Source

Description and Objective

• Illustrates the basic structure of a Java application.
• Discusses the development cycle of a Java program

Notes

• A Java application is a Java program that runs standalone
  • This is in constrast to an applet which we shall see runs within a Web page.
• A Java application must contains a main method
  • The main method is the entry (i.e. starting) point of execution of the program.

• A Java program can be developed using
  • An IDE (integrated development environment)
    • A set of (graphic interface) tools to allow one to manage the entire development cycle
      • Editor
      • Project builder
      • Debugger
    • Examples
      • Eclipse
      • NetBeans
  • A text editor and the command-line compiler/interpreter
    • The text editor is up to you
    • The command-line compiler is the javac executable
    • The command-line interpreter is the java executable

• All code in Java appears within the context of a class
• A class containing a main method is often called an application class (as we can use it as the entry point of the execution of an application).

This Application's Behavior

• The Class Header
  • If a class is declared public, the name of the source file MUST match the class name
    • Thus our (public) class AJavaConsoleApplication must be contained within a file named AJavaConsoleApplication.java
• The main Method
  • The main method accepts a single parameter of a String array
    • This array contains the command-line arguments passed to the program
    • This is analogous to the argc/argv pair sent to the main function in C/C++.
  • Invoking the println method on System.out sends the argument to the standard output device
    • There is a corresponding System.err. which sends its output to the standard error device

The Java Application Programmers Interface (API)

• A hyperlinked documentation of the Java libraries
• Three basic frames (windows)
  
  
  • packages (think of them as individual libraries-- like iostream in C++) (upper left)
  • Classes (and other beasts) within the selected package (lower left)
  • Main documentation frame (right)
• There's also an alphabetical index (see banner at top of main doc frame)
• The hyperlinks allow for quick navigation through the documentation

Things to Do

• The System class
  • Find the class in the API
  • What package is it in?
  • Find the out 'field'
  • Find the err 'field'
  • The types of the 'out' and 'err' fields are the same. What is their type?
    • This class provides basic output capability
• The PrintStream class
  • Find the class in the API
  • What package is it in?
  • Find the println method(s) in the class
    • There should be several overloaded versions of this method
    • Also, check out the print method(s)
• What is happening in the following statement>:

  System.out.println("Hello World");
  				

  • i.e., what roles do System, out, and println play in the above statement?