Kotlin - Referential equality and structural equality

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Introduction

For objects, there are two types of equality.

TypeMeaning
referential equalitytwo separate references to the exact same instance in memory.
structural equality two objects are separate instances in memory but have the same value.

To test whether two references point to the same instance, we use the === operator or !== for negation: 

val a = File("/a.doc") 
val b = File("/a.doc") 
val sameRef = a === b //false

To test whether two objects have the same value, we use the == operator or != for negation.

These function calls are translated into the use of the equals function that all classes must define. 

val a = File("/a.doc") 
val b = File("/a.doc") 
val structural = a == b //true

It is up to the creator of a class to determine what structural equality means for that class.

The == operator is null safe. We don't need to worry if we are testing a null instance as the compiler will add the null check.