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Organizing classes with packages

From: Java Essential Training

Video: Organizing classes with packages

So far in this course, I've been creating applications where every class was placed in the root folder of its project, but Java developers very seldom actually do that. Most classes in a Java application are placed in packages or subfolders of the project. Because of the way Java application is structured it could have dozens, hundreds or even thousands of classes, and as an application gets larger it's very important to organize these classes into groups. We call these groups packages but they represent actual physical folders on disk.

Organizing classes with packages

So far in this course, I've been creating applications where every class was placed in the root folder of its project, but Java developers very seldom actually do that. Most classes in a Java application are placed in packages or subfolders of the project. Because of the way Java application is structured it could have dozens, hundreds or even thousands of classes, and as an application gets larger it's very important to organize these classes into groups. We call these groups packages but they represent actual physical folders on disk.

I'm going to start by creating a sub package to store my helper classes. I'm working in a version of the Calculator application where all of the classes are stored in the root folder or default package and I'm first going to create a custom package and then move my helper classes into it. I'll go to the default package in the package Explorer view, right-click and choose New>Package. Initially, I'm going to use a shallow package structure meaning that each package will be just a subfolder of the root.

So I'm going to call this package, helpers. Now, when I create a package in Eclipse, I am actually creating a physical folder on disk as well. Next, I'm going to move my helper classes into the helpers package. I'll do this one at a time. I'll start with InputHelper.java, I'll right-click on the file and I'll choose Refactor>Move. I'll select my new helpers package and click OK. The file is moved into the package and a couple of changes are made to the code.

First of all, in the InputHelper.java file, a new package declaration is added to the top of the file. This is a requirement in Java. When a class is anywhere but the default package, it has to be declared that way in the code. The syntax is the keyword package and then the name of the package ending with a semicolon. Also, in the main application, where I am using that class, a new import statement has been added to the top of that code. Once again, this is a requirement. If the class isn't a member of the same package as the one that's calling it, then it has to be imported.

In this case, I am referring not just to the package but also the class name, helpers.InputHelper. Now I'll move my second helper class. I'll go to MathHelper.java in the package Explorer, right-click and choose Refactor>Move and once again, I'll choose the helpers package and click OK. My main application now has imports for both helper classes and both InputHelper and MathHelper have the package declaration. I'll run the application, I'll click the Run button and if prompted, I'll choose Java Application.

I'll enter some values and an operation and everything is working just fine. So organizing your classes into packages doesn't do anything to change the functionality for application; it just gives you a way of organizing the code so you can find it more easily later on. In large scale Java development environments package names typically are structured using a prefix of the company's domain in reverse domain order. So for example, if a company is named lynda.com then you would start off with a package prefix of com.lynda.

The reason to do this is so that you can eliminate any possible conflicts with other classes in the Java environment. Remember that Java compilation doesn't create a single monolithic file. Instead your runtime application consists of a whole bunch of compiled classes and it's up to the JVM to figure out which classes needed at runtime. By using reverse domain notation in your classes, you guarantee universal uniqueness. So I'm going to do a little bit more refactoring. I'll go to my default package and once again I'll add a new package, and I'll name this package com.lynda.calc.

So I start off with the domain name first and then something to say which application I'm working on. You can add more levels to your packages, it's up to you. I'll click Finish, then I'll go to my Calculator class, I'll right click and choose Refactor>Move and I'll choose com.lynda.calc. Notice that because I have dot notation, each of these is seen as its own individual package. I won't be placing anything in com or com.lynda; those are just prefixes for the way I am organizing my code.

But my main class will go in this calc package. I'll click OK and my source code file is moved and I'll look at my code and I'll see that the package declaration has been added at the top of the code. Next, I'll rename my helpers package, I'll right-click, I'll choose Refactor>Rename and I'll change the package name to com.lynda.calc.helpers. I am still distinguishing this package from the package that contains my main application and keeping all my helper classes in their own unique package.

But they're all deeply buried now in my uniquely named primary package. This time I'll choose Preview and show you that Eclipse can show you what your code will look like before and after refactoring. I'll click OK and the change is made. My main application still has the imports for the two classes, InputHelper and MathHelper. If I prefer, I can change these and just put in an asterisk and take out the second import, and this now means import everything in that package.

If you have classes in the package that you're not using that's okay; it won't affect performance. But note that if you use the Organize Imports tool in Eclipse that's the tool that kicks in when you press Ctrl+Shift+ O on Windows or Command+Shift+O on Mac, Eclipse will convert from an asterisk , a wildcard import to explicit imports for each class that you're using. That's the preferred approach for Java development because it helps you know by looking at the top of your code exactly which classes are actually in use in your application.

