Both variables have same contents but show different hashcodes,same happens with Lists but it works-CodePudding

    void main(){
    var x=Object(33);
    var y=Object(33);
    print(x.hashCode);    
    print(y.hashCode);
              }
    class Object{
    int a;
    Object(this.a);
                }

Both have different hashcodes even if both are same instantiations

CodePudding user response：

You can copy the code below into dart pad to check it on your own!

   void main() {  
  
    var x = MyObject(33); // Instance X
    var y = MyObject(33); // Instance Y
    var z = x;            // Instance X
    print(x.hashCode);  // hashCode: 329225559  
    print(y.hashCode);  // hashCode: 758565612
    print(z.hashCode);  // hashCode: 329225559
    bool result1 =  identical(x, y);
    bool result2 =  identical(x, z);
    print(result1); // false - They are NOT the same instance!
    print(result2); // true - They are the same instance!
  
   var a =  const MyConstantObject(33);
   var b =  const MyConstantObject(33);
   print(a.hashCode); // 479612920
   print(b.hashCode); // 479612920
   bool result3 =  identical(a, b);
   print(result3); // true - They are the same instance!

   var c = const MyConstantObject(33); // Creates a constant
   var d = MyConstantObject(33); // Does NOT create a constant
   print(c.hashCode); // 479612920
   print(d.hashCode); // 837581838
   bool result4 =  identical(c, d);
   print(result4);// false - NOT the same instance!
}
  
class MyObject{int a; MyObject(this.a);}
class MyConstantObject{ final int a ; const MyConstantObject(this.a);}

If you check out the flutter api for the hashCode property you can find out the following:

The default hash code implemented by Object represents only the identity of the object, the same way as the default operator == implementation only considers objects equal if they are identical (see identityHashCode).

In conclusion, yes both variables (or in my example all three) have the same "content", however, only x and z are identical as these variables hold the exact same instance.

If you dive deeper into the resource linked above (esp. in how the == operator works), you will find out why your approach works with int, String and double types (instead of your CustomObjects).

Note - As you can learn here, if you are constructing two identical compile-time constants, it results in a single, so-called canonical instance. See example above with var a, b, c, d (adapted from the dart language tour).