Packages and Imports

Scala uses packages to create namespaces which allow you to modularize programs.

Creating a package

Packages are created by declaring one or more package names at the top of a Scala file.

package users

class User

One convention is to name the package the same as the directory containing the Scala file. However, Scala is agnostic to file layout. The directory structure of an sbt project for package users might look like this:

- ExampleProject
  - build.sbt
  - project
  - src
    - main
      - scala
        - users
          User.scala
          UserProfile.scala
          UserPreferences.scala
    - test

Notice how the users directory is within the scala directory and how there are multiple Scala files within the package. Each Scala file in the package could have the same package declaration. The other way to declare packages is by nesting them inside each other:

package users {
  package administrators {
    class NormalUser
  }
  package normalusers {
    class NormalUser
  }
}

package users:
  package administrators:
    class NormalUser

  package normalusers:
    class NormalUser

As you can see, this allows for package nesting and provides greater control for scope and encapsulation.

The package name should be all lower case and if the code is being developed within an organization which has a website, it should be the following format convention: <top-level-domain>.<domain-name>.<project-name>. For example, if Google had a project called SelfDrivingCar, the package name would look like this:

package com.google.selfdrivingcar.camera

class Lens

This could correspond to the following directory structure: SelfDrivingCar/src/main/scala/com/google/selfdrivingcar/camera/Lens.scala.

Imports

import clauses are for accessing members (classes, traits, functions, etc.) in other packages. An import clause is not required for accessing members of the same package. Import clauses are selective:

import users._  // import everything from the users package
import users.User  // import the class User
import users.{User, UserPreferences}  // Only imports selected members
import users.{UserPreferences => UPrefs}  // import and rename for convenience

import users.*  // import everything from the users package except given
import users.given // import all given from the users package
import users.User  // import the class User
import users.{User, UserPreferences}  // Only imports selected members
import users.UserPreferences as UPrefs  // import and rename for convenience

One way in which Scala is different from Java is that imports can be used anywhere:

def sqrtplus1(x: Int) = {
  import scala.math.sqrt
  sqrt(x) + 1.0
}

def sqrtplus1(x: Int) =
  import scala.math.sqrt
  sqrt(x) + 1.0

In the event there is a naming conflict and you need to import something from the root of the project, prefix the package name with _root_:

package accounts

import _root_.users._

package accounts

import _root_.users.*

Note: The scala and java.lang packages as well as object Predef are imported by default.