Code listings

Listing of Hello1.java

/ / This is a `comment'. You can tell because the

// lines start with `//'. These lines are all

// ignored by the Java compiler, so you can write

// whatever you like. At the very least you should

// start with a comment saying what the program

// does

// Note that this program has far more comments than

// real program statements; this would probably not

// be the case in most real programs.

// Hello1.java

// A program that displays a message

// Kevin Boone, May 1999

// The first real (i.e., non-comment) line of the program is `import'

// This tells the compiler what classes to use apart

// from any defined in this program. In this case

// I tell the compiler to consider using any of the

// classes that are part of the `package' java.awt.

// and the class `Applet'

// I have to do that because the program uses the

// Class `Applet' to do most of the work, and this

// class is defined in the java.applet package and

// the java.awt package. 

// Most Java programs will start with an `import'

// statement

import java.applet.Applet;

import java.awt.*;

// So now down to work. Define a class. All java programs

// have at least one class. In this case we are writing an

// applet, so the new class is a type of applet. `extends'

// means, essentially, `is a type of'.

// Note that Java rules stipulate that a `public' class

// must be defined in a file of the same

// name, i.e., Hello1 must be defined in `Hello1.java'

public class Hello1 extends Applet

// The open brace below denotes that all the statements that

// follow are part of the class Hello1, until the matching

// closing brace at the end of the program

{

// Now we define an method called `paint'. Providing this

// method ensures that when the program starts up,

// something useful will happen. 

// If we do not provide a `paint' method, then the program

// will compile and run correctly, but it won't display

// anything at all.

// The concept of an `method' will be covered in more detail

// later in the course

public void paint (Graphics g)

{

// Now the program text is `indented', this is,

// all the lines start a few spaces from the

// left margin. Doing this makes it easy for the

// (human) reader to identify all the lines that 

// are part of `paint'. The computer does not

// care about this, but people will find the

// program easier to understand

// `drawString' is an method in the class called

// Graphics. Its job is simply to display text in

// an area of the screen. In this case the text is

// positioned 20 pixels across, and 20 pixels down 

g.drawString ("Welcome to INT4120", 20, 20);

}

// This final brace denotes the end of the class `Hello1' and,

// in this case, the end of the program

}

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