Types and Null Safety

Lessons

1. 01 Null Safety A measurement of how nullable and non-nullable types are separate types, how a type narrows after a null check, and which criterion null equal to null produces. 15 min read
2. 02 Safe Calls and the Elvis Operator A measurement of where a safe call chain stops when it meets null, when the Elvis operator evaluates its right side, and which exception a not-null assertion throws. 15 min read
3. 03 Classes and Constructors A measurement of the execution order of the primary constructor, the init block, and the secondary constructor, and of why instances of a class without equals fall to the identity criterion. 14 min read
4. 04 Data Classes A measurement of the methods a data class generates from the properties in its primary constructor, of the separate object built with copy, and of how the type of a field affects the criterion. 15 min read
5. 05 Sealed Types and Enumerations A measurement of which criterion an enum entry, a data object, a data class, and an ordinary class fall to in the same sealed hierarchy, and of how an exhaustive when rejects a missing branch. 14 min read
6. 06 Object Declarations Measures the object declaration, the companion object, the `data object`, and the object expression with the criterion that makes two values equal, and shows that a declaration is a singleton while an expression builds a new object on every evaluation. 15 min read
7. 07 Interfaces and Delegation Measures the interface, the default implementation, and delegation with `by` using the criterion that makes two values equal, and shows that delegation forwards the members of the interface but does not forward `equals`. 14 min read
8. 08 Extension Functions Writes an extension function, an extension property, and an extension on a nullable receiver, and measures that an extension cannot shadow a member and is chosen by the static type. 16 min read
9. 09 Generics and Variance Measures a generic class, a generic function, an upper-bound constraint, `out` and `in` variance, and type erasure; shows that the type parameter is erased at run time and that equality looks at the value. 15 min read