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Revised: Sun Mar 27 23:30:58 CDT 2016
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It is traditional in introductory programming courses to write and explain a simple program that prints the text "Hello World" on the computer screen.
This module continues that tradition.
I recommend that you open another copy of this module in a separate browser window and use the following links to easily find and view the Figures and Listings while you are reading about them.
This module introduces you to Java programming by presenting and discussing a traditional Hello World program.
Two approaches
Java programs can be written and executed in several different ways, including the following:
It is also possible in many cases to write applets, which can be run in a stand-alone mode from the command line, or can be run under control of aJava-capable browser. An example of such an applet will be presented in a future module.
Applets vs. applications
Programming an "application" in Java is significantly different from programming an "applet." Applets are designed to be downloaded and executed on-line under control of a browser.
Restrictions on applets
Their functionality of an applet is usually restricted in an attempt to prevent downloadedapplets from damaging your computer or your data. No such restrictions apply to the functionality of a Java application.
Class definitions
All Java programs consist of one or more class definitions. In this course, I will often refer to the primary class definition for a Java application as the controlling class .
The main method
A stand-alone Java application requires a method named main in its controlling class .
An Applet does not require a main method. The reason that a Java Applet does not require a main method will be explained in a future module.
Getting started
Figure 1 shows the steps for compiling and running a Java application.
| Figure 1. How to compile and run a Java application. |
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Here are the steps for compiling and running a Java application, based on the assumption that you are running under Windows. If you are running under someother operating system, you will need to translate these instructions to that OS. 1. Download and install the JDK from Oracle. Also consider downloading and installing the documentation, which is a separate download. 2. Using any editor that can produce a plain text file (such as Notepad), create a source code file with the extension on the file name being .java Thisfile contains your actual Java instructions. (You can copy some sample programs from the early lessons in this collection to get started.) 3. Open a command-line window and change directory to the directory containing the source file. It doesn't really matter which directory the sourcefile is in, but I normally put my Java files in a directory all their own. 4. Assume that the name of the file is joe.java , just to have something definitive to refer to. 5. To compile the file, enter the following command at the prompt: javac joe.java
6. Correct any compiler errors that show up. Once you have corrected all compiler errors, the javac program will execute and return immediately to the prompt with no output. At that point, the directory shouldalso contain a file named joe.class and possibly some other files with a .class extension as well. These are the compiled Java files. 7. To run the program, enter the following command: java joe
8. If your program produces the correct output, congratulations. You have written, compiled, and executed a Java application. If not, you will need todetermine why not. |
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