What is Dependency Inversion Principle?

Dependency Inversion Principle (DIP) means that the most flexible systems are those in which dependencies refer only to abstractions, not to concretions. That is because an abstract interface is less volatile than its implementations meaning that it is less likely to change and thus less likely to cause problems to those that refer to it.

image from Clean Architecture

Sample code

Clean architecture is a popular architecture used to implement modern software; app architecture for Android. In Clean Architecture the flow of dependencies should be :

• Presentation → Data → Domain

Presentation

class Presentation(
   private val repository : Repository
){
   val name = repository.getUserName()
   /* show name to UI */
}

Data

class RepositoryImpl : Repository(
   val dataSource : DataSource
){
  override fun getUserName(){
    /* concrete implementation */
  }
}

interface DataSource(
){
   /* remote or cache data */
}

Domain

interface Repository(){
   fun getUserName()
}