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Caught in the Net (Applets)

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Caught in the Net (Applets)

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Caught in the Net

A sip of Networking

" Oh good! You're awake"

If you thinks a web is that weelittle fly-catching thing in the corner of your room, you've probably been living in the Himalayas. Today, each and every little Tyke (and his dog), are on the World Wide web. And if you are reading this, then more certainly than your dog's name (Bruno?), you are conversant with the powers of this blown-up scaffold of computer networks. The bad news is that if you feel writing programs in the field of networking, say for something like a live world-wide chat, is a nightmare, then you are probably feeling right. The good news, however, is that before a couple of shakes of Bruno's tail, you will be writing your first Networking program! The program that we'll build on right now, is to yank in a document lurking somewhere on the web (doesn't everything just lurk somewhere out there?) into our computer. And if its kinda hard-hitting, try not to weasel out midway; it will all begin to make sense soon.

Albeit an elaborate explanation is absconding now, we expect you to try this out. Meanwhile, we will do some more slogging :)

Before we swoop down on the actual clutter of Resource Locators, servers, streams etc., let us remind ourselves of a basic list and a button.
 


Program 1

import java.awt.*;
import java.applet.*;
public class zzz extends Applet
{	public void init()
	{	setLayout(new BorderLayout());
		List l;
		l = new List(3000, false);
		add("Center", l);
		add("North", new Button("Start"));
	}
}

Now, let us write the novice program.
Program 2

import java.awt.*;
import java.applet.*;
public class zzz extends Applet
{	URL u;
	public void init()
	{	setLayout(new BorderLayout());
		List l;
		l = new List(3000, false);
		add("Center", l);
		add("North",new Button("Start"));
	}
	public boolean action ( Event e, Object o)
	{	if ( "Start".equals(o) )
		{	try
			{	u=new URL("http://www.neca.com/~vmis/java.html");
				showStatus(" URL is initialised ");
			}
			catch(MalformedURLException x)
			{	showStatus("error in URL");
			}
		}
		return true;
	}
}

Not as bed-wettingly scary as you thought it would be, was it? Only the vocabulary of the programming luminaries seems to be bloated with jargon. If you've been using the web, the kinda highfalutin word URL (Uniform Resource Locator) is perhaps at least halfway memorable by now (we wonder why it is called a URL, but we understand one can live out a full and relatively cheerful life without ever needing to know) Anyway, you will get these errors as of now...

class URL not found in type declaration
class URL not found in new
class MalformedURLException not found in type declaration
class MalformedURLException not found in catch

Therefore, we use the inevitable import , which, incidentally, is mandatory if you want to get past the compilation stage!

import java.net.*;

With that wisdom up our sleeve, let us give it another try. We first have to define a URL connection . Right after the line that says URL u , let us add this line

URLConnection c;

And later, in the action() , we write the following...

			
try
{	c = u.openConnection();
	showStatus("Open Connection successful");
}
catch(IOException x)
{	showStatus ("Connection failed");
}

But again, there are three errors lobbed at us which very rhetorically go as...

Exception :java.io.IOException must be caught , or it must be declared in the throws clause of this method
Class IOException not found in type declaration
Class IOException not found in catch

And that brings us to yet another import statement. This one is for the IO (and for those who came in late or didn't come at all, that stands for "Input Output").

 import java.io.*;

As yet, whatever we are doing with our code is far from spectacular. While we are taking care of the URL and the URL connection, we conveniently forget about fetching in the file. Towards that end, we have a class called InputStream . Hence, we include the following line...

	InputStream n;

Besides, our action() will now look like this...

	public boolean action ( Event e, Object o)
	{	if ( "Start".equals(o) )
		{	try
			{	u=new URL("http://www.neca.com/~vmis/java.html");
				showStatus(" URL is initialised ");
			}
			catch(Exception x)
			{	showStatus("URL not initialised");
				return true;
			}
			try
			{	c = u.openConnection();
				showStatus("Open Connection successful");
			}
			catch(Exception x)
			{	showStatus("Error in Open Connection");
				return true;
			}
			try
			{	n = c.getInputStream();
				showStatus("getInputStream successful");
			}
			catch(Exception x)
			{	showStatus("error in getInputStream");
				return true;
			}
		}
		return false;
	}

 


Next we have the DataInputStream class which will likewise be inserted in the following way...

	DataInputStream d;


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The Java Lesson 1:
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Identifiers and primitive data types
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Variables, constants, and literals
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Arithmetic operations, conversions, and casts
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Boolean expressions and operations
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Bitwise operations
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Flow control with if and else
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switch statements
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for, while, and do-while statements
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Using break and continue
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Class methods and how they are called
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Using the Math class
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Creating and calling custom class methods
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Overloading class methods
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The StringBuffer class
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Initializing and processing arrays of primitives
The Java Lesson 20:
Initializing and processing arrays of objects
The Java Lesson 23:
Inheritance and overriding inherited methods
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abstract classes and polymorphism
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Interfaces, instanceof, and object conversion and casting
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The Component class
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The Color and Font classes
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Choice, List, and Checkbox controls
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The Java Lesson 35:
Essential HTML to launch an applet and pass it parameters
The Java Lesson 36:
Mouse event processing
Java Lesson 37:
Menus and submenus
Java Lesson 38:
The WindowListener interface and the WindowAdapter class
Java Lesson 39:
An introduction to GridBagLayout
Java Lesson 40:
An introduction to the Java Collections API
Java Lesson 41:
Exception handling with try, catch, and finally blocks
Java Lesson 42:
Claiming and throwing exceptions
Java Lesson 43:
Multithreading, the Thread class, and the Runnable interface
Java Lesson 44:
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