Default interface methods are included in a new feature proposal for C# 8, which will allow developers to use the traits programming technique on the .NET platform.

For example, InetAddress (in the java.net package), which represents an Internet Protocol address, declares a private Addresses interface that’s implemented by private CachedAddresses and ...

Java also lets you nest interfaces in interfaces and classes in interfaces. Check out this article on nested interfaces to learn about this strange but occasionally useful Java language feature.