Chapter 13 Java AWT – Part II (Optional) Lecture Slides to Accompany An Introduction to Computer Science Using Java (2nd Edition) by S.N. Kamin, D. Mickunas, E. Reingold

Chapter Preview In this chapter we will: discuss the use of Java in animating World Wide Web applications show how to write Java applets introduce Java graphical components (e.g. Textfields, Buttons, and Labels) show how Java programs can be made to react to user actions on Java graphical components show how Java applications can be run from inside web browsers

Frames A frame is a window with a title bar and a border The Frame class is a subclass of Container class Container class objects may have other components (e.g. Buttons) added to them using the add method A typical Java GUI will create and display one or more frames To make a frame visible the message setVisbible(true) must be sent to the frame

Layout Managers Governs how components are arranged inside a Container object The Container method setLayout allows the programmer to specify which layout manager (e.g. FlowLayout ) is desired for a particular container object The size of a Container object should be set explicitly by the programmer as well Can use pack() to set size to accommodate preferred sizes of components)

Frame Example public class TwoButtons { Frame f; Button redButton, blueButton; public TwoButttons() { f = new Frame(“Two Buttons Frame”); redButton = new Button(“Red”); blueButton = new Button(“Blue”); f.setLayout(new Flowlayout()); f.add(redButton); f.add(blueButton); f.pack(); f.setVisible(true); }

UML Notation The filled diamond represents composition This shows that the class TwoButtons contains a class Frame which contains a class Buttons A good UML diagram only shows the classes and associations that are important to understanding the architecture This UML diagram does not include super classes like Component or Container

Using Inheritance It is possible to use inheritance to allow TwoButtons to become a frame in its own right Note that the UML diagram will show TwoButtons as a subclass of Frame You will need to use super() to set the frame title

Frame Inheritance Example public class TwoButtons extends Frame { Button redButton, blueButton; public TwoButttons() { super(“Two Buttons Frame”); redButton = new Button(“Red”); blueButton = new Button(“Blue”); f.setLayout(new Flowlayout()); f.add(redButton); f.add(blueButton); f.pack(); f.setVisible(true); }

Other Simple Java Components Label –contains some text that can be set using a constructor or the method setLabel TextField –a box into which the user can type text –text can be edited (backspace, delete, etc.) –text can be retrieved using getText() –text can be set using setText()

Java AWT Event Model Including reactive program components involves: 1.Having the class header declare itself as implementing the ActionListener interface 2.Typically in the class constructor the class instance registers itself as being interested in listening for events from a newly created component 3.One method (e.g. actionPerformed ) of the ActiveListener interface is defined

Frame Inheritance Example public class classname extends Frame implements ActionListener { … Button buttonname; … public classname() { … buttonname = new Button(“button label”); … add(buttonname); … buttonname.ActionListener(this); … } … public void actionPerformed(ActionEvent e) { … what to do when button is pushed … }

UML Notation for Button Program Abstract class indicated by italicizing the class name Implements association (concrete class) indicated by line with a closed arrowhead UML stereotype (constructors, accessor, interface implementation methods) indicated by enclosing a descriptive word inside guillements >

Closing the Window You must implement the WindowListener and its seven required methods windowActivated, windowClosed, windowClosing, windowDeactivated, windowIconified, windowDeiconified, windowOpened This is usually done in a programmer defined class like ClosableFrame and all classes needing these services would extend ClosableFrame

Using Conditionals with Reactive Components public void actionPerformed(ActionEvent e) { double fahr, cent; // check for user input in tFahr TextField if (e.getSource() == tFahr) { fahr = new Double(tFahr.getText()),doubleValue(); cent = 5.0 * (fahr – 32.0) / 9.0; tCent.setText(cent + “”); // update tCent TextField } else { cent = new Double(tCent.getText()),doubleValue(); fahr = 9.0 * cent / ; tFahr.setText(fahr + “”); }

