A Simple Applet 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 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 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 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 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 it s drawn on the screen then java awt is probably involved 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 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 Java s import gives you access to the library It does not make your program bigger It s OK to use lots of include directives The applet so far import java applet Applet import java awt 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 extracted from your program and used in documentation 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 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 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 Java s Applet just makes an empty window We are going to draw in that window 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 The applet so far import java applet Applet import java awt CIT 591 example public class Drawing extends Applet we still need to put some code in here 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 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 doesn t return any result 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 there s only one is inside parentheses The method s commands are inside braces By the way names are parentheses are braces are brackets 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 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 can t pet dog but I can pet Fido Fido is an object of type Dog 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 The applet so far import java applet Applet import java awt CIT 591 example public class Drawing extends Applet public void paint Graphics g we still need to put some code in here Colors The java awt package defines a class named Color There are 13 predefined colors here are their fullyqualified names Color black Color green Color darkGray Color gray Color lightGray Color white Color pink Color red Color orange Color yellow Color magenta Color cyan Color blue 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 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 The paint method so far public void paint Graphics g g setColor Color blue draw a rectangle g setColor Color red draw another rectangle 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 Java s 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 …
View Full Document
Unlocking...