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Creating a Hello World application

From: Java Essential Training

Video: Creating a Hello World application

Now it's time for that traditional first programing task: building an application that says Hello World. If you don't already have Eclipse open, you should open it now. I'm going to show you a little trick on Windows and you can do something similar on Mac OS X. I am going to create a shortcut on my Desktop to make it easier to start Eclipse in the future. I'll go to the Eclipse folder under Program files, and if you are working on Mac, go to \applications\eclipse and locate the Eclipse command with the little graphic.

Creating a Hello World application

Now it's time for that traditional first programing task: building an application that says Hello World. If you don't already have Eclipse open, you should open it now. I'm going to show you a little trick on Windows and you can do something similar on Mac OS X. I am going to create a shortcut on my Desktop to make it easier to start Eclipse in the future. I'll go to the Eclipse folder under Program files, and if you are working on Mac, go to \applications\eclipse and locate the Eclipse command with the little graphic.

Now I am going to right click on it. And on Windows I'll create a shortcut and on Mac create an alias. I'll then drag that shortcut or alias out to the Desktop and then I am going to rename it. On Windows, I'll press F2, on Mac I'll press Enter, and I'll simply call it Eclipse. And now I'll be able to start up Eclipse easily from my Desktop whenever I need it. Now, double click the shortcut or alias and that starts up Eclipse on my system.

Before you create the Hello World application, you should set up your workspace. A workspace is a folder that tracks all of your Java projects. It contains all of your user configurations, pointers to your projects regardless of whether they are actually physically in the workspace folder, and adds other capabilities to your Java development environment. To switch your workspace, go to the Menu and choose File, Switch Workspace>Other, it will show you your current workspace.

I am going to change my workspace to my Exercise Files folder. If you have access to the Exercise Files for this course you can do the same, or you can create your own Exercise Files folder. I'll Browse, I'll go to my Desktop, and from there I will go to Exercise Files, and click OK. Now, when you click OK from this screen, Eclipse is going to close and then reopen. This allows Eclipse to let go of any file locks it may already have.

And when it opens it will be ready to get started building your applications. I'll maximize Eclipse and once again close the Welcome Screen, and now I am ready to create my first application. When you create Java applications in Eclipse, they are always inside Java project. So the first thing to do is to create a project. From the Menu choose File>New>Java Project. Give your project a name. I'll name it HelloWorld. I recommend creating your project names without any spaces or special characters.

The default location for your project will be a folder of the same name under your Workspace folder, in my case under the Exercise Files folder. If you prefer, you can uncheck the option, Use default location and place the project anywhere you like. For the JRE, you can choose either a specific JRE, or I recommend using the default JRE. This will give your project maximum portability between systems. So for example, if you say Use default JRE and your system currently has JRE 6.0, and then you take that same project and move it to a system where the default JRE is JRE 7.0, the project will still compile and run just fine and you won't have to go into your Java project settings and change the environment, accept all the other default settings and click Next.

On this screen you are asked for Java build settings. The default setting is to create your source code files in a folder named SRC under the project folder. And down at the bottom it shows that the output folder, that is the folder where you will be creating your output binary files or class files, is named bin. So your files aren't all stored in one single folder. Accept the default and click Finish. Your project starts out empty, that is, there aren't any Java code files in it.

You have to create the applications explicitly. Every Java application is built inside a class. This makes it very different from other programming environments such as say Perl or Python, where an application can be as a little as a single line of code. In Java, everything is an object, and an object is created from a class, so you have to define a class. The good news is Eclipse makes this pretty simple to do. Right-click on your source folder and choose New>Class.

This is the new Java Class wizard. The Source folder is set explicitly to HelloWorld/src, that is the project folder and the SRC subfolder. For now, leave the Package blank. You'll see a warning when you do that, Eclipse thinks that you should put every class in a subfolder or package of some kind, but to keep things simple for your Hello World application we're going to leave it in the default Package or the project route. Give the class a name of HelloWorld, you could also name this class Main or anything else you want, but to follow good Java conventions, the first character of the class name must be uppercase.

