Kotlin中的Lambda表达式

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!")
    }
})