Packages in practice

This last step looks at packages outside this site: how real projects arrange their folders, how packages are named, and the messages javac gives when something about packages goes wrong.

The folders of a real project

On this site, a program's package folders start at the top, next to Main.java. A project on your own computer has a few more folders around them. Many Java projects are built with a build tool such as Maven or Gradle (a later module of this course sets up Maven), and both expect this layout:

• src/main/java/quiz/domain/Question.java
• src/main/java/quiz/logic/Quiz.java
• src/main/java/quiz/ui/TextUI.java
• src/test/java/quiz/logic/QuizTest.java

What the layout says:

• src/main/java holds the program's own classes, and the package folders start inside it. Question.java is still in quiz/domain/, and its package line is still package quiz.domain;.
• src/test/java holds the tests, in a folder tree of its own. A test class is in the same package as the class it tests: QuizTest is in quiz.logic too, so it can test Quiz's package-private parts as well.
• In a real program every class is in a package, Main too, often in the program's top package: quiz.Main. A class in a named package can't use a class of the default package at all, since no import can name it. The default package suits small programs, like the ones on this site, where nothing needs to use Main.
• A programming editor, such as NetBeans, IntelliJ IDEA or VS Code, shows the packages as a tree, and creates the folders and the package line when you make a class in a package.

Naming packages

• Package names are all small letters, one short word for each part: quiz.logic, not Quiz.Logic or quiz.gameLogic.
• Many start with the internet domain of the organization that writes the code, turned around, so that the packages of two organizations never share a name. JUnit's classes, which module 26 imported, are in org.junit, and a company with the domain example.com would use packages like com.example.quiz.logic. Names that start with java belong to Java's own library.
• A package is named for what its classes do together. A small program splits into domain, logic and ui, as in this module. Bigger programs often have a package for each feature instead, such as quiz.players and quiz.questions. Either way, each package has one clear job.

Messages you'll meet

javac says	It usually means
package quiz.domian does not exist	a typo in an import or a full name
cannot find symbol, with symbol: class Question	an import is missing
Question is not public in quiz.domain; cannot be accessed from outside package	the class needs public
getPoints() is not public in Question; cannot be accessed from outside package	the method or constructor needs public, unless it's meant only for its own package
getAnswer() has protected access in Question	only its package and its subclasses may use it (a subclass only through its own type)
file does not contain class quiz.domain.Question	the file's package line doesn't match its folders

The last one comes with more messages around it, such as duplicate class and bad source file. They all go away when the package line is right.

This module in short

• A package line puts a class in a package, and the file's folders match it. A class's full name is its package and its own name. A class without a package line is in the default package.
• An import lets a file use a class of another package by its simple name, and * imports a whole package. java.lang is imported into every file.
• From another package, only public classes, constructors and methods can be used, and protected also lets in subclasses. What has no word in front in a class stays inside its package.
• A program splits into packages by responsibility, such as domain, logic and ui, which use each other in one direction.