COMP 14 Introduction to Programming

1. COMP 14Introduction to Programming Mr. Joshua Stough February 2, 2005 Monday/Wednesday 11:00-12:15 Peabody Hall 218

2. Announcements • Assignment simpler than I thought (darn). • Office hours (I’m lonely). • Precedence of % operator. • Why reference variables?

3. Review Assignment Operators count += 5; // count = count + 5; count -= 5; // count = count - 5; count *= 5; // count = count * 5; count /= 5; // count = count / 5;

4. Review • Primitive Variables • primitive data types (int, double, ...) • stores the data in the memory location • Reference Variables • stores an address in the memory location • "points to" another memory location

5. Today in COMP 14 • Input/Output using GUI • String Tokenization • Format Floating-Point Output • Read from and Write to Files

6. Using Dialog Boxes for I/O • Use a graphical user interface (GUI) • class JOptionPane • Contained in package javax.swing • showInputDialog • allows user to input a string from the keyboard • showMessageDialog • allows the programmer to display results • Program must end with System.exit(0);

7. JOptionPane Methods • showInputDialog str = JOptionPane.showInputDialog(strExpression); • stores what the user enters into the String str • showMessageDialog JOptionPane.showMessageDialog(parentComponent, strExpression, boxTitleString, messageType);

8. showMessageDialog • parentComponent • parent of the dialog box • we'll use null • StrExpression • what you want displayed in the box • boxTitleString • title of the dialog box • messageType • what icon will be displayed

9. messageType • JOptionPane.ERROR_MESSAGE • error icon • JOptionPane.INFORMATION_MESSAGE • information icon • JOptionPane.PLAIN_MESSAGE • no icon • JOptionPane.QUESTION_MESSAGE • question mark icon • JOptionPane.WARNING_MESSAGE • exclamation point icon

10. JOptionPane Example JOptionPane.showMessageDialog (null, "Hello World!", "Greetings", JOptionPane.INFORMATION_MESSAGE);

11. UsingGUI.java example

12. User Input • BufferedReader • reads everything as a string • Integer.parseInt • only handles one integer in the string • How to handle? Enter 3 numbers: 34 15 75

13. The StringTokenizer Class • tokens • elements that comprise a string • tokenizing • process of extracting these elements • delimiters • characters that separate one token from another • StringTokenizer class • defined in the java.util package • used to separate a string into tokens

14. The StringTokenizer ClassTokens and Delimiters "Four score and seven years ago" delimiter: ' ' tokens: "Four" "score" "and" "seven" "years" "ago" "Bart:Lisa:Homer:Marge" delimiter: ':' tokens: "Bart" "Lisa" "Homer" "Marge"

15. The StringTokenizer Class • Default delimiters: • space, tab, carriage return, new line • Methods • StringTokenizer (String str) • StringTokenizer (String str, String delimits) • String nextToken() • boolean hasMoreTokens() • int countTokens()

16. Tokenize.java example • separated by spaces • separated by commas • gotcha

17. Formatting the Output of Decimal Numbers • float: defaults to 6 decimal places • double: defaults to 15 decimal places

18. class DecimalFormat • Import package java.text • Create DecimalFormat object and initialize DecimalFormat fmt = new DecimalFormat (formatString); • FormatString • "0.00" - limit to 2 decimal places, use 0 if there's no item in that position • "0.##" - limit to 2 decimal places, no trailing 0 • Use method format • rounds the number instead of truncating • Result of using DecimalFormat is a String

19. Examples DecimalFormat twoDecimal = new DecimalFormat("0.00"); DecimalFormat fmt = new DecimalFormat("0.##"); System.out.println (twoDecimal.format(56.379)); System.out.println (fmt.format(56.379)); System.out.println (twoDecimal.format(.3451)); System.out.println (fmt.format(.3451)); System.out.println (twoDecimal.format(.3)); System.out.println (fmt.format(.3)); 56.38 56.38 0.35 0.35 0.30 0.3

20. Reading From Text Files • Similar to reading from the keyboard • Create a BufferedReader object, but use a FileReader object instead of InputStreamReader • Create BufferedReader object inside the main method instead of outside • Substitute the name of the file for System.in • When finished reading from the file, we need to close the file: • BufferedReader close() method

21. Exceptions • FileNotFoundException • if the file specified to open was not found • IOException • some other I/O exception public static void main (String[] args) throws FileNotFoundException, IOException

22. Reading From Text Files String file = "data.dat"; BufferedReader inFile = new BufferedReader (new FileReader (file)); String line = inFile.readLine(); inFile.close();

23. Writing To Text Files • Similar to reading from text files • Use FileWriter and PrintWriter instead of FileReader and BufferedReader • PrintWriter • methods include print() and println(), which we use just like those in System.out • Like reading, we need to close the file when we're done • PrintWriter close() method

24. Writing To Text Files String file = "outfile.dat"; PrintWriter outFile = new PrintWriter (new FileWriter (file)); outFile.print ("Hi"); outFile.println(" There!"); outFile.close();

25. FileGUI.java Example

26. Summary • JOptionPane • showInputDialog • showMessageDialog • StringTokenizer • tokens are separated by a delimiter • DecimalFormat • pattern tells how many digits after the decimal point • 0 - fill in with trailing 0s • # - don't fill in trailing 0s

27. Summary • Reading Data from File • BufferedReader • FileReader • readLine() • Writing Data to a File • FileWriter • PrintWriter • print(StrExpression) • println(StrExpression) • Close files after using them

28. Next Time in COMP 14 Reading Assignment Chapter 4 (pgs. 147-164) • Relational and Logical Operators and Expressions