when expression

object Bob {
    fun hey(statement: String): String =
        when {
            statement.isQuestion() && statement.isYelling() -> "Calm down, I know what I'm doing!"
            statement.isQuestion() -> "Sure."
            statement.isYelling() -> "Whoa, chill out!"
            statement.isSilence() -> "Fine. Be that way!"
            else -> "Whatever."
        }

    private fun String.isSilence(): Boolean = this.isBlank()
    private fun String.isQuestion(): Boolean = this.trim().endsWith('?')
    private fun String.isYelling(): Boolean = any(Char::isLetter) && toUpperCase() == this
}

In this approach you have a when expression containing different so-called branches that can be matched using the corresponding patterns. As soon as one of these patterns on the left side of the arrow (->) is matched ...

1. the value on the right side
2. the block on the right side is executed and the value retuned from the block

... is returned from the when expression.

Only one branch is matched in one execution of the when expression. The branches are matched from top to bottom and the first branch in which the condition evaluates to true is selected. If none of the given conditions matches the else branch is selected and returned.

An object declaration is used to define Bob as essentially a singleton object instantiation of the class. This is sufficient, since there is no object state that needs to change with each call of the hey method.

Inside this object there are some private extension methods as members of the object. This adds these methods to the String data type for private usage of these methods in this object. This allows calling the methods directly on the String instead of passing the String to the method. (More about extension methods in general here)

These extension methods check for:

1. isSilence(): a blank string
2. isQuestion(): a string with the last non-whitespace-character being a question mark
3. isYelling(): a string with all letters in uppercase

When combining these methods as in the when expression above you can map all the cases required by the exercise.