Welcome!

Eclipse Authors: Pat Romanski, Elizabeth White, Liz McMillan, David H Deans, JP Morgenthal

Related Topics: Java IoT, Industrial IoT, Microservices Expo, IBM Cloud, Weblogic, Linux Containers, Containers Expo Blog, IT SOLUTIONS GUIDE, Eclipse

Java IoT: Article

Java Basics: Lesson 11, Java Packages and Imports (Live Video Education)

Lesson 11 in the Hugely Popular "Java Basics" Series by JDJ Editorial Board Member Yakov Fain

Java comes with thousands of classes that are organized in packages (similar to files and directories on you disk). Some packages include classes responsible for drawing, while other have classes for the Internet access, and so on. For example the class String is located in the package called java.lang, and the fully qualified name of this class is java.lang.String.

The Java compiler only knows where to find classes that are located in the package java.lang, but there are many other packages with useful classes, and it's your responsibility to let the compiler know where the classes that are used in your program live. For example, the package java.io contains classes responsible for input/output operations, while most of the Swing classes live in the following two packages:


javax.swing
javax.swing.event

It would be annoying to write a full class name every time you use it, for example:

javax.swing.JButton myButton = new javax.swing.JButton();
javax.swing.JFrame myFrame = new javax.swing.JFrame();

To avoid this you can use import statements right above the class declaration line, for example:

import javax.swing.JFrame;
import javax.swing.JButton;

class Calculator{
JButton myButton = new JButton();
JFrame myFrame = new JFrame();
}

These import statements allow you to use short class names like JFrame or JButton, and the Java compiler will know where to find them. Please note, that nothing is actually imported into your program: it's just a name resolution mechanism that helps the compiler to find classes and make your program more readable. If your need to use several classes from the same package, you do not have to list each of them in the import statement, just use the wild card. In the following example the asterisk (*) makes all classes from the package javax.swing visible to your program:

import javax.swing.*;

Still, it's better to use separate import statements, so you can see clearly which classes are imported from a particular package.

When programmers work on large projects that have lots of classes, they usually organize them in different packages. For example, one package can have all classes that display graphical windows, while another can contain data access classes.

Let's create a new project called PingPong in the Eclipse IDE. This project will have classes in two packages: screens and engine. Now create a new class PingPongTable and enter the word screens in the field Package:  

Press the button Finish and Eclipse will generate the code that will include the line with the package name.

package screens;

public class PingPongTable {

public static void main(String[] args) {
}
}

By the way, if your class includes the line with the keyword package, you are not allowed to write anything but the program comments above this line.

Since each package is stored in a different folder on a disk, Eclipse creates the folder called screens and puts the file PinPongTable.java there. Check it out - there should be a folder c:\eclipse\workspace\PingPong\screens on your disk with files PingPongTable.java and PingPongTable.class.

Now create another class called PingPongEngine and enter the word engine as the package name. The PingPong project has two packages now:

Since our two classes are located in two different packages (and folders), the class PingPongTable won't see the class PingPongEngine unless you add the appropriate import statement.

package screens;

import engine.PingPongEngine;

public class PingPongTable {

public static void main(String[] args) {
  PingPongEngine gameEngine = new
      PingPongEngine();
}
}

Java packages not only help better organize your classes, but also can be used to hide their classes from the "foreigners" living in other packages. In Java you can use public, private and protected keywords to specify the access level to a particular method or a class. But if you do not use any of these keywords in the method or class declaration, you'll be able to access them only from the classes located in the same package. We've discussed access levels briefly in Lesson 3 of this series.

More Stories By Yakov Fain

Yakov Fain is a Java Champion and a co-founder of the IT consultancy Farata Systems and the product company SuranceBay. He wrote a thousand blogs (http://yakovfain.com) and several books about software development. Yakov authored and co-authored such books as "Angular 2 Development with TypeScript", "Java 24-Hour Trainer", and "Enterprise Web Development". His Twitter tag is @yfain

Comments (5)

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


IoT & Smart Cities Stories
At CloudEXPO Silicon Valley, June 24-26, 2019, Digital Transformation (DX) is a major focus with expanded DevOpsSUMMIT and FinTechEXPO programs within the DXWorldEXPO agenda. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive over the long term. A total of 88% of Fortune 500 companies from a generation ago are now out of business. Only 12% still survive. Similar percentages are found throug...
AI and machine learning disruption for Enterprises started happening in the areas such as IT operations management (ITOPs) and Cloud management and SaaS apps. In 2019 CIOs will see disruptive solutions for Cloud & Devops, AI/ML driven IT Ops and Cloud Ops. Customers want AI-driven multi-cloud operations for monitoring, detection, prevention of disruptions. Disruptions cause revenue loss, unhappy users, impacts brand reputation etc.
At CloudEXPO Silicon Valley, June 24-26, 2019, Digital Transformation (DX) is a major focus with expanded DevOpsSUMMIT and FinTechEXPO programs within the DXWorldEXPO agenda. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive over the long term. A total of 88% of Fortune 500 companies from a generation ago are now out of business. Only 12% still survive. Similar percentages are found throug...
Atmosera delivers modern cloud services that maximize the advantages of cloud-based infrastructures. Offering private, hybrid, and public cloud solutions, Atmosera works closely with customers to engineer, deploy, and operate cloud architectures with advanced services that deliver strategic business outcomes. Atmosera's expertise simplifies the process of cloud transformation and our 20+ years of experience managing complex IT environments provides our customers with the confidence and trust tha...
The Japan External Trade Organization (JETRO) is a non-profit organization that provides business support services to companies expanding to Japan. With the support of JETRO's dedicated staff, clients can incorporate their business; receive visa, immigration, and HR support; find dedicated office space; identify local government subsidies; get tailored market studies; and more.
At CloudEXPO Silicon Valley, June 24-26, 2019, Digital Transformation (DX) is a major focus with expanded DevOpsSUMMIT and FinTechEXPO programs within the DXWorldEXPO agenda. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive over the long term. A total of 88% of Fortune 500 companies from a generation ago are now out of business. Only 12% still survive. Similar percentages are found throug...
As you know, enterprise IT conversation over the past year have often centered upon the open-source Kubernetes container orchestration system. In fact, Kubernetes has emerged as the key technology -- and even primary platform -- of cloud migrations for a wide variety of organizations. Kubernetes is critical to forward-looking enterprises that continue to push their IT infrastructures toward maximum functionality, scalability, and flexibility. As they do so, IT professionals are also embr...
In his general session at 19th Cloud Expo, Manish Dixit, VP of Product and Engineering at Dice, discussed how Dice leverages data insights and tools to help both tech professionals and recruiters better understand how skills relate to each other and which skills are in high demand using interactive visualizations and salary indicator tools to maximize earning potential. Manish Dixit is VP of Product and Engineering at Dice. As the leader of the Product, Engineering and Data Sciences team at D...
As you know, enterprise IT conversation over the past year have often centered upon the open-source Kubernetes container orchestration system. In fact, Kubernetes has emerged as the key technology -- and even primary platform -- of cloud migrations for a wide variety of organizations. Kubernetes is critical to forward-looking enterprises that continue to push their IT infrastructures toward maximum functionality, scalability, and flexibility.
Today's workforce is trading their cubicles and corporate desktops in favor of an any-location, any-device work style. And as digital natives make up more and more of the modern workforce, the appetite for user-friendly, cloud-based services grows. The center of work is shifting to the user and to the cloud. But managing a proliferation of SaaS, web, and mobile apps running on any number of clouds and devices is unwieldy and increases security risks. Steve Wilson, Citrix Vice President of Cloud,...