Challenge 3: Object Expression Implementing an Interface — Solution

interface Validator {
    fun isValid(input: String): Boolean
}

fun checkInput(text: String, validator: Validator) {
    println("\"$text\" is valid: ${validator.isValid(text)}")
}

fun main() {
    val lengthValidator = object : Validator {
        override fun isValid(input: String): Boolean {
            return input.isNotBlank() && input.length >= 3
        }
    }

    checkInput("hi", lengthValidator)
    checkInput("hello", lengthValidator)
    checkInput("   ", lengthValidator)
}

/*
Output:
"hi" is valid: false
"hello" is valid: true
"   " is valid: false

Notes:
- "object : Validator { ... }" creates a one-off, unnamed class that
  implements Validator on the spot, with no separate named class needed
  anywhere in the file.
- checkInput() only knows about the Validator interface — it has no idea
  the actual implementation is an anonymous object expression, which is
  exactly the point: it's usable anywhere a Validator is expected.
- "hi" fails (length 2 < 3), "hello" passes, and "   " fails isNotBlank()
  even though its length is 3.
*/