Create Software Components 

Using Java Level 2

 

 

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Understanding Your First Program

Hello World Application Explained

Hello World Applet Explained

Comparing the HelloWorld Application and Applet


Hello World Application Explained

 

Lets look at the code for Hello World

 

 /*  The HelloWorld class implements an application that simply displays "Hello World!" to the standard output.  */


public class HelloWorld {


    public static void main(String[] args) {


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


    }

}

 

Now let us examine each line

 

 /*  The HelloWorld class implements an application that   simply displays "Hello World!" to the standard output.  */

By convention we always give a brief explanation of our class at the beginning.

The  /* tells the java compiler that the lines following are to be ignored and are treated as comments.

The */ tells the Java compiler that we have finished our comment lines

 
public class HelloWorld All java programs are written in sets of one or more classes.

Each class is saved to it's own file

We will ignore the keyword public for now

The keyword class marks the beginning of our class definition.

HelloWorld is the name of this class.

The first letter of your class name should be in capitals.

When saving the source code the file name must be the same as the class name and remember, Java is case sensitive.  So we would save this file as HelloWorld.java

 
{ The curly brace indicates the beginning of our class.

In Java, the { curly brace indicates the start of a block of code.

The } curly brace indicates the end of a block of code.

 
public static void main { We will ignore the keyword public, static and void for now.

The keyword  main is the name of a method.  All Java applications must have at least one  main method amongst it's set of classes.

When Java executes this class it executes the main method first.

If we didn't have a main method then Java would not know where to start executing our lines of code.  

Notice the curly brace again.  This indicates the beginning of our  main method block of code.

 
(String[] args) This specifies any parameters being passed to the main method.  We are going to ignore this for now, but we must always type this in.
 
System.out.println This term sends output to the screen.  You must use a capital S for System or you will get an error.
 
("Hello World!") The string we which to display to screen is enclosed within parenthesis  ( )

You will note that " marks surrounding the text  Hello World!  This is a way of specifying a string.

 
; The semicolon ends the  System.out.println("Hello World!") statement.  Most lines in Java end with a semicolon.

If you forget the semicolon you will get an error

 
} The curly brace indicates the end of our main method.
 
} The final curly brace indicates the end of our class..
 

Summary

  • A Java program is put together using classes

  • A class is a structure that contains methods.

  • Methods are blocks of code that carry out a specific functions.

  • An application must have a main method in one of it's classes

  • The class containing the main method should be saved to a file with the same name as the class.

  • The main method is the first code to be executed when your application is run.

  • The System.out.println  method is used to output strings to the display.

 


Hello World Applet Explained

Lets look at the code for the Hello World applet

 

/* 

The HelloWorldApplet class implements an applet that simply displays "Hello World!".
 */
import java.applet.Applet;
import java.awt.Graphics;

public class HelloWorldApplet extends Applet {
    public void paint(Graphics g) {
        g.drawString("Hello World!",0,50);
    }
}

 

Now let us examine each line

 

 /* 

The HelloWorldApplet class implements an applet that simply displays "Hello World!".
 */

By convention we always give a brief explanation of our class at the beginning.

The  /* tells the java compiler that the lines following are to be ignored and are treated as comments.

The */ tells the Java compiler that we have finished our comment lines

 


import java.applet.Applet;
import java.awt.Graphics;
Compiled classes can be held together in packages.  Java comes with lots of pre-written classes held in packages that we can use.

To make our own applets work we must also include a pre-defined class called Applet. The is held in the java.applet package and the import statement tells Java to include this class in our own applet program.

We also have to import the java.awt.Graphics class so we can draw the "Hello World!" string onto our Applet window.

 
public class HelloWorldApplet  All java programs are written in sets of one or more classes.

Each class is saved to it's own file

We will ignore the keyword public for now

The keyword class marks the beginning of our class definition.

HelloWorldApplet is the name of this class.

The first letter of your class name should be in capitals.

When saving the source code the file name must be the same as the class name and remember, Java is case sensitive.  So we would save this file as HelloWorldApplet.java

 
extends Applet We will ignore this for now.

However you must always include this when writing an applet

 
{ The curly brace indicates the beginning of our class.

In Java, the { curly brace indicates the start of a block of code.

The } curly brace indicates the end of a block of code.

 
public void paint(Graphics g)  We will ignore the keyword public and void for now.

The keyword  paint is the name of a method.  

The method allows us to draw onto the applet window

We will ignore the Graphics g  terms for now.

 
{ The curly brace indicates the beginning of our paint method 
 
g.drawString

Applets have various ways of drawing, or displaying, their content. The drawString method (part of the Graphics class) is used to display text to the appet window. 

 
("Hello World!",0,50)

The drawString method takes three arguments: 

(1) the string to be displayed.  In this case the string is "Hello World!"

(2) the x coordinate, indicating the horizontal starting point for displaying the string.  In this case the x coordinate is 0.

(3) the y coordinate, indicating the vertical starting point for displaying the string (which is below the text).  In this case the y coordinate is 50.

 
; The semicolon ends a statement.  Most lines in Java end with a semicolon.

If you forget the semicolon you will get an error

 
} The curly brace indicates the end of our paint method.
 
} The final curly brace indicates the end of our class.
 

 

Summary

  • A Java program is put together using classes

  • A class is a structure that contains methods.

  • Methods are blocks of code that carry out a specific functions.

  • An application needs a main method so Java has a starting point for running your program. 

  • An applet does not have a main method.  This is because applets do not start themselves, they are added to an already running program, your browser or appletviewer.

  • An applet uses a Paint method to draw thing to the applet window

  • The drawString method is used to draw strings onto an applet window

 


Comparing the HelloWorld Application & Applet

 

  • The HelloWorld application outputs the string "Hello World!" to the screen display. The System.out.println method is used to do this.

  • The HelloWorld applet outputs the string "Hello World!" to the applet window. The drawstring method is used to do this.

  • An application must have a method called main. An applet does not have a main method.

  • An applet uses a paint method to draw to the applet window

  • The source code for a class is saved in a .java file.  This is true for both application and applet classes.

  • Compiled source code is saved in .class files. This is true for both application and applet classes.

  • An extra HTML file is needed for running applets.

 


That is folks!!

Now try the My Second App exercise

 

 

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