How to validate class String property is not empty in Kotlin?

Given:

class JWTToken(val token: String) {
// ...
var email: String = jwt.claims["email"]?.asString()?: 
        throw Exception("null email")

}

The exception is thrown if wt.claims["email"] is null, but not if the is string empty.

How can the check for the empty string be concisely added?

>Solution :

Kotlin’s require built-in would be a good candidate here:

val aNullableOrBlankValue: String? = "..."

class JWTToken(val token: String) {
    var email: String = aNullableOrBlankValue
        .let { requireNotNull(it) { "Email should not be null..." } }
        .apply { require(isNotBlank()) { "Email should not be blank..." } }
}

The methods require and requireNotNull will throw an IllegalArgumentException if the value is false for require, or null for requireNotNull.

Another concise way to write this would be:

var email2: String = aNullableOrBlankValue
        .apply { require(!isNullOrBlank()) { "Email should not be blank or null..." } }!!

Albeit less readable (notice the !!), and error not as clear.

Do note that isBlank is not the same as isEmpty and it really depends on your use-case. (In your case, isBlank is more appropriate as emails should not be blank)

isBlank checks that a char sequence has a 0 length or that all indices are white space. isEmpty only checks that the char sequence length is 0.

See: stackoverflow.com/a/45337014/5037430 for further detalis.