I have been educating myself in Kotlin recently, and one of the most powerful features that Kotlin provides is extension functions. The simplest example of an extension function that I can come up with is: fun String.addOne() = this + "1" 

And then, println("Boba".addOne()) leads to an output of: Boba1

So above we have added a new function to the String class.

Generic Extension Functions

With the basic introduction out of the way, I wanted to write a generic extension function. We use Netflix OSS at work and are constantly getting property values via code that looks like this:

DynamicPropertyFactory.getInstance().getStringProperty("SOME_CONFIG_PROP_NAME", "I am a good default value").get()

Basically, the above code allows us to fetch (dynamically) values of a property, SOME_CONFIG_PROP_NAME in this example, and provide a default value, I am a good default value in this case.

We have several variations of this, for example:

DynamicPropertyFactory.getInstance().getIntProperty 
DynamicPropertyFactory.getInstance().getFloatProperty 
DynamicPropertyFactory.getInstance().getDoubleProperty 
DynamicPropertyFactory.getInstance().getLongProperty 
DynamicPropertyFactory.getInstance().getBooleanProperty

So on and so forth. I wanted to have a generic extension function available on the String class because property names are always of the String type. Here's the generic function:

fun  String.getConfig(default: T): T {  
    val dynamicPropertyFactory = DynamicPropertyFactory.getInstance()
    return 
      when (default) {
         is String  -> dynamicPropertyFactory.getStringProperty(this, default).get()
         is Int     -> dynamicPropertyFactory.getIntProperty(this, default).get()
         is Float   -> dynamicPropertyFactory.getFloatProperty(this, default).get()
         is Double  -> dynamicPropertyFactory.getDoubleProperty(this, default).get()
         is Boolean -> dynamicPropertyFactory.getBooleanProperty(this, default).get()
         is Long    -> dynamicPropertyFactory.getLongProperty(this, default).get()
         else       -> default
      }
}

Kotlin would know based on the input param what "type" it is. And we can use that power to use the when expression, along with is, to check what type it is and call the appropriate function based on the type of the default value.

So now our code can become as simple as:

"some.config.name".getConfig("I am a good default value")

"call.external.service".getConfig(false)

Boom. Done.