Kotlin 101 - Extension Function

There is some code that we write everywhere. It doesn’t have state and usually small enough to become a class. We often call it a helper function and in Java we used to save it in some *Utils class.

For example i have a StringUtils class that have a function called isNullOrEmpty. It have no state and small enough.

public class StringUtils {
    public static boolean isNullOrEmpty(String text){
        return text == null || text.isEmpty();
    }
}

And to call it

StringUtils.isNullOrEmpty("text"); // return false

The downside of this pattern is we used to forget that we wrote it. Hence when the times come, we sometimes create the helper class again and again.

Extension Function

Just like C# and Swift extension function, (or Javascript prototype if you like) Kotlin has extension function too.

With extension function you have the ability to extend with new functionality without having to inherit from the class

For the example we’re gonna convert the StringUtils above to Kotlin’s extension function

// in StringExt.kt

fun String?.isNullOrEmpty = null || isEmpty()

And to call it

"text".isNullOrEmpty() // return false

And it will be converted to utility style in Java

StringExt.isNullOrEmpty(\"test\");

In Kotlin, we can define the extension function as a top level function, it means we don’t have to create it inside a class. Much simpler, eh?

The motivation is to tackle the problem above. Another pros is we have a completion after the object typed not beforehand.

I think that’s it for extension function, see you next time in another Kotlin 101 series. Cao ~ 👋

Published 2 Aug 2017

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