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For Friday

For Friday. Finish reading chapter 9 WebCT quiz 17. Program 6. Any questions?. Handling Exceptions. Files. What are files? Why are they important?. File Processing Programs. What needs to be accomplished in a file processing program?. The Scanner Class. Reading. Lines of text

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For Friday

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  1. For Friday • Finish reading chapter 9 • WebCT quiz 17

  2. Program 6 • Any questions?

  3. Handling Exceptions

  4. Files • What are files? • Why are they important?

  5. File Processing Programs • What needs to be accomplished in a file processing program?

  6. The Scanner Class

  7. Reading • Lines of text • Single words • Integers • Doubles • Etc.

  8. Opening a File • Creating the File object. • The FileNotFoundException

  9. import java.io.File; import java.io.FileNotFoundException; import java.util.Scanner; /** Read census data, printing all those lines containing the string "AK" (Alaska). * * @author Byron Weber Becker */ public class ReadCountyData { public static void main(String[ ] args) { // Open the file. Scanner in = null; try { File file = new File("2000US_County_data.txt"); in = new Scanner(file); } catch (FileNotFoundException ex) { System.out.println(ex.getMessage()); System.out.println("in " + System.getProperty("user.dir")); System.exit(1); }

  10. // Read and process each record. while (in.hasNextLine()) { String record = in.nextLine(); if (record.indexOf("AK") >= 0) // records containing “AK” (Alaska) { System.out.println(record); } } // Close the file. in.close(); } }

  11. Writing Files • Writer classes • Each classes has a purpose • FileWriter – connects to a file • BufferedWriter – makes output more efficient • PrintWriter – lets you use print and println

  12. Using the FileWriters FileWriter fw = new FileWriter(fileName); BufferedWriter bw = new BufferedWriter(fw); PrintWriter out = new PrintWriter(bw); out.println(“the first line”);

  13. Alternative PrintWriter out = new PrintWriter (new BufferedWriter (new FileWriter(fileName))); out.println(“the first line”);

  14. Notes • Using files always potentially throws IOExceptions. • Sometimes we can handle these gracefully. • Sometimes we need to allow the program to report an error and end.

  15. File Practice • Write code to read a file of integers and write a new file with one integer per line of the new file.

  16. File Practice • The files input1.txt and input2.txt each contain a list of numbers of type int in ascending order. Write a program to create a new file, output.txt, that contains all of the numbers from both input files in sorted order. Use a function that will accept as parameters any two open input streams and an open output stream.

  17. Objects and Records • Approaches to reading objects • Approaches to writing objects • Issues to deal with • New lines • Multiple word fields

  18. The File Class • Contains a number of methods • Useful for answering questions about files

  19. Interacting with the User • How is this similar to files? • How is it different?

  20. System.in

  21. Practice Problem • Write Java code to ask for a student’s exam score out of 100. Your program is then to match the exam score to a letter grade and print the grade to the screen. The letter grade is to be calculated as follows: 90 and above A 80-89 B 70-79 C 60-69 D below 60 F

  22. Switch Practice • A program is required to read a customer’s name, a purchase amount and a tax code. The tax has been validated and will be one of the following: 0 tax exempt (0%) 1 state sales tax only (3%) 2 federal and state sales tax (5%) 3 special sales tax (7%)The program must then compute the sales tax and the total amount due and print the customer’s name, purchase amount, sales tax and total amount due.

  23. Practice • Design and write a program that will prompt for, receive, and total a collection of payroll amounts entered by the user until a sentinel amount of -99 is entered. After the sentinel has been entered, display the total payroll amount on the screen.

  24. Problem 3 • Write a program that sums a sequence of integers. Assume that the first integer read specifies the number of values remaining to be entered. Your program should read only one value at a time. A typical input sequence might be 5 100 200 300 400 500

  25. Problem 4 • Write a program that finds the smallest of several integers. Assume that input will end when a sentinel value of –999 is read. Do not count –999 as one of the integers to consider.

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