Digital Edition

SYS-CON.TV
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.

About 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

In order to post a comment you need to be registered and logged in.

Register | Sign-in

Reader Feedback: Page 1 of 1

I'm glad you mentioned the use of interfaces as repositories of constants. While it may not be recommended in the strict sense of why interfaces exist, I've used this technique for years on large projects as a good way of organizing and reusing constants.

So you can use wildcards in the import? Can you use static import for static classes?

Will the other 10 lessons be on SYS-CON.TV?

Is it Yakov Fain who is speaking at the moment? A live Java lesson - cool!

I used this lesson in conjunction with the last one, lesson ten on eclipse IDE and it really helped me




ADS BY GOOGLE
Subscribe to the World's Most Powerful Newsletters

ADS BY GOOGLE

DX World EXPO, LLC, a Lighthouse Point, Florida-based startup trade show producer and the creator of...
SYS-CON Events announced today that Conference Guru has been named “Media Sponsor” of the 22nd Inter...
DevOps is under attack because developers don’t want to mess with infrastructure. They will happily ...
The next XaaS is CICDaaS. Why? Because CICD saves developers a huge amount of time. CD is an especia...
Companies are harnessing data in ways we once associated with science fiction. Analysts have access ...
Widespread fragmentation is stalling the growth of the IIoT and making it difficult for partners to ...
"Akvelon is a software development company and we also provide consultancy services to folks who are...
Enterprises are adopting Kubernetes to accelerate the development and the delivery of cloud-native a...
In his session at 21st Cloud Expo, James Henry, Co-CEO/CTO of Calgary Scientific Inc., introduced yo...
The question before companies today is not whether to become intelligent, it’s a question of how and...
In his session at 21st Cloud Expo, Carl J. Levine, Senior Technical Evangelist for NS1, will objecti...
22nd International Cloud Expo, taking place June 5-7, 2018, at the Javits Center in New York City, N...
"Cloud Academy is an enterprise training platform for the cloud, specifically public clouds. We offe...
Agile has finally jumped the technology shark, expanding outside the software world. Enterprises are...
"MobiDev is a software development company and we do complex, custom software development for everyb...
"Codigm is based on the cloud and we are here to explore marketing opportunities in America. Our mis...
High-velocity engineering teams are applying not only continuous delivery processes, but also lesson...
"CA has been doing a lot of things in the area of DevOps. Now we have a complete set of tool sets in...
While some developers care passionately about how data centers and clouds are architected, for most,...
"NetApp is known as a data management leader but we do a lot more than just data management on-prem ...