Chapter 2: Java Fundamentals cont’d

1. Chapter 2: Java Fundamentals cont’d Lecture 5: Wednesday Sept 13, 2006

2. Outline • 2.1 The Parts of a Java Program • 2.2 The print and println Methods, and the Java Standard Class Library • 2.3 Variables and Literals • 2.4 Primitive Data Types • 2.5 Arithmetic Operators • 2.6 Combined Assignment Operators • 2.7 Conversion Between Primitive Types • 2.8 Creating Named Constants with final • 2.9 The String Class • 2.10 Scope • 2.11 Comments • 2.12 Programming Style • 2.13 Reading Keyboard Input • 2.14 Dialog Boxes • 2.15 Common Errors to Avoid

3. Scope • The variable scope is the part of the program that has access to it public class Scope { public static void main(String[] args) { System.out.println(value); // ERROR! int value = 100; } }

4. Scope public class Scope{ public static void main(String[] args){ int number = 100; System.out.println(number); int number = 200; //ERROR } }

5. Comments • Java provides three methods for commenting code.

6. Programming Style • Although Java has a strict syntax, whitespace characters are ignored by the compiler. • The Java whitespace characters are: • space • tab • newline • carriage return • form feed

7. Programming Style public class Compact {public static void main(String[] args){int shares=220; double averagePrice=14.67; System.out.println("There were "+shares+" shares sold at $"+averagePrice+ " per share.");}} Compiles !!!

8. Indentation • Programs should use proper indentation. • Each block of code should be indented a few spaces from its surrounding block. • Two to four spaces are sufficient

9. Programming Style /** This example is much more readable than Compact.java. */ public class Readable { public static void main(String[] args) { int shares = 220; double averagePrice = 14.67; System.out.println("There were " + shares + " shares sold at $" + averagePrice + " per share."); } }

10. Dialog Boxes • A dialog box is a small graphical window that displays a message to the user or requests input. • A variety of dialog boxes can be displayed using the JOptionPaneclass. • Two of the dialog boxes are: • Message Dialog - a dialog box that displays a message. • Input Dialog - a dialog box that prompts the user for input.

11. Using the import Statement • The JOptionPane class is not automatically available to your Java programs. • The following statement must be before the program’s class header: import javax.swing.JOptionPane; • This statement tells the compiler where to find the JOptionPane class.

12. Dialog Boxes The JOptionPane class provides static methods to display each type of dialog box.

13. Message Dialogs • JOptionPane.showMessageDialog method is used to display a message dialog. JOptionPane.showMessageDialog(null, "Hello World"); • The second argument is the message that is to be displayed.

14. Input Dialogs • An input dialog is a quick and simple way to ask the user to enter data. • The dialog displays a text field, an Ok button and a Cancel button. • If Ok is pressed, the dialog returns the user’s input. • If Cancel is pressed, the dialog returns null.

15. Input Dialogs String name; name = JOptionPane.showInputDialog( "Enter your name."); • The argument passed to the method is the message to display. • If the user clicks on the OK button, name references the string entered by the user. • If the user clicks on the Cancel button, name references null.

16. NamesDialog.java import javax.swing.JOptionPane; public class NamesDialog { public static void main(String[] args) { String firstName; // The user's first name String middleName; // The user's middle name String lastName; // The user's last name // Get the user's first name firstName = JOptionPane.showInputDialog("What is " + "your first name? ");

17. NamesDialog.java // Get the user's middle name. middleName = JOptionPane.showInputDialog( "What is " +"your middle name? "); // Get the user's last name. lastName = JOptionPane.showInputDialog("What is " +"your last name? ");

18. Example // Display a greeting JOptionPane.showMessageDialog(null, "Hello " +firstName + " " +middleName +" " + lastName); System.exit(0); } }

19. The System.exit() Method • A program that uses JOptionPane does not automatically stop executing when the end of the mainmethod is reached. • Java generates a thread, which is a process running in the computer, when a JOptionPane is created. • If the System.exit method is not called, this thread continues to execute.

20. The System.exit() Method • The System.exitmethod requires an integer argument. System.exit(0); • This argument is an exit code that is passed back to the operating system. • This code is usually ignored, however, it can be used outside the program: • to indicate whether the program ended successfully or as the result of a failure. • The value 0 traditionally indicates that the program ended successfully.

21. Converting a String to a Number • The JOptionPane’sshowInputDialog method always returns the user's input as a String • String containing a number, such as “127.89, can be converted to a numeric data type.

22. The Parse Methods • Parse methods convert strings to numeric data types • They are: • Byte.parseByte • Integer.parseInt • Short.parseShort • Long.parseLong • Float.parseFloat • Double.parseDouble

23. The Parse Methods- Examples • byte bVar = Byte.parseByte("1"); • int iVar = Integer.parseInt("2599"); • short sVar = Short.parseShort("10"); • long lVar = Long.parseLong("15908"); • float fVar = Float.parseFloat("12.3"); • double dVar = Double.parseDouble("7945.6");

24. PayrollDialog.java import javax.swing.JOptionPane; public class PayrollDialog { public static void main(String[] args) { String inputString; // For reading input String name; // The user's name int hours; // The number of hours worked double payRate; // The user's hourly pay rate double grossPay; // The user's gross pay

25. PayrollDialog.java // Get the user's name. name = JOptionPane.showInputDialog("What is " + "your name? "); // Get the hours worked. inputString = JOptionPane.showInputDialog( "How many hours” + “did you work this week? "); // Convert the input to an int. hours = Integer.parseInt(inputString);

26. PayrollDialog.java // Get the hourly pay rate. inputString = JOptionPane.showInputDialog("What is” +" your hourly pay rate? "); // Convert the input to a double. payRate =Double.parseDouble(inputString); // Calculate the gross pay. grossPay = hours * payRate;

27. PayrollDialog.java // Display the results. JOptionPane.showMessageDialog(null, "Hello " + name + ". Your gross pay is $" + grossPay); // End the program. System.exit(0); } }