Android Development with Kotlin

Nullability

When we define a class with an unbounded type parameter, we can use both non-nullable and nullable types as type arguments. Occasionally, we need to make sure that a particular generic type will not be parametrized with nullable type arguments. To block the ability to use nullable types as type arguments, we need to explicitly define a non-nullable type parameter upper bound:

    class Action (val name:String)    class ActionGroup<T : Action>     // non-nullable type parameter upper bound    var actionGroupA: ActionGroup<Action>    var actionGroupB: ActionGroup<Action?> // Error

Now we can't pass a nullable type argument (Action?) to the ActionGroup class.

Let's consider another example. Imagine that we want to retrieve the last Action in