Checkboxes Used to allow user to select one or more items Always has a label Program often needs to react to the user selection(s) Declaration example: Checkbox powerBrakes = new Checkbox(“Power Brakes”); Checkbox powerSteering = new Checkbox(“Power Steering”); Checkbox ac = new Checkbox(“Air Conditioning”); … add(powerBrakes); add(powerSteering); add(ac);

Programming Checkboxes Program is declared with implements ItemListener Checkbox is registered by calling addItemListener Event is handled using itemStateChanged argument type ItemEvent The ItemEvent argument is used to tell which item triggered the event by calling getSource

Radio Buttons A group of Checkboxes in which only one item can be selected at a time Implemented using a Java CheckboxGroup Items are declared initially as selected (true) or unselected (false) Example: CheckboxGroup gender; Checkbox maleCheck = new Checkbox(“Male”, gender, true); Checkbox femaleCheck = new Checkbox(“Female”, gender, true);

Processing Radio Buttons public void compute() { … boolean female = (gender.getSelectedCheckbox() == femaleCheck); … if ((bodyMassIndex > 27.8) || (female && (bodyMassIndex > 27.3)) lHigh.setText(“This is considered high”); else 1High.setText(“this is not considered high”); }

Drawing in a Frame To draw in a Frame you need to the override the frame’s paint method: public void paint(Graphics g) Graphics objects are defined by the Java runtime system and are used for drawing operations The Frame should be considered to be a grid with upper left coordinates (0,0) and positive coordinates (x,y) for the lower right

Typical Drawing Code public class MyDrawing extends ClosableFrame { … public MyDrawing() { … } … public void paint(Graphics g) { … g.drawLine(…); … g.drawImage(…); … }

Repaint and Update The paint method requires an argument and actionPerformed does not know what to supply when called The method repaint will clear the Frame and then call paint along with supplying the needed missing argument You can prevent clearing the Frame when using repaint by overriding the method update before calling repaint

Panel and Canvas Classes The Panel class is container subclass that is used to reserve a rectangular portion of a Frame to place other components The Canvas class is not a Container subclass, but does allow you to reserve a portion of a Frame to draw in

Comparing Layout Managers FlowLayout –Default frame layout –Components are placed on new line only when needed GridLayout –Frame is declared as a grid of fixed size (e.g. two rows and three columns) –Components are placed in the grid left to right and top to bottom BorderLayout –Frame is divided into north, south, east, west, and center –Components are placed by the programmer in the desired location using the add method

Typical GUI Frame Panel p1 = new Panel(); p1.setLayout(new GridLayout(2,1)); p1.add(component); Panel p2 = new Panel(); p2.setLayout(…); p2.add(component); … setLayout(new BorderLayout()); add(“North”, p1); add(“Center”, p2);

Scrollbars The leftmost position of a horizontal scrollbar corresponds to some integer value and the rightmost position corresponds to a larger integer value Scrollbars can be displayed vertically User movement options –Clicking either end button causes bubble to move in unit increments –Clicking the are between bubble and end button causes movement in 10 unit increments –Clicking and dragging the bubble in the desired direction

Hypertext Markup Language HTML – Skeleton Page Title Text and graphics to display in browser

HTML Text Displays Displays a centered heading in browser window Naploean I To identify text paragraphs to be displayed In my spare time I like to do origami and spend quiet evenings at home with my wife Josephine.

HTML Using Pictures and Links Displays a centered picture To display a text link to another web page Josephine To display a picture link to another web page

Applet Tag and HTML To include a Java applet as part of an HTML document Java applet shell for BodyMass.class import java.applet.*; public class BodyMass extends Applet { … public_html void start() { new BodyMass(); }

Transforming a Java Program into an Applet 1.Set the size you want the applet to be in the HTML file 2.Add the import statement to the Java code java.applet.*; 3.Change header from extends Frame to extends Applet 4.Change constructor heading to public void start(); 5.Eliminate any Java code that is meaningless in applets