765025
List
false
false
61814395
515290226