Java Applications

Java programs consist of pieces called classes and methods. Although you can program the classes and methods yourself, you can also utilize collections of existing classes and methods in Java class libraries. Thus, there are really two pieces to learning Java--learning the Java language itself so that you can program your own classes and methods, and learning how to use the classes and methods in the Java class libraries. Class libraries are provided both by compiler vendors and independent software vendors. 

Java systems generally consist of several parts: an environment, the language, the Java Applications Programming Interface, and various class libraries. 

An Example Application 

 1  // A first application in Java
 2  public class Welcome1 {
 3     public static void main (String args[ ])
 4     {
 5         System.out.println("Welcome to Java Programming");
 6     }
 7  }

Note that the line numbers are not part of a Java program, but have been added for reference.

Various components of the above program will be discussed below.

For a Java application class, one of the methods must be called main and must be defined as shown in line 3, otherwise the Java interpreter will not execute the application.

An application begins execution at the main method (line 3).  Applications are executed using the Java interpreter (java). 


Comments

• The first line begins with a //, indicating that the remainder of the line is a comment. 
• Comments beginning with // are single line comments, and can begin in the middle of a line.
• Multi-line comments begin with /* and end with */. 
• Comments are ignored by the Java compiler and do not cause any bytecodes to be generated. 

Classes

• The line  
          public class Welcome1 {  
  begins a class definition for the Welcome1 class. 
• The class keyword introduces a class definition in Java and is immediately followed by the class name (Welcome1 in this program). 
• Every Java program is composed of at least one class definition. 
• The left brace, {, begins the body of every class definition. A corresponding right brace, }, must end each class definition. 
• The line 
      public static void main (String args[ ]) 
  introduces a program building block called a method. 
  • Methods are able to perform tasks and return information when they complete their tasks. 
  • Java programs contain at least one method that performs tasks during the program's execution. 
• The program above contains the definition for method main (lines 3 through 6). 
  • The public keyword indicates that the main method can be called by any object. 
  • The static keyword indicates that the main method is a class method.
  • The void keyword indicates that this method will perform a task, but will not return any information when it completes its task. 
  • The set of parentheses after main defines the method's parameter list, which is where methods receive information required to complete their tasks. 
  • The main method accepts a single argument: an array of elements of type String.  This array is the mechanism through which the runtime system passes information to the application. Each String in the array is called a command-line argument. Command-line arguments let users affect the operation of the application without recompiling it.
• System.out is the standard output object, and allows Java applications to display strings and other information in the command window from which the application is executed.
  • Method System.out.println displays a line of text in the command window.  
  • Method println advances the cursor to the next line.
  • Method print does not advance the cursor.  
  • A single statement can display multiple lines by using the newline character (\n), one of several special characters.

A modified program

Rather than making use of the command window, most Java applications use windows or dialog boxes to display their output.

 1  // Printing multiple lines in a dialog box
 2  import javax.swing.JOptionPane;   // import class JOptionPane
  3
  4  public class Welcome2 {
 5     public static void main (String args[ ])
 6     {
 7         JOptionPane.showMessageDialog (
  8                   null, "Welcome\nto\nJava\nProgramming");
  9
10         System.exit (0);   // terminate the program
11     }
12  }

Import

• Java contains many predefined pieces called classes that are grouped by directories on disk into categories of related classes called packages. 
• The packages are referred to as the Java class library or the Java applications programming interface (Java API).
• Import statements (line 2) load classes required to compile a Java program. 
• Line 2 tells the interpreter to load the JOptionPane class from the javax.swing package.
  • The JOptionPane displays a dialog box that can be used to display output.

The exit method is required in any application that displays a GUI in order to terminate the application.

The 0 argument passed to the exit method indicates that the application terminated normally.

Identifiers that start with capital letters normally represent class names, so you can assume that System is a class.

Class System is part if the package java.lang, but does not require an import statement because it is imported automatically into every Java program.

Recap:

Java applications require the statement public static void main (String args[ ]).

Java applications display their output in a command window unless a dialog box or other type of window is specified.

Java applications that use the GUI must include the line System.exit(0);