What does a sealed class do and explanation of the class below

Question

I have been doing android development for a while but I always looking forward to learning new things.

I came across the code below on codelab for viewmodel unit testing. I really like the code base is arranged but do not understand some codes like the one below.

I will like some guidance on creating a class that has a type of map as below.

Basically I will like to know how Result<*> still relate to Result and why the class is just called/implemented as Success(it).

I will appreciate a kind guidance.

sealed class Result<out R> {


    data class Success<out T>(val data: T) : Result<T>()
    data class Error(val exception: Exception) : Result<Nothing>()
    object Loading : Result<Nothing>()

    override fun toString(): String {
        return when (this) {
            is Success<*> -> "Success[data=$data]"
            is Error -> "Error[exception=$exception]"
            Loading -> "Loading"
        }
    }
}

/**
 * `true` if [Result] is of type [Success] & holds non-null [Success.data].
 */
val Result<*>.succeeded
    get() = this is Success && data != null

//implementation
 override fun observeTask(taskId: String): LiveData<Result<Task>> {
        return tasksDao.observeTaskById(taskId).map {
            Success(it)
        }
    }

Answer 1

Result<*>.succeeded is an extension property of the sealed class Result.

Check the guide on extensions in Kotlin:

https://kotlinlang.org/docs/reference/extensions.html