A Tweak of Lemon...

Blowing away the clouds in the coffee..Brewing the first Applet

"Ours is the age of substitutes; instead of principles, we have slogans; instead of genuine ideas, bright ideas; instead of languages, jargon. "

When it comes to bright jargon, Sun Microsystems has left no stone unturned. And the creative terminology goes a long way (if not all the way) in making it like the tidal wave it has been. Java programs are called 'Applets'. In the directory in which the whole downloaded software of Java is saved, there is a subdirectory called Bin. This is where we have the Java compiler called 'Javac.exe'. The first thing we do is to create a subdiretory under the root called, say, "xxx". Then set the path to \java\bin. We are going to write our Applets here. Each applet or program has to be written in a file with the extension of .Java. In keeping with the ritual, let us consider this one-liner code first...

a1.java
class zzz
{
}

You'll agree that even an ice-hockey player or a visitor to Disneyland will love this sort of a program because it is looks manageably concise and absolutely simple. Well, we do not expect fireworks with that sort of a code, but let us compile it all the same. Save the file a1.java. It is noteworthy to mention here that like C/C++, Java is case-sensitive too (and preposterously so!). Type the command javac a1.java. The compiler gives no errors. If you key in the DIR command now, you will see that after successful compilation, a file called zzz.class is made automatically. That is because we specified zzz as the name of our class. This makes it adequately clear that the name of the applet file (the .java file) and the class can very well be different.

Now comes the question of viewing how our applet works. If it is a Java applet, you can call it only from an HTML file (An HTML file, in case you don't know, is nothing but a text file with tags that link it to other documents). Since all the documents on the Internet use this format, an applet has to be specified in an HTML file. Consider the following HTML file.

aa.html
< applet code = "zzz"  width =120 height =120></applet>

As is evident, the .class file (zzz.class in our case) that is formed after successful compilation of the .java file (a1.java) is specified in the HTML. The width and the heigth This is the way in which you can incorporate an applet in the HTML format. The applet could be a program to display animation or perform any other dynamic task and thus, enliven a page on the net.

But it still remains to be seen how the applet works or what the output looks like. Towards this end, we have a program called the appletviewer (in the same directory as javac). This is a program that allows you to view how your applet works. Hence, we key in the command appletviewer aa.html. Sorry folks, but there is no output (if your stars are not happy with you, your machine might even hang!).

There are a hundred and sixty five things to be done in the background before our class begins work (like the initialization routines et al). Our class does not have any property towards that end. As one does not know what routines are to be initialized internally, the safest thing to do is to derive a class from another class provided with the language. Now edit your a1.java file in the following way.

class zzz extends Applet
{
}

Applet is a class that has all the required functions necessary for this purpose, so we derive zzz from Applet or extend zzz to Applet. This means, in plain English, that we can now use all the functions and variables of the Applet class and also add our own ones. The only hitch now is that compiler refuses to recognize Applet, unless we mention specifically the name of the class library in which it is contained. So we now have to formally introduce the compiler to the Applet class. We do this by inviting the Applet class into our program (how else?). Only, the invitation here is in the form of importing a file called Applet.class from the classes\ java\Applet subdirectory.

import java.applet.*
class zzz extends Applet
{
}

import java.applet.*;
class zzz extends Applet
{
}

import java.applet.*;
public class zzz extends Applet
{
}

import java.applet.*;
public class zzz extends Applet
{	public void init()
	{	resize(300, 500);
	}
}

import java.applet.*;
import java.awt.*  ;
public class zzz extends Applet
{	public void init()
	{	resize(300, 500);
	}
	public void paint(Graphics g)
	{	g.drawString("Hello", 10, 50);
	}
}

Now that we are aware of the method of displaying, let us write a program that is a little more interesting. We propose to display a string and a value everytime the Mouse is clicked. Key in the following code...

import java.applet.*;
import java.awt.*;
public class zzz extends Applet
{	int i;
	public void init()
	{	resize(300, 500);
	}
	public void paint(Graphics g)
	{	g.drawString("i....." + i, 10, 50);
	}
	public boolean mouseUp(Event e, int x, int y)
	{	i++;
		repaint();
		return true;
	}	
}

They said programming in Java is object-oriented and event-driven. That sent my brains for a toss when I first heard it, but the above program bears a glittering testimony to that. The function mouseUp() gets activated everytime the mouse button is clicked, the paint() is called whenever a window or a screen has to be redrawn etc.

Let us alter this code to display our usual "Hello", but now at the coordinates where the user clicks. Change the code in zzz.java as the following..

import java.applet.*;
import java.awt.*  ;
public class zzz extends Applet
{	int a; int b;
	public void init()
	{	resize(300, 500);
	}
	public void paint(Graphics g)
	{	g.drawString("Hello", a, b);
	}
 	public boolean mouseUp(Event evt, int x, int y) 
	{	a=x; b=y;
		repaint();
		return true;
	}
}

Our next program draws a line between two clicks of the mouse. That is, a line is drawn between the first and the second click, the third and the fourth click and so on.

import java.applet.*;
import java.awt.*  ;
public class zzz extends Applet
{	int a; int b; int c; int d; int e;

	public void init()
	{	resize(300, 500);
	}
	public void paint(Graphics g)
	{	g.drawLine(a, b, d, e);
	}
 	public boolean mouseUp(Event evt, int x, int y) 
 	{	if (c==0)
 	 	{
			a=x;
			b=y;
			c=1; 
    		}
		else
		{
			d=x;
			e=y;
			c=0;
			repaint();
		}
		return true;
	}
}

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