Interfaces

An interface is nothing more than a contract; it contains definitions for a set of related functionalities. The implementer of the interface has to adhere to the interface of the contract and implement the required methods. Just like Java 8, a Kotlin interface contains the declarations of abstract methods as well as method implementations. Unlike abstract classes, an interface cannot contain state; however, it can contain properties. Any Scala developer reading this book will find this similar to traits in Scala:

    interface Document { 
      val version: Long   
      val size: Long 
 
      val name: String     
      get() = "NoName" 
 
      fun save(input: InputStream) 
      fun load(stream: OutputStream) 
      fun getDescription(): String { 
         return "Document $name has $size byte(-s)"} 
    } 

This interface defines three properties...