CSCI A348/A548
Lecture Notes 15

Fall 1999

Java, applets, clients and servers, a feedback applet with Java.

1. Using Java applets (Chapter 11)

Java Basics (pages 645-659)

The Life of a Java Applet
Obtaining and Installing Applets
- Installing Precompiled Applets
Getting Applets
- Installing Applets Distributed as Source Code
- Embedding Remote Applets in Your Pages
  Example from O'Reilly
- Testing Applets

The <APPLET> and <PARAM> Tags

<APPLET
  CODE     = name of applet (.class file)
  WIDTH    = width of applet (pixels) 
  HEIGHT   = height of applet (pixels) 
  CODEBASE = URL of applet 
  ALT      = alternate text to display 
  NAME     = applet's name 
  ALIGN    = alignment 
  VSPACE   = extra whitespace above and below the applet 
  HSPACE   = extra whitespace to either side of applet 
>
<PARAM NAME="param 1" VALUE="value 1">
<PARAM NAME="param 2" VALUE="value 2">
  
  ...
  Alternative HTML code to display 
  
</APPLET>

Attributes of the <APPLET> Tag
- CODE (required)
- WIDTH, HEIGHT (required)
- CODEBASE (optional)
- ALT (optional)
- NAME (optional)
- ALIGN (optional)
- VSPACE, HSPACE (optional)
The <PARAM> Tag
Unpaired and contains two required attributes, NAME and VALUE:
```
<PARAM NAME=score VALUE=120>
```
Each PARAM corresponds to a named parameter.
Alternative HTML Content

Lab notes 8 contain in-lab assignment 3.

2. Writing Java applets and applications

See the on-line resources for a complete, outstanding tutorial on Java.

Java development cycle:

create a source code (.java)
compile it with javac and
run it with java (if standalone application) or
1. insert an applet tag for it in an (.html) file then
2. look at the file with a web browser or with the appletviewer.

Empty file does not compile: need class definitions.

Empty class compiles but doesn't run: needs special main method.

Simplest program:

class Empty {
  public static void main(String[] args) {
 
  } 
}

It compiles, runs, but doesn't do anything.

The "Hello, world!" program.

class Hello {
  public static void main(String[] args) {
    System.out.println("Hello, world!"); 
  } 
}

It compiles, runs, and prints the message.

Who's System? How about out?

Basic objects: a BasicCounter class.

class BasicCounter {
  int n = 0; 
  void addOne() {
    n += 1; 
    System.out.println("addOne: n is now " + n); 
  } 
} 
 
class Tester {
  public static void main(String[] args) {
    BasicCounter c = new BasicCounter(); 
    c.addOne(); 
    c.addOne(); 
  } 
}

A CustomCounter:

class CustomCounter extends BasicCounter {
  void show() {
    System.out.println("show: n is currently " + n); 
  } 
  public static void main(String[] args) {
    CustomCounter c = new CustomCounter();
    c.show();
    c.addOne();
    c.addOne();
    c.show(); 
    c.addOne(); 
    c.show();
    c.show(); 
  } 
}

Hierarchy (inheritance) diagram:

java.lang.Object
   |
   +----BasicCounter           // defines n, addOne();
           |
           +----CustomCounter  // defines show();

Same kind of diagram for java.applet.Applet:

java.lang.Object
   |
   +----java.awt.Component
           |
           +----java.awt.Container
                   |
                   +----java.awt.Panel
                           |
                           +----java.applet.Applet

Here's a simple applet:

import java.applet.Applet;
import java.awt.Graphics;
            
public class HelloWorld extends Applet {
    public void paint(Graphics g) {
        g.drawString("Hello world!", 50, 25);
    }
}

(How and when is the paint() method called?)

To see this applet we need an .html front end:

<html>
<head><title>Applet One</title></head>
<body bgcolor=white>
<applet code="HelloWorld" width=100 height=100>
</applet>
</body> 
</html>

Then we look at it with the appletviewer or a web browser.

Networking Experiments

1. telnet to the time of day service

telnet to a machine, on port 13.

2. telnet to your web server

First prepare a file:

tucotuco.cs.indiana.edu% pwd
/nfs/paca/home/user2/dgerman/httpd/htdocs
tucotuco.cs.indiana.edu% vi myfile
tucotuco.cs.indiana.edu% cat myfile
 
    ***( this is my test )***

tucotuco.cs.indiana.edu% pwd
/nfs/paca/home/user2/dgerman/httpd/htdocs

Then connect with telnet to your host on your port and

Trying 129.79.251.110...
Connected to tucotuco.cs.indiana.edu.
Escape character is '^]'.
GET /myfile HTTP/1.0
 
HTTP/1.1 200 OK
Date: Thu, 01 Oct 1998 17:18:06 GMT
Server: Apache/1.3.1 (Unix)

2. POST to a script of yours with telnet

We have a script that can be used to mail a comment to the webmaster.

2.1 the script

#!/usr/bin/perl
 
print qq{Content-type: text/plain\n\n}; 
 
if ($ENV{'REQUEST_METHOD'} eq 'POST' && 
    open(MAIL, "| mail dgerman\@indiana.edu")) {
  while (<STDIN>) {
    print MAIL; 
  } 
  close(MAIL);      
  print "Mailto.pl";
} else {
  print "Error";   
}

2.2 the experiment

From school telnet to your web server and


Trying 129.79.251.110...
Connected to tucotuco.cs.indiana.edu.
Escape character is '^]'.
POST /cgi-bin/lecture9/mailto.pl HTTP/1.0
Content-type: application/octet-stream
Content-length: 10
 
Greetings! 
HTTP/1.1 200 OK
Date: Thu, 01 Oct 1998 18:03:22 GMT

3. the feedback applet

It's a combination of three things:

an .html file to deliver the applet
the .class file
the mailto.pl script

But first let's write our 'telnet'.

3.1 a network client

This should essentially be able to send a stream of bytes (a stream of characters) over the network, to a server. Here's the code for one such network client:

import java.io.*;
import java.net.*;
      
public class Client {
  public static void main(String[] args) {
    try {
      Socket con = new Socket("tucotuco.cs.indiana.edu", 19904);
      PrintWriter out = new PrintWriter(con.getOutputStream(), true);
      
      out.println(
        "POST /cgi-bin/lecture15/mailto.pl HTTP/1.0\r\n" + 
        "Content-type: application/octet-stream\r\n" + 
        "Content-length: " + args[0].length() + "\r\n\r\n" +
        args[0]
      );
      out.flush();
      System.out.println("Sent: (" + args[0].length() + ") " + args[0]);    
    } catch (Exception e) {
      System.out.println("E: " + e);
    }
  }     
}

Last updated: October 19, 1999 by Adrian German

CSCI A348/A548 Lecture Notes 15

CSCI A348/A548
Lecture Notes 15