Scala Design Patterns - Second Edition

Inheritance leaking functionality

If we use inheritance, we would also leak functionality to subclasses that we do not want. Let's look at the following code:

trait DB {  def connect(): Unit = {    System.out.println("Connected.")  }  def dropDatabase(): Unit = {    System.out.println("Dropping!")  }  def close(): Unit = {    System.out.println("Closed.")  }}trait UserDB extends DB {  def createUser(username: String): Unit = {    connect()    try {      System.out.println(s"Creating a user: $username")    } finally {      close()    }  }  def getUser(username: String): Unit = {    connect()    try {      System.out.println(s"Getting a user: $username")    } finally {      close()    }  }}trait UserService extends UserDB {  def bad(): Unit = {    dropDatabase()  }}

This could be a real-life scenario. Because ...