I'll go take look at the InputHelper and MathHelper classes and I'll see that the package declarations have been modified there. And finally, I'll save everything and run the application again and make sure I haven't broken anything. I'll enter the numeric value and another numeric value and this time, I'll multiply and I get back a correct answer. So, organizing your coding packages is a good thing to do. As your applications become larger and more complex and you have more and more classes, you'll find organization of your classes through packages to be invaluable.

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This video is part of

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Java Essential Training

71 video lessons · 68626 viewers

David Gassner
Author

 
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  1. 10m 8s
    1. Welcome
      1m 3s
    2. Is this course for you?
      5m 35s
    3. Using the exercise files
      3m 30s
  2. 31m 24s
    1. The history of Java
      5m 19s
    2. Java compilation and syntax
      8m 54s
    3. Understanding the principles of Java
      8m 28s
    4. Choosing a development environment
      8m 43s
  3. 19m 5s
    1. Installing Java on Windows
      6m 42s
    2. Installing Eclipse on Windows
      3m 19s
    3. Exploring Java on Mac OS X Leopard and Snow Leopard
      2m 27s
    4. Installing Java on Mac OS X Lion
      3m 27s
    5. Installing Eclipse on Mac OS X
      3m 10s
  4. 46m 10s
    1. Creating a Hello World application
      11m 7s
    2. Exploring the Eclipse IDE
      8m 55s
    3. Compiling and running from the command line
      8m 2s
    4. Passing arguments to the application
      8m 17s
    5. Using the Java API documentation
      4m 5s
    6. Memory management and garbage collection
      5m 44s
  5. 58m 57s
    1. Everything is an object
      5m 59s
    2. Declaring and initializing variables
      9m 15s
    3. Working with numbers
      8m 32s
    4. Converting numeric values
      6m 40s
    5. Understanding operators
      7m 58s
    6. Working with character values
      5m 14s
    7. Working with boolean values
      5m 13s
    8. Outputting primitive values as strings
      5m 33s
    9. Creating a simple calculator application
      4m 33s
  6. 53m 40s
    1. Writing conditional code
      5m 35s
    2. Using the switch statement
      8m 50s
    3. Repeating code blocks with loops
      7m 35s
    4. Creating reusable code with methods
      6m 31s
    5. Declaring methods with arguments
      5m 41s
    6. Overloading method names with different signatures
      5m 53s
    7. Passing arguments by reference or by value
      5m 35s
    8. Creating a more complex calculator application
      8m 0s
  7. 20m 30s
    1. Using the String class
      5m 44s
    2. Building strings with StringBuilder
      3m 34s
    3. Parsing string values
      3m 19s
    4. Working with date values
      7m 53s
  8. 20m 44s
    1. Understanding compile-time vs. runtime errors
      4m 5s
    2. Handling exceptions with try/catch
      4m 55s
    3. Throwing exceptions in methods
      2m 50s
    4. Using the debugger
      8m 54s
  9. 32m 22s
    1. Using simple arrays
      4m 47s
    2. Using two-dimensional arrays
      6m 17s
    3. Managing resizable arrays with ArrayList
      7m 14s
    4. Managing unordered data with HashMap
      6m 5s
    5. Looping through collections with iterators
      7m 59s
  10. 52m 2s
    1. Understanding encapsulation
      5m 59s
    2. Creating and instantiating custom classes
      8m 8s
    3. Organizing classes with packages
      6m 47s
    4. Creating and using instance methods
      6m 52s
    5. Storing data in instance variables
      6m 56s
    6. Using constructor methods
      5m 40s
    7. Managing instance data with getter and setter methods
      8m 26s
    8. Using class variables and Enum classes
      3m 14s
  11. 41m 15s
    1. Understanding inheritance and polymorphism
      9m 12s
    2. Extending custom classes
      9m 1s
    3. Overriding superclass methods
      3m 8s
    4. Casting subclass objects
      5m 3s
    5. Understanding interfaces and implementing classes
      4m 2s
    6. Creating your own interfaces
      4m 14s
    7. Using abstract classes and methods
      6m 35s
  12. 32m 17s
    1. Managing files with the core class library
      7m 46s
    2. Managing files with Apache Commons FileUtils
      7m 32s
    3. Reading a text file from a networked resource
      7m 52s
    4. Parsing an XML file with DOM
      9m 7s
  13. 17m 39s
    1. Creating your own JAR files
      4m 54s
    2. Understanding the classpath
      5m 2s
    3. Documenting code with Javadoc
      7m 43s
  14. 47s
    1. Goodbye
      47s

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