Internet Software Development

1. Internet Software Development Java with added Swing Lecture 3

2. // file: Welcome.java public class Welcome { public static void main(String[ ] args) { System.out.println(“Welcome to JavaWorld!”); } }

3. javac Welcome.java • java Welcome Welcome to JavaWorld!

4. // file: Welcome.java // Always introduce the class public class Welcome { // Class name must match file name for public classes public static void main(String[ ] args) { // So, “args” is an array of Strings. // “void” means no value is returned System.out.println(“Welcome to JavaWorld!”); // System is a class that interfaces to system facilities // “out” is the standard output stream of System

5. String Handling is Simple System.out.println(“Welcome to JavaWorld\n” + “\tObject-Oriented Programming\n” + “\tPLUS\n” + “\tConnectivity!”); • Java Welcome Welcome to JavaWorld Object-Oriented Programming PLUS Connectivity!

6. Rectangle length height area circumference A Java Class // Rectangle.java // A straightforward implementation of the Java Rectangle class public class Rectangle { public double length; public double height; public Rectangle(double length, double height) { this.length = length; this.height = height; } public double area( ) { return length*height; } public double circumference( ) { return 2*(length+height); } }

7. public class MakeRectangle { public static void main(String[ ] args) { double length = Double.parseDouble(args[0]); // parse first argument to double double height = Double.parseDouble(args[1]); // parse second argument to double Rectangle rectangle = new Rectangle(length, height); double area = rectangle.area(); // calculate area System.out.println(area); // display it to the user } }

8. javac Rectangle.java • javac MakeRectangle.java • java MakeRectangle 3.0 2.0 6.0

9. Java Foundation Classes • Provides Graphics and GUI capability for Java applications • See: Java Foundation Classes In a Nutshell, by David Flanagan • Provides a reference, but its not good as an introduction

10. A Class in Swing that can be used to display simple dialog boxes to the user A method in JOptionPane that displays a simple data entry dialog Data Entry with Swing import javax.swing.*; public class MakeRectangle { public static void main(String[] args) { String input = JOptionPane.showInputDialog("Enter length");

11. Modified MakeRectangle import javax.swing.*; public class MakeRectangle { public static void main(String[ ] args) { String input = JOptionPane.showInputDialog("Enter length"); double length = Double.parseDouble(input); input = JOptionPane.showInputDialog("Enter height"); double height = Double.parseDouble(input); Rectangle rectangle = new Rectangle(length, height); double area = rectangle.area(); System.out.println(area); } // This will turn out to be an unsatisfactory termination }

13. GUI application termination import javax.swing.*; public class MakeRectangle { public static void main(String[ ] args) { String input = JOptionPane.showInputDialog("Enter length"); double length = Double.parseDouble(input); ... Rectangle rectangle = new Rectangle(length, height); double area = rectangle.area(); System.out.println(area); System.exit(0); // terminates the program - this is required // for any program using a GUI } }

14. The message to be displayed Note use of “+” to concatenate strings Parent component if relevant (“null” here) Prettier Output • Swing to the rescue again! • Need to display a different kind of dialog: • present a message • no field for the user to edit • Use JOptionPane again: JOptionPane.showMessageDialog(null, "The area is " + area);

15. // MakeRectangle.java // Application with “complete” GUI import javax.swing.*; public class MakeRectangle { public static void main(String[ ] args) { String input = JOptionPane.showInputDialog("Enter length"); double length = Double.parseDouble(input); input = JOptionPane.showInputDialog("Enter height"); double height = Double.parseDouble(input); Rectangle rectangle = new Rectangle(length, height); double area = rectangle.area(); JOptionPane.showMessageDialog(null, "The area is " + area); System.exit(0); } }

17. The message Title bar string Icon type Improving the dialog • This was the simplest method for creating a message dialog • But we can: • alter the icon • specify the title bar string JOptionPane.showMessageDialog( null, "The area is " + area, "Result", JOptionPane.PLAIN_MESSAGE);

19. Message dialog types • JOptionPane.ERROR_MESSAGE • Indicates an error to the application user • JOptionPane.INFORMATION_MESSAGE • Displays an informational message - the user simply dismisses the dialog when ready • JOptionPane.WARNING_MESSAGE • Warns the user of a potential problem

20. Message dialog types • JOptionPane.QUESTION_MESSAGE • Poses a question - normally requires a Yes/No response from the user • JOptionPane.PLAIN_MESSAGE • Dialog that simply contains a message with no icon

22. public class Circle { protected double radius; protected void checkRadius(double radius) { if (radius < 0.0) { JOptionPane.showMessageDialog(null, "Radius must not be negative", "Illegal argument", JOptionPane.ERROR_MESSAGE); System.exit(-1); } } public Circle(double radius) { checkRadius(radius); this.radius = radius; } public double area() {return PI*radius*radius; } public double circumference() {return 2*PI*radius; } }

24. More robust Classes • Declare fields as Private or Protected • Protected fields can be accessed by subclasses or members of the same package • Declare public “get” and “set” methods • with appropriate checks on the “set” methods • E.g. public void setRadius(double radius) { checkRadius(radius); this.radius = radius; }

25. public class Circle { public static final double PI = 3.14159; // a constant protected double radius; protected void checkRadius(double radius) { if (radius < 0.0) { throw new IllegalArgumentException("radius must not be negative"); } } public Circle(double radius) { checkRadius(radius); this.radius = radius; } public double getRadius() {return radius;} public void setRadius(double radius) { checkRadius(radius); this.radius = radius; } public double area() {return PI*radius*radius; } public double circumference() {return 2*PI*radius; } }