Thread by @vandadnp

🔥What are #equality (==) and #hashCode in #Dart / #Flutter objects and why would you need to override these? 💙 A thread to explain it all 🧵
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Object equality in Dart is defined through the == operator. This is an operator that has a return value of bool and has only one parameter which is another object of the same type this operator is implemented on
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The == operator's implementation in Dart has to have 4 important qualities.
Quality #1 is that this operator should be "total" meaning that it should only return a boolean and it should never throw
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Quality #2 is that it should be "reflexive" meaning that for the same object instance, it should return true. So if you instantiate an object and compare it to itself, it should always return true.
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Quality #3 is that it should be "symmetric" meaning that if object 1 is equal to object 2, then object 2 should also be equal to object 1 in the reverse manner
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Quality #4 of the == operator and hence the last quality is that it should be "transitive" meaning that if object 1 is equal to object 2 and object 2 is equal to object 3 then object 1 should also be equal to object 3
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The default implementation of the == operator only implements quality #1 and #2 meaning that it never throws and it only checks for the objects being compared to be the same instance, or to be "identical" in other words
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If you have a custom class, you want to always ensure that you are implementing the == operator on it. Let me show you an example
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In this example, the LoginHandle class has 1 property called "token" of type "String". We've implemented the == operator to ensure that this.token == other.token. Note "covariant". This means our operator override always expects another instance of LoginHandle
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When overriding this operator, ensure that the 4 qualities named above are met and that your parameter is a covariant of the same type as "this"
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Equality implementation is only half of the equation for equality in Dart. When implementing equality with the == operator, you also need to ensure that you've implemented the hashCode property.
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hashCode property is of type int. hashCode is an integer that represents the state of an object inside collections such as Map. Map in Dart is implemented so that it uses an object's hashCode to store that object inside itself
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That means if two objects have the same hashCode, they are going to consume the same space in the map and over-write each other. Here is an example of two instances being equal and used as keys to a Map
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So whereas the == operator is used to test for equality, the hashCode property is used to represent the state of an object. If two objects are equal from == operator, they should have the same hashCode
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