A Simple Applet 1 Applets and applications An application is an ordinary program Examples Notepad MS Word Firefox Halo etc An applet is a Java program that runs within another program usually a browser Applets can be run within any browser To run Java applets browsers need an up to date Java plugin appletviewer is a program that can run applets When you download the Java SDK appletviewer comes with it appletviewer is always up to date with your Java system Eclipse has an built in applet viewer 2 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 For this applet you will need to import two drawing packages awt and swing 3 Importing some things we need To create an applet you will import the JApplet class Capitalization matters The JApplet class is in the javax swing package This is the only class we will need from the swing package import javax swing JApplet Japplet and JApplet are different names be careful Since you will want to use many classes from the java awt package we will just import them all import java awt 4 Your first class part 2 public class Drawing extends JApplet Drawing is the name of your class Class names should always be capitalized extends JApplet says that our Drawing is a kind of JApplet but with added capabilities Java s JApplet just makes an empty window We are going to draw in that window 5 Your first class part 3 public class Drawing extends JApplet the code for your class goes in here The braces mark the beginning and ending of your code 6 The applet so far import javax swing JApplet import java awt CIT 591 example public class Drawing extends JApplet we still need to put some code in here 7 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 method s parameters or arguments The result is just called a result 8 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 In applets this is a special method name This will be the only parameter it needs paint doesn t return any result 9 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 Method names should begin with a lowercase letter The argument or parameter there s only one is inside parentheses The method s commands are inside braces 10 By the way names are parentheses are braces are brackets 11 The paint method part 3 public void paint Graphics g A Graphics short for Graphics context is an object 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 but it doesn t remember what you have painted In this program The type of the parameter is Graphics The name of the parameter is g 12 The applet so far import javax swing JApplet import java awt CIT 591 example public class Drawing extends JApplet public void paint Graphics g we still need to put some code in here 13 Colors The java awt package defines a class named Color There are 13 predefined colors here are their fully qualified names Color BLACK Color DARK GRAY Color GRAY Color BLUE Color LIGHT GRAY Color WHITE Color PINK Color RED Color ORANGE Color GREEN Color CYAN Color YELLOW 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 14 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 15 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 16 The paint method so far public void paint Graphics g g setColor Color BLUE draw a rectangle g setColor Color RED draw another rectangle 17 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 18 Java s coordinate system 0 0 0 0 50 0 50 0 0 20 20 20 50 0 50 20 w 1 h 1 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 19 Drawing rectangles There are two ways to draw rectangles g drawRect left top width height g fillRect left top width height 20 Drawing strings A String is a sequence of characters enclosed in double quote marks A double quote mark in a String must be preceded by a backslash Hello World He said Please don t go To draw a string you need to specify not only what you want to say but where to say it g drawString string left top For example g drawString Example JApplet 20 80 21 The complete applet import javax swing JApplet import java awt CIT 591 example public class Drawing extends JApplet 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 g setColor Color BLACK g drawString Example JApplet 20 80 22 More java awt Graphics 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 arcWidth arcHeight arcWidth arcHeight define the roundedness of corners g fillRoundRect left top width height arcWidth arcHeight g drawArc left top width height startAngle arcAngle Angles are in degrees 0 degrees is the 3 o clock position Positive angles are to the right g FillArc left top width height startAngle arcAngle 23 Still more Graphics methods g drawPolygon xPoints yPoints n g fillPolygon xPoints yPoints n xPoints and yPoints are int arrays of size n One way to write an int array is new int value1 value2 …
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