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Packages and Imports

As a codebase grows, it becomes impractical to keep every class in one giant, flat list. Java solves this using packages, a way of grouping related classes together, similar to how folders organize files on a computer.

package-box

The package Declaration​

Every Java file belongs to a package, declared with the package keyword at the very top of the file, before anything else (even before import statements).

Package Formula​

package [package.name];
package frc.robot.subsystems;

class Flywheel {
    // ...
}

Packages are typically named in reverse-domain style, and mirror the folder structure of the project. A class declared in frc.robot.subsystems would physically live inside a frc/robot/subsystems/ folder in the project.

Importing Classes​

Classes automatically have access to every other class in the same package, but to use a class from a different package, it must first be imported.

Recall from the Input/Output section that reading user input required this line at the top of the file:

import java.util.Scanner;

This tells Java that the Scanner class lives in the java.util package, and makes it available for use throughout the rest of the file, without needing to write its full name (java.util.Scanner) every single time.

Import Formula​

import [package.name].[ClassName];

Wildcard Imports​

Instead of importing classes one at a time, a wildcard (*) can be used to import every class in a package at once.

import java.util.*; // Imports every class in java.util, including Scanner

While convenient, wildcard imports are generally discouraged, since they make it less clear exactly which classes a file actually depends on, and can occasionally cause naming conflicts if two different packages contain classes with the same name.

Why Packages Matter​

Packages provide two main benefits:

  • Organization: Related classes (e.g. all of a robot's subsystems) can be grouped together, making a large codebase easier to navigate.
  • Namespacing: Two classes in different packages are allowed to share the same name, since their full, qualified names (package.name.ClassName) are still unique.

Java's own built-in classes are organized this way too. String and Math live in java.lang (which is automatically imported into every file, no import required), while Scanner lives in java.util, and must be explicitly imported before use.