How to idiomatically test for non-null, non-empty strings in Kotlin?

Question

I am new to Kotlin, and I am looking for help in rewriting the following code to be more elegant.

var s: String? = "abc"
if (s != null && s.isNotEmpty()) {
    // Do something
}

If I use the following code:

if (s?.isNotEmpty()) {

The compiler will complain that

Required: Boolean
Found: Boolean?

Thanks.

Answer 1

You can use isNullOrEmpty or its friend isNullOrBlank like so:

if(!s.isNullOrEmpty()){
    // s is not empty
}

Both isNullOrEmpty and isNullOrBlank are extension methods on CharSequence? thus you can use them safely with null. Alternatively turn null into false like so:

if(s?.isNotEmpty() ?: false){
    // s is not empty
}

you can also do the following

if(s?.isNotEmpty() == true){ 
    // s is not empty
}

Answer 2

Although I like the answer by @miensol very much, my answer would be (and that is why I do not put it in a comment): if (s != null && s.isNotEmpty()) { … } actually is the idiomatic way in Kotlin. Only this way will you get a smart cast to String inside the block, while with the way in the accepted answer you will have to use s!! inside the block.

Answer 3

Or you can create an Extension Function:

public inline fun String?.ifNotEmpty(crossinline block: (String) -> Unit): Unit {
    if (this != null && this.isNotEmpty()) {
        block(this)
    }
}

See it in action

I find this approach more readable in context of Jetpack Compose which is a UI Toolkit.

handles.twitter.ifNotEmpty {
    SocialMediaItem(handle = it, type = SocialMedia.TWITTER)
}

In this case the intention is to only display that block of UI if Twitter handle is not null, and is not an empty string.

Answer 4

or create an extension method and use it as a safe call:

fun String?.emptyToNull(): String? {
    return if (this == null || this.isEmpty()) null else this
}

fun main(args: Array<String>) {
    val str1:String?=""
    val str2:String?=null
    val str3:String?="not empty & not null"

    println(str1.emptyToNull()?:"empty string")
    println(str2.emptyToNull()?:"null string")
    println(str3.emptyToNull()?:"will not print")
}