A Simple Applet

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A Simple Applet
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Applets and applications

• An applet is a Java program that runs on a web
  page
• Applets can be run from
• Internet Explorer
• Netscape Navigator (sometimes)
• appletviewer
• An application is a Java program that runs all by
  itself

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Packages and classes

• Java supplies a huge library of pre-written
  code, ready for you to use in your programs
• Code is organized into classes
• Classes are grouped into packages
• One way to use this code is to import it
• You can import a single class, or all the classes
  in a package

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The Applet class

• To create an applet, you must import the Applet
  class
• This class is in the java.applet package
• The Applet class contains code that works with a
  browser to create a display window
• Capitalization matters!
• applet and Applet are different names

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Importing the Applet class

• Here is the directive that you need
• import java.applet.Applet
• import is a keyword
• java.applet is the name of the package
• A dot ( . ) separates the package from the class
• Applet is the name of the class
• There is a semicolon ( ) at the end

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The java.awt package

• awt stands for Abstract Window Toolkit
• The java.awt package includes classes for
• Drawing lines and shapes
• Drawing letters
• Setting colors
• Choosing fonts
• If its drawn on the screen, then java.awt is
  probably involved!

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Importing the java.awt package

• Since you may want to use many classes from the
  java.awt package, simply import them all
• import java.awt.
• The asterisk, or star (), means all classes
• The import directives can go in any order, but
  must be the first lines in your program

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C and C programmers only

• C and C have an include directive that copies
  a library function into your program
• This makes your program bigger
• Javas import gives you access to the library
• It does not make your program bigger
• Its OK to use lots of include directives!

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The applet so far
import java.applet.Appletimport java.awt.
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Comments

• A comment adds information for the reader
• Java ignores everything inside comments
• There are three kinds of comments
• // starts a comment that goes to the end of the
  line
• / starts a comment that can extend over many
  lines, and ends at /
• / is a javadoc comment that can be
  extractedfrom your program and used in
  documentation /

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Classes

• In Java, all code occurs in classes
• Except for the package and import directives
• We will talk about package some day
• The code that you import is in classes
• Your code will also be in classes
• For now, a class is a bundle of code
• We will talk about what it really is very soon

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Your first class

• public class Drawing extends Applet the
  code for your class goes in here
• public says your class is not hidden
• This makes your class visible to BlueJ
• We will talk later about why we hide code
• class says we are making a class (Duh!)

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Your first class, part 2

• public class Drawing extends Applet
• Drawing is the name of your class
• Class names should always be capitalized
• extends Applet says that our Drawing is a kind of
  Applet, but with added capabilities
• Javas Applet just makes an empty window
• We are going to draw in that window

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Your first class, part 3

• public class Drawing extends Applet the
  code for your class goes in here
• The braces, , mark the beginning and ending of
  your code

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The applet so far
import java.applet.Appletimport java.awt. //
CIT 591 example public class Drawing extends
Applet we still need to put some code in
here...
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Methods

• A method is a group of commands that tell the
  computer to do something
• C programmers methods are similar to functions
• A method takes information in, does something
  with it, and returns a result
• The input information is called the methods
  parameters, or arguments
• The result is just called a result

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The paint method

• Our applet is going to have a method to paint
  some colored rectangles on the screen
• This method must be named paint
• paint needs to be told where on the screen it can
  draw
• this will be the only parameter it needs
• paint doesnt return any result

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The paint method, part 2

• public void paint(Graphics g)
• public says that anyone can use this method
• void says that it does not return a result
• paint is the name of the method
• The argument (theres only one) is inside
  parentheses
• The methods commands are inside braces

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By the waynames

• ( ) are parentheses
• are braces
• are brackets

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The paint method, part 3

• public void paint(Graphics g)
• A Graphics is something that holds information
  about a painting
• It remembers what color you are using
• It remembers what font you are using
• You can paint on it, and it remembers what you
  have painted

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Classes and objects

• A class is a description of some objects
• An object is a member of a class
• The type of an object is the class it belongs to
• Classes are more abstract than objects
• If I have a dog named Fido, I cant pet dog,
  but I can pet Fido
• Fido is an object of type Dog

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The paint method, part 4

• public void paint(Graphics g)
• g is the parameter, or argument
• we could use any name we wanted for it
• but it should not begin with a capital letter
• The type of a name tells what kind of thing the
  name refers to
• Graphics g says g is an object of type Graphics
• We can paint on g

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The applet so far
import java.applet.Appletimport java.awt. //
CIT 591 example public class Drawing extends
Applet public void paint(Graphics g)
we still need to put some code in here

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Colors

• The java.awt package defines a class named Color
• There are 13 predefined colors--here are their
  fully-qualified names

Color.BLACK Color.PINK Color.GREENColor.DARK_G
RAY Color.RED Color.CYANColor.GRAY Color.ORANGE
Color.BLUEColor.LIGHT_GRAY Color.YELLOWColor.W
HITE Color.MAGENTA

• For compatibility with older programs (before the
  naming conventions were established), Java also
  allows color names in lowercase Color.black,
  Color.darkGray, etc.

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New colors

• Every color is a mix of red, green, and blue
• You can make your own colors new Color(
  red , green , blue )
• Amounts range from 0 to 255
• Black is (0, 0, 0), white is (255, 255, 255)
• We are mixing lights, not pigments
• Yellow is red green, or (255, 255, 0)

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Setting a color

• To use a color, we tell our Graphics g what color
  we want
• g.setColor(Color.RED)
• g will remember this color and use it for
  everything until we tell it some different color

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The paint method so far
public void paint(Graphics g)
g.setColor(Color.BLUE) draw a
rectangle g.setColor(Color.RED)
draw another rectangle
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Pixels

• A pixel is a picture (pix) element
• one pixel is one dot on your screen
• there are typically 72 to 90 pixels per inch
• java.awt measures everything in pixels

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Javas coordinate system

• Java uses an (x, y) coordinate system
• (0, 0) is the top left corner
• (50, 0) is 50 pixels to the right of (0, 0)
• (0, 20) is 20 pixels down from (0, 0)
• (w - 1, h - 1) is just inside the bottom right
  corner, where w is the width of the window and h
  is its height

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Drawing rectangles

• There are two ways to draw rectangles
• g.drawRect( left , top , width , height )

• g.fillRect(left , top , width , height )

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The complete applet
import java.applet.Appletimport java.awt. //
CIT 591 example public class Drawing extends
Applet public void paint(Graphics g)
g.setColor(Color.BLUE) g.fillRect(20,
20, 50, 30) g.setColor(Color.RED)
g.fillRect(50, 30, 50, 30)
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Some more java.awt methods

• g.drawLine( x1 , y1 , x2 , y2 )
• g.drawOval( left , top , width , height )
• g.fillOval( left , top , width , height )
• g.drawRoundRect( left , top , width , height )
• g.fillRoundRect( left , top , width , height )
• g.drawArc( left , top , width , height ,
  startAngle , arcAngle )

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Drawing Strings

• A String is a sequence of characters enclosed in
  double quote marks
• "Hello, World!"
• A double quote mark in a String must be preceded
  by a backslash ( \ )
• "He said, \"Please don't go.\""
• g.drawString( string , x , y )

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The End