Kotlin out (even with <out T> "? extends" is needed in Java)

Consumer.kt

interface Consumer<in T> {

    fun put(param: T)
}

class ConsumerImpl<in T> : Consumer<T> {

    override fun put(param: T) {
        println("put param: $param")
    }
}

SampleJavaTest.java

package ru.maksim.sample;

import org.junit.jupiter.api.Test;

public class SampleJavaTest {

    @Test
    void foo() {
        Source<Integer> intSource = new SourceImpl<>(11);
        Source<? extends Number> source = intSource;
        final Number o = source.nextT();
        System.out.println(o);

        // consumer
        final Consumer<? super Number> consumer = new ConsumerImpl<>();
        consumer.put(12);
    }
}

Source.kt

interface Source<out T> {

    fun nextT(): T
}