Exercise 1: “Hello World” Arch 486
Spring, 2008
Exercise 1: Hello World,
graphically!
The starting point for learning a new language is often a very simple program that
simply says “Hello” to you. These “Hello World” programs usually reveal basic
operations, introduce syntax, and give you a chance to get started.
Such a program might look like this:
package ex1;
public class HelloWorld{
public static void main (String[] args){
System.out.println(“Hello World”);
}
}
However, even this simple program raises the question: where? That is, where does
this message appear? to whom or what is it delivered? As it happens, the program
is sending the message to the System output device, or console. If all we cared
about were text input and output, we would use this framework, but we want to do
this as part of a Java web applet, and we want to do it graphically, which means we
have to think about it as constructing a graphic display containing some text, and we
have to answer questions like “what color?” “how big?” and “what font?”.
We do this in the applet by sending messages (commands) to a “graphics” object,
telling it about the graphic we are creating. That’s why the source on the next page
includes “import java.awt.*;”
Exercise 1: Hello World,
extends Applet!
The programs created for this course are considered “fragments” of a larger
application (the web browser), with which they can interact to some extent. Thus,
they are not “applications” (which have a “main” method, as shown above), but
“applets”, which “extend” (a Java concept) a pre-existing framework known as
“Applet”. Unlike “main” programs, applets go through certain preset life-stages. That
is, they receive certain “messages” (method calls) based on events in the containing
browser’s universe—events like “init”, “start”, “stop” and “destroy”. And because
applets are primarily graphical creatures, they also receive a “graphics environment”
(aka a “Graphics” object) and messages instructing them to “paint” and “update” the
graphics environment (due to browser window scrolling, resizing, etc.) That’s why
the source on the next page includes “import java.applet.*;”
Exercise 1: Hello World,
going beyond the basics!
So, our HelloWorld (no spaces!) applet receives a “draw here” message (via the
paint method and associated Graphics object) when the browser is ready to display
the applet’s results. For static output, all we have to do is write “paint”, as shown in
the source on the next page. However, for maximum learning, investigate alternative
values for font, color, and position, and consider what else might be fun to do. Then
look and see if you can find out how to do it. If you can, share the “how” with us. If
you can’t, ask.
