2.1. Introduction

As you transition from small programming projects to larger, real-world applications (sometimes with hundreds of Java files), it becomes increasingly important to organize the files in your project. Just as you may might organize your personal photos into separate folders, we organize code into packages.

Note

Java packages allow programmers to organize classes that have related functionality. Proper code organization becomes increasingly important as the size of the project increases.

In this chapter, you will see that organizing code is similar to organizing other files on your computer, as we will need to place our source code files (.java files) into separate directories. However, we will also need to slightly modify the source code and adjust our compilation commands.

2.1.1. Existing Java Packages

You may not have realized it, but classes that you have used in the past from the standard Java libraries are already organized into packages. For example, the Scanner class and the Math class are in two separate packages. The Scanner class is included in the utility package java.util and the Math class is part of the fundamental language package java.lang.

When a class is contained within a package, its fully qualified name (or FQN for short) consists of two parts: the name of the package followed by the class name (sometimes referred to as the simple class name to differentiate it from the FQN). By convention, the simple class name will always begin with a capital letter and the package name will be all lowercase.

Simple Class Name	Package Name	Fully Qualified Name	Common Uses
Scanner	java.util	java.util.Scanner	Used to parse input from various sources. Typically used for reading user input.
Math	java.lang	java.lang.Math	Provides common math functions such as sqrt, sin, cos, pow, among others.

Note

When you import a class, you have to use its FQN, but once the class is imported, you can use its simple class name.

You have to import classes from other packages in order to use them in your programs. The only exception is that you don’t have to import classes from java.lang as those are automatically included.

2.1.2. Primary Benefits

The two primary benefits of packages that we will see in this course are:

1. Name Space Management:

   Packages allow you to give a common name to a group of related types. For example, java.util.Scanner and java.util.Random are two utility classes provided in the java.util package. You and other programmers can easily determine that these types are related based on their package. They are both utilities that you have probably used in your code.

   Note

   You could have two types with the same simple name that are located in different packages. For example, you could have a java.util.Random class and a edu.cs.uga.Random class. Both classes have the same simple class name (Random) but they can be differentiated by their package names.

2. Access Protection:

   Visibility in Java is not limited to public and private. Packages and additional visibility modifiers enable programmers to declare entities as visible only within a package. We will discuss this in more detail when we cover visibility.

Further Reading

For more information about packages in Java see [1]

In this chapter, you will create multiple classes, group them into a package, then compile and run them. The expectation is that you will follow along with this tutorial in a terminal emulator on Odin or some Unix machine. You should ask questions on Piazza if you are unable to proceed or if some aspect of the tutorial is particularly confusing.

Quick Review Questions

1. Given the FQN java.util.ArrayList, what is the simple class name and package name?

2. If you want to import the ArrayList class, what is the exact line of code you would need to add to your program and where would that line need to be located?

3. Given the FQN javafx.scene.control.Button, what is the simple class name and package name?

4. Identify two syntax errors in the code below.

   Note: The code you see is the full program. Do not assume any additional lines.

   public class AbsoluteNum {
   
      public static void main(String[] args) {
         Scanner input = new Scanner(System.in);
         int number = input.nextInt();
         System.out.println(abs(number));
      } // main
   
   } // AbsoluteNum
   

Quick Review Answers (Don’t open until you complete the questions above)

1. The simple class name is ArrayList and the package name is java.util.

2. The exact line would need to be import java.util.ArrayList; and it would need to be at the top of the Java file.

3. The simple class name is Button and the package name is javafx.scene.control.

4. Scanner is not recognized because the java.util package is not imported. Also abs() is not recognized because it is not declared in this class. We intended to call the static abs method in the Math class, so the call should be Math.abs(number).