Why Java introduced private interface methods? [on hold]

I am realy curious to hear your thoughts on that strange decision in Java 9 design. For me, it is step backwards as it violates idea of an interface with is supposed to be a open contract. If we add possibility of providing default method implementation since Java 8 I feel that interfaces are becoming more and more like inheritance, which is completely different animal. Even worse, because you can implement multiple interfaces, so it opens up a possibility for creating realy bad code.