Invoke Operator & Operator Overloading in Kotlin

Question

I get to know about the Invoke operator that,

a() is equivalent to a.invoke()

Is there anything more regarding Invoke operator then please explain. Also, I did not get any example of Invoke operator overloading.

Is Invoke operator overloading is possible? If possible then can anyone please explain about the Invoke operator overloading with an example. I did not get anything regarding this.

Thanks in advance.

Answer 1

Yes, you can overload invoke. Here's an example:

class Greeter(val greeting: String) {     operator fun invoke(target: String) = println("$greeting $target!") }  val hello = Greeter("Hello") hello("world")  // Prints "Hello world!" 

In addition to what @holi-java said, overriding invoke is useful for any class where there is a clear action, optionally taking parameters. It's also great as an extension function to Java library classes with such a method.

For example, say you have the following Java class

public class ThingParser {     public Thing parse(File file) {         // Parse the file     } } 

You can then define an extension on ThingParser from Kotlin like so:

operator fun ThingParser.invoke(file: File) = parse(file) 

And use it like so

val parser = ThingParser() val file = File("path/to/file") val thing = parser(file)  // Calls ThingParser.invoke extension function