Java Unit 5: Applets and Graphics Web Pages and Viewing Applets

What is an Applet?  An applet is a special Java program that can be included on a web page.  The inclusion of an applet would be written in the html file of the page.  Most applets are graphical in nature.  Here, we’re going to take a very basic approach to applets and graphics.

HTML!? Oh noooooo….  We don’t have to become masters of HTML in order to write an applet page. In fact, this is so basic, we could write this in Notepad.  The code for the inclusion of an applet would go something like this: 

No HTML? No Problem!  For the purposes of debugging and testing out applets, there is an applet viewer available.  Whenever an applet class is present, the applet viewer will run that code when you press the run button in Eclipse.

Java Unit 5: Applets and Graphics Geometric and Graphics classes

Graphics  Java includes various Graphics classes.  Ex: Point, Rectangle2D  We need to keep in mind that since Java has evolved over time, there are newer and older versions of classes, each with differing features.

It’s a starting… POINT!  The Point class would be a useful place to start.  There are four subclasses to this: Point, Point2D, Point2D.Double, Point2D.Float  All the Point classes come from java.awt.geom.Point2D.  For all of these classes, Point2D is a superclass.

Point Hierarchy

Superclasses? Inheritance?  Superclasses can be thought of as a class that other ‘subclasses’ model themselves off of. That is, the subclass ‘inherits’ from the superclass.  A superclass provides all of its variables and methods to its subclasses.  A subclass can also have its own unique variables and methods in addition to the ones in the superclass.

Point and Inheritance  It can be said that all of these Point classes are in an inheritance hierarchy.  The subclasses of Point2D are reliant on Point2D’s constructor, and they also inherit all of Point2D’s variables and methods.

Point and Point2D  Historically, ‘Point’ came before ‘Point2D’.  This is an example of new classes replacing old ones.  When using Point2D.Double and Point2D.Float, you must use their full names.  Besides, referring to Point2D.Double as ‘Double’ would create ambiguity with the ‘Double’ class.

Using Points  Import into the code:  import java.awt.geom.Point2D;  Use in the code:  Point2D.Double myPoint = new Point2D.Double();  This kind of pattern of simple geometric classes integrated into more complex classes is repeated in Java.

But Points have zero dimensions!  You cannot literally draw points, but you would use it to draw other shapes.  Circles, for a basic point representation.  You could also draw a line using two points, despite a line having only one dimension.

Graphics  Like with Point and Point2D, Java has evolved and now has a Graphics and Graphics2D class.  Graphics2D is the newer class which we will be using.  In order for older applications to continue working, the inner machinery still relies on Graphics.

Using Graphics2D  Suppose you have a paint(Graphics g) method, which absolutely has to take in a Graphics as a parameter.  We get a Graphics2D from it by casting like so:  Graphics2D g2D = (Graphics2D) g;

Java Unit 5: Applets and Graphics Applets

LineApplet.java import javax.swing.JApplet; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.geom.Point2D; import java.awt.geom.Line2D;

LineApplet.java public class LineApplet extends JApplet { public void paint(Graphics g) { Graphics2D g2 = (Graphics2D) g; Point2D.Double start = new Point2D.Double(10, 20); Point2D.Double end = new Point2D.Double(110, 120); Line2D.Double myLine = new Line2D.Double(start, end); g2.draw(myLine); } }

LineApplet.java

The Cartesian Grid: Defied  The coordinate system in Java is a little different from what you might expect.  The X value increases from left to right, so higher values will appear further to the right.  However, the Y value increases from top to bottom, so higher values of Y will appear further down instead of up.  So, our ‘starting point’ (0, 0) for drawing is in the upper-left hand corner.

Looking at the code  public class LineApplet extends JApplet  Here, our ‘LineApplet’ is said to extend ‘JApplet‘, or it inherits from JApplet. JApplet is a superclass which all applets we build will subclass from.  public void paint(Graphics g)  Rather than having a main() method, applets have a paint() method, which will be used to draw onto the screen.

Looking at the code  Graphics2D g2 = (Graphics2D) g;  An applet’s paint method has to use a Graphics for its parameter. We use Graphics2D instead, so we cast it into another Graphics2D variable.  Point2D.Double start = new Point2D.Double(10, 20); Point2D.Double end = new Point2D.Double(110, 120);  Start and end points are made using numerical input for their x and y values.

 Line2D.Double myLine = new Line2D.Double(start, end);  A new line is constructed using the start and end points.  g2.draw(myLine);  This draws the line that we have defined for it.  It turns out that draw() takes in many different parameters, and a line object is just one of them.

Producing the results  Unlike a regular application, applets use a paint() method instead of a main() method.  If a class extends JApplet, Java knows that it has to have a paint() method, so the system calls that method, and passes in the Graphics object.  Once the paint() method is run, our ‘g2’ object causes things to appear on screen.  It would be useful to think of Graphics and Graphics2D as pens which we use to draw.