Lambda expressions in Kotlin

Question

Consider the following Java class:

public class SomeClass {     public interface Something {         void doSomething();     }      public void call(Something something) {} } 

In Kotlin, I can use a lambda expression as follows:

SomeClass().call {      // do something } 

But if I define the following method in Kotlin (using the same interface):

fun call(something: Something) {} 

Then this call:

call {     // do something } 

Would generate a type mismatch error. Why?

Answer 1

Kotlin only supports SAM conversions for Java methods, because Kotlin itself has function types. Normally, Kotlin functions should be taking () -> Unit rather than Something. If you really need it to take Something, you can use a SAM constructor:

call(Something { /* do something */ }) 

Any SAM type (Java interface with one abstract method) automatically gets such a constructor function that converts a lambda to its instance.

Answer 2

Kotlin requires that call takes something of interface something.

() -> Kotlin.Unit 

Does not satisfy the type requirement.

You can do this

call (object : Something {     override fun doSomething() {         println("Passing an interface to call in kotlin!")     } })