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.

Could you please edit your title to better reflect your question? Having many questions called How to write this code in a Kotlin way? doesn't really make it easy to look up later. — marstran
– marstran, Commented Dec 15, 2016 at 11:34
@marstran Yes sure. Do you have any suggestions about the title? I cannot find a good way to explain my problem. Thanks. — iForests
– iForests, Commented Dec 15, 2016 at 12:35
Something like How to idiomatically test for non-null, non-empty strings in Kotlin? maybe? Basically something that describes the actual question. — marstran
– marstran, Commented Dec 15, 2016 at 12:38
Since Kotlin 1.3, isNullOrEmpty and orEmpty are available on collections, maps, and array of objects. Not just on CharSequence. (kotlinlang.org/docs/reference/whatsnew13.html) — Aldo Wachyudi
– Aldo Wachyudi, Commented Dec 22, 2019 at 18:46

miensol · Accepted Answer · 2018-04-17 08:16:33Z

60

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
}

edited Apr 17, 2018 at 8:16

answered Dec 15, 2016 at 11:16

miensol

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8 Comments

mfulton26 Over a year ago

I would add that if (s != null && s.isNotEmpty()) { ... } is idiomatic Kotlin for this scenario if you want to proceed to actually use s as non-null using smart cast.

Kirill Rakhman Over a year ago

The question is not silly at all. Another possible form would be if (s?.isNotEmpty() == true) {}

miensol Over a year ago

@mfulton26 note that s was declared as var in which case if (s != null && s.isNotEmpty()) will not compile nor will it result in smart cast.

Michael Piefel Over a year ago

@miensol I notice that I get a smart cast very well even for a var. As long as it’s a local variable, it cannot have changed in between. Has Kotlin gotten smarter about this in the meantime?

rozina Over a year ago

@miguel Why do you have to keep track of that? If it was a member the code would not compile on a nullable type. No confusion imo.

|

Michael Piefel · Accepted Answer · 2017-03-29 07:27:05Z

6

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.

answered Mar 29, 2017 at 7:27

Michael Piefel

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3 Comments

Love Over a year ago

The suggestion by @Kirill Rakhman will also produce a smart cast: if (s?.isNotEmpty() == true) {}

Michael Piefel Over a year ago

That’s true. It’s a matter of personal preference here. When I see the suspicious == true part I immediately think that it could be removed, which it cannot in this case. You save 4 characters, but for me it’s less readable.

Michael Piefel Over a year ago

Note that my answer is outdated. !s.isNullOrEmpty() will perform a smart cast.

mtuulu · Accepted Answer · 2018-03-07 18:09:10Z

3

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")
}

answered Mar 7, 2018 at 18:09

mtuulu

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1 Comment

IgorGanapolsky Over a year ago

Isn't something similar offered in Kotlin lang?

Nevercom · Accepted Answer · 2021-08-18 11:47:11Z

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.

answered Aug 18, 2021 at 11:47

Nevercom

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3 Comments

Gober Over a year ago

This doesnt work with compose code unfortunately (@Composable invocations can only happen from the context of a @Composable function)

Gober Over a year ago

With some small changes it works. Probably been som updated to kotlin/compose: public inline fun String?.ifNotEmpty(block: (String) -> Unit) { if (!this.isNullOrEmpty()) { return block(this) } }

Nevercom Over a year ago

@Gober if you want this function to accept Composable functions inside, the signature should be changed to crossinline block: @Composable (String) -> Unit

Pipo · Accepted Answer · 2022-12-21 11:42:14Z

Another extension option. I was looking for a way of validating a bunch of strings and then using them, and the nested blocks are a pain for reading in that aspect.

fun String?.notNullOrEmpty(illegalArgumentExceptionMsg: String): String {
    return if (this == null || this.isEmpty()) throw IllegalArgumentException(illegalArgumentExceptionMsg) else this
}

simple use

    val myNullOrEmptyString: String? = "bar"
    val myNotNullString = myNullOrEmptyString.notNullOrEmpty("myNullOrEmptyString should not be empty")

use

    val myNullOrEmptyString: String? = "bar"
    val myNotNullString: String = myNullOrEmptyString.notNullOrEmpty("myNullOrEmptyString should not be empty")

Test or uses

@Test
fun emptyNotNullExtension() {
    val msg = "foo"
    assertThatThrownBy {
        val emptyNotNullString: String = "".notNullOrEmpty(msg)
    }
        .isExactlyInstanceOf(IllegalArgumentException::class.java)
        .hasMessageContaining(msg)
    assertThatThrownBy {
        val emptyNotNullString: String = null.notNullOrEmpty(msg)
    }
        .isExactlyInstanceOf(IllegalArgumentException::class.java)
        .hasMessageContaining(msg)

    val myNullOrEmptyString: String? = "bar"
    val myNotNullString: String = myNullOrEmptyString.notNullOrEmpty("myNullOrEmptyString should not be empty")

}

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