Kotlin "in"

Consumer.kt

interface Consumer<in T> {

    fun consume(param: T)
}

SampleTest.kt

class SampleTest {

    @Test
    fun foo() {
        val numberConsumer = object : Consumer<Number> {
            override fun consume(param: Number) {
                println("number: $param")
            }
        }

        val intConsumer = object : Consumer<Int> {
            override fun consume(param: Int) {
                println("Int: $param")
            }
        }

        demo(numberConsumer) // not possible without "in" in Consumer
        demo(intConsumer)
    }

    private fun demo(consumer: Consumer<Int>) {
        consumer.consume(1)
    }
}

SampleTest2.kt

class Baz<in T> {

    private var otherData: T? = null

    fun set2(data: T) {
        otherData = data
    }
}

// Usage 
class SampleTest2 {

    @Test
    fun check() {
        val baz = Baz<Int>()
        val bazN = Baz<Number>()
        handleBaz(baz, 1)
        handleBaz(bazN, 2)
    }
}