Chapter 19: Generic Variance in Kotlin

Key Concepts

• Generic Variance: Explaining why an array of integers is not a subtype of an array of numbers.
• Variance in Type Hierarchies: Variance doesn't only apply to generics but also to regular type hierarchies.

Vending Machine Interface Example

• Type Associations: Example with vendingMachine interface with two types:
  • Parameter type: coin
  • Return type: snack
• Variance describes the relationship between the container type and its associated types.
  • Vending machine interacting with coin and snack.

Covariance

• Definition: As the container type becomes more specific, so can the return type.
  • candyBar is a subtype of snack.
  • Demonstrated through SimpleVendingMachine always returning a candyBar which is a subtype of snack.
• Rule: Return types can become more specific in subtypes (covariance).
• Error Prevention: Kotlin prevents replacing with a more general type (e.g., product instead of snack).

Contravariance

• Definition: As the container type becomes more specific, the parameter type can become more general.
  • money type includes more than just coin.
  • Function overloading in Kotlin affects covariance for parameters.
• Function Overloading Workaround: Change function to a property with a function type to utilize more general parameters.
• Summary: Return types (covariance) and parameter types (contravariance).

Application to Generics

• Kotlin Syntax:
  • out: indicates covariant relationship (appears as return type).
  • in: indicates contravariant relationship (appears as parameter type).
• Generic Interface: vendingMachine converted into a generic interface.
  • Rules for type arguments in generics paralleling type relationships in regular hierarchies.

Final Thoughts

• Understanding variance in type hierarchies helps clarify generic variance.
• Encouragement to experiment and code with these concepts.
• Availability of more detailed content in upcoming Chapter 19 of