to-string

// Test.java
public class Test {
  public String foo() {
    return null;
  }
}

// Main.kt
fun main() {
  print(Test().foo().toString())
}

// output
Exception in thread "main" java.lang.NullPointerException
	at MainKt.main(Main.kt:2)
	at MainKt.main(Main.kt)