Kotlin interfaces are essentially the same as Java interfaces. But Traits in Scala are more like (slightly) restricted multiple inheritance. They can have field

Kotlin interfaces are essentially the same as Java interfaces. But Traits in Scala are more like (slightly) restricted multiple inheritance. They can have fields, they can be stacked (http://www.artima.com/scalazine/articles/stackable_trait_pattern.html http://www.artima.com/scalazine/articles/stackable_trait_pat...), you can restrict the types where a trait can be mixed in. Value types are useful to enforce certain types which helps to avoid million dollar mistakes like the NASA made when mixing inch and meter as simple floating point numbers. All in all I think, that Kotlin is a good example of the 80:20 rule. You get the most useful concepts and convenience with a low investment. Maybe Kotlin hits a sweet spot in the JVM landscape.