Generic Type variance (in Kotlin) The topic that killed my brain 

Invariance 

Invariance Why? List strs = new ArrayList(); 

Invariance Why? List strs = new ArrayList(); List objs = strs; // !!! The cause of the upcoming problem sits here. Java prohibits this! 

Invariance Why? List strs = new ArrayList(); List objs = strs; // !!! The cause of the upcoming problem sits here. Java prohibits this! objs.add(1); // Here we put an Integer into a list of Strings 

Invariance Why? List strs = new ArrayList(); List objs = strs; // !!! The cause of the upcoming problem sits here. Java prohibits this! objs.add(1); // Here we put an Integer into a list of Strings String s = strs.get(0); // !!! ClassCastException: Cannot cast Integer to String 

Invariance Why? List strs = new ArrayList(); List objs = strs; // !!! The cause of the upcoming problem sits here. Java prohibits this! objs.add(1); // Here we put an Integer into a list of Strings String s = strs.get(0); // !!! ClassCastException: Cannot cast Integer to String Note: All generic types in Java are invariant! 

Invariance A <: B so, for List: List 

Covariance interface Producer { fun produce(): T } 

Covariance interface Producer { fun produce(): T } val p = object : Producer { override fun produce(): String = "Hello" } 

Covariance interface Producer { fun produce(): T } val p = object : Producer { override fun produce(): String = "Hello" } val gp: Producer = p 

Covariance val gp: Producer = p 

Covariance val gp: Producer = p p.produce() // -> String gp.produce() // -> Any 

Covariance val gp: Producer = p p.produce() // -> String gp.produce() // -> Any String <: Any Producer <: Producer 

Covariance val gp: Producer = p p.produce() // -> String gp.produce() // -> Any String <: Any Producer <: Producer -> You can always read Any from a Producer 

Covariance A <: B so, for Producer // Producer: Producer <: Producer 

Contravariance interface Consumer { fun consume(t: T) } 

Contravariance interface Consumer { fun consume(t: T) } val c = object : Consumer { override fun consume(t: Any): Any = print(t) } 

Contravariance interface Consumer { fun consume(t: T) } val c = object : Consumer { override fun consume(t: Any): Any = print(t) } val gc: Consumer = c 

Contravariance val gc: Consumer = c 

Contravariance val gc: Consumer = c c.consume("hello") // <- Any gc.consume("hello") // <- String 

Contravariance val gc: Consumer = c c.consume("hello") // <- Any gc.consume("hello") // <- String String <: Any Consumer <: Consumer 

Contravariance val gc: Consumer = c c.consume("hello") // <- Any gc.consume("hello") // <- String String <: Any Consumer <: Consumer -> You can always write Strings into a Consumer 

Covariance A <: B so, for Consumer // Consumer: Consumer <: Consumer 

Links https://kotlinlang.org/docs/reference/generics.html#varianc e https://medium.com/@tpolansk/understanding-generics-and- variance-in-kotlin-714c14564c47 

Thank you Q&A