If you create a class name with a lowercase initial character it will still work, but it will violate conventions that are very strongly held in the Java development world. For the Modifiers, accept the default of public. Don't check the checkboxes for abstract or final, again, we'll talk about those later in the course, and leave the Superclass set to Java.lang.Object. In fact, there is only one other change to make here. To make this a valid startup class, check the option labeled public static void main.

This will create a method or function that will be called automatically when you start up the class by the Java Virtual Machine. That's it. Click Finish. Once the class has been created, it will open in an editor. Now, to make this a little easier to read, I am going change my font. I'll go to my Preferences dialog, which on Windows you can get to through Window>Preferences, and on Mac you can get to through Eclipse>Preferences. In the Preferences dialog, I'll click on General>Appearance>Colors and Fonts, and then in the Colors and Fonts list I'll choose Basic, and scroll down to the bottom of that list and choose Text Font, and then I'll click Edit.

I am just going to expand the size of my Font to 14 pixels and click OK, and click OK again. And that should make the code easier to read on the screen. I am going to clean up the code a little bit, removing things I don't need. I don't need this commenting section, so I am just going to select and delete it, and I don't need this TODO comment either. This is added to the main method by Eclipse when you generate the code. You can either select and delete it or you can use this little icon over on the left with the checkbox.

I'll click the icon, and from the Task List I'll choose Remove task tag, double clicking. I'll press tab once and now I am ready to add some executable code. Here is the code to output a string to the console. Start with System with an uppercase S. System is a Java class and it's a part of the core Java Class Library that's included with your installation of Java. Now type a period and you will see a list of all of the properties and objects that are a part of the system class. Choose out; you can either type it or simply double click it.

Out represents an object which implements something called a PrintStream interface. A PrintStream object allows you to output text to some sort of environment. In this case, it will output to the console. Type another dot or period, and now you will see a list of all of the functions that are a part of the PrintStream named out. I am going to choose one called Println. There is a Print and a Println; the Println function or method will output something to the screen and add a line feed at the end.

I'll type in an opening parenthesis and then a double quote and notice that Eclipse is closing the parenthesis and the quotes as I type, and then I'll type in the string "Hello World!" I'll move the curser to the end of the line and put in a semicolon. In Java, the semicolon is like a period in English, it completes the statement, and it's required. In some C style languages, such as ActionScript, if you don't put in the semicolon you will be okay.

In Java you have to put the semicolon in or the compiler will complain. That's my finished Hello World application. I'll save my changes by pressing Ctrl + S on Windows or command S on Mac. Now, I am going to run the application for the first time. To run an application, go to the Toolbar and look for the Run button, it's a little arrow pointing to the right. Click the Down Arrow next to the Run button and choose Run Configurations. In the Run Configurations screen choose Java Application, and then up on the Toolbar click the Plus button.

This will create a new Run Configuration for this application. The name of the Run Configuration will match the name of the class, HelloWorld. There are many options for the Run Configuration, but you don't need any of them for now. Just go down to the bottom of the dialog box and click Run. If all of your code works okay, you should see a rectangular area down at the bottom of the screen called the Console View appear, and it should contain the output of your application, Hello World! Now, once you've created the Run Configuration for the first time on a particular application, you don't need to go through the whole process again.

So I am going to add a couple of exclamation marks here so that I can just modify the application in some way. I'll save the changes by pressing, once again, Ctrl+S or Command+S, and this time to run the application I don't need to pull down the list and go into the Configuration screen, I just need to click the Run button. And down at the bottom in the Console I see the output. So that's your HelloWorld Java application. As I've previously mentioned, Java is a little more complex than certain scripting languages that allow you to say Hello World with a single line of code.

In Java, all code must be wrapped in class definitions and code that you want to execute must be wrapped inside a function of some kind. This main function is called automatically by the Java Virtual Machine. It must have the words public, static, and void at the beginning, and it must accept this value between the parenthesis, String, open and closed bracket, and the name of a variable, which is traditionally named args. I'll talk more about that main method and specifically about how those values are being passed into the function, known as arguments, in a later video.

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This video is part of

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Java Essential Training

71 video lessons · 69994 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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