Package

Introduction

One of the problems in programming is identifier conflicts. This conflict arises when two or more elements have the same identifier or name. This is not allowed in programming because the compiler or interpretor cannot determine which element is being referenced.

It is up to the programmers it make sure that the same identifier is not used for multiple elements within the program. The problem comes when a program uses third party libraries or Java libraries. The programmer has no control of what identifiers were used in third party code.

For example, let say the program has a class called Shape which is the parent or base class for classes such as Square, Triangle, and Circle. The class Shape may also be declared in third party code. When this occurs, the source code cannot be compiled successfully.

Package

Java solution to identifier conflict also known as identifier resolution is to place source code into packages. A package can be thought as a container of related software. The compiler will automatically use the package name and the identifier together to create a unique name for the element.

Note: Other programming languages may use a similar technique for preventing duplicate identifiers from happening. C++ uses the keyword namespace instead of package to do the same thing.

Unique Package Naming

The is no way to strictly enforce the use of unique package names. therefore there is a convention used by Java Programmers to hopefully come up with a unique package name.

In most cases, every programming project is associated with an Internet Domain. The convention is to use that reverse of the domain name. This does not mean completely reversing the letters and dots. It means that each component of the domain name is used and appear in the reverse order. Added to the end of that sequence, could be team and/or project name, followed by subproject names.

<reverse domain>.<team>.<project>.<subproject>

For example, let the domain name used for a project is my.domain.com, the team name is SCAROB, the project is Cresendo, and the subproject is the drive system. This could result in a package name of:

com.domain.my.scarob.cresendo.drivesystem

Using Packages

When creating a new file for Java source code, begin by placing the package statement as the first line of source code. This first line should follow the file comment.

The syntax of the package statement is:

package <your package name>;

In a file, it will look like this:

/**
 * File Header Comment...
 */

package com.domain.my.scarob.cresendo.drivesystem;

import ...

Each source code file in the package must have the same package statement.

All new elements declared after the package statement will belong to that package. The compiler will then use their full name when resolving their identifiers.

For example, assume that the package statement in a source code file is package frc.robotic.scarob and the class MyDrive is declared in that same file. When the compiler reads MyDrive in that file, the compiler will use the identifier of frc.robotic.scarob.MyDrive. The code can still refer to the element as MyDrive. The only time the code might need to use the full identifier is when the compiler cannot resolve the identifier because there are more than one element with the same one.