A Simple AppletApplets and applicationsPackages and classesThe Applet classImporting the Applet classThe java.awt packageImporting the java.awt packageC and C++ programmers onlyThe applet so farCommentsClassesYour first classYour first class, part 2Your first class, part 3Slide 15MethodsThe paint methodThe paint method, part 2By the way…namesThe paint method, part 3Classes and objectsThe paint method, part 4Slide 23ColorsNew colorsSetting a colorThe paint method so farPixelsJava’s coordinate systemDrawing rectanglesThe complete appletSome more java.awt methodsDrawing StringsThe EndA Simple AppletApplets 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 itselfPackages 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 packageThe 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 namesImporting 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 endThe 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 programC 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 farimport 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 extractedfrom 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 soonYour 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 windowYour 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 codeThe applet so farimport java.applet.Applet;import java.awt.*;// CIT 591 examplepublic 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 resultThe 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 resultThe paint method, part 2public 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 bracesBy the way…names• ( ) are parentheses• { } are braces• [ ] are bracketsThe paint method, part 3public 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 paintedClasses 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 DogThe paint method, part 4public 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 gThe applet so farimport java.applet.Applet;import java.awt.*;// CIT 591 examplepublic 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 fully-qualified names:•For compatibility with older programs (before the naming conventions were established), Java also allows color names in lowercase: Color.black, Color.darkGray, etc.Color.BLACK
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