Equality and identity in Dart

Question

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.

Answer 1

The example below might help to illustrate the usage of the operator == and the function identical.

• By default, == and identical return the same result. See first example, where two objects of type A are compared. The hashCode can be thought of as an object id. Note: Classes may override the == operator and the getter hashCode. For this reason it makes sense to have the function identical which is the equivalent of Python's is.

• The second example is slightly different since both variables s1 and s2 are assigned a literal, in this case a String literal. (To achieve a similar behaviour with class A, we could define a const constructor and use that constructor when assigning the values of a1 and a2.)

• The third example is similar to the second one. In this case, l1 and l2 are 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