javalambdakotlinanonymous

How to create an instance of anonymous interface in Kotlin?


I have a third party Java library which an object with interface like this:

public interface Handler<C> {
  void call(C context) throws Exception;
}

How can I concisely implement it in Kotlin similar to Java anonymous class like this:

Handler<MyContext> handler = new Handler<MyContext> {
   @Override
   public void call(MyContext context) throws Exception {
      System.out.println("Hello world");
   }
}

handler.call(myContext) // Prints "Hello world"

Solution

  • Assuming the interface has only a single method you can make use of SAM.

    val handler = Handler<String> { println("Hello: $it") }
    

    Since version 1.4 Kotlin supports SAM for interfaces defined in Kotlin. That requires prefixing the interface keyword with fun

    fun interface Handler<C> {
      fun call(context: C);
    }
    

    If you have a method that accepts a handler then you can even omit type arguments:

    fun acceptHandler(handler:Handler<String>){}
    
    acceptHandler(Handler { println("Hello: $it") })
    
    acceptHandler({ println("Hello: $it") })
    
    acceptHandler { println("Hello: $it") }
    

    If the interface has more than one method the syntax is a bit more verbose:

    val handler = object: Handler2<String> {
        override fun call(context: String?) { println("Call: $context") }
        override fun run(context: String?) { println("Run: $context")  }
    }