Announcements

1. Announcements • Program 5 Milestone 1 was due today • Program 4 has been graded

2. Today in COMP 110 • Review Exceptions • Basic File I/O • Programming Demo

3. Review • Exceptions

4. Exceptions • An exception is an object that signals the occurrence of an unusual (exceptional) event during program execution • Exception handling is a way of detecting and dealing with these unusual cases in principled manner i.e. without a run-time error or program crash

5. The try Block • A try block contains the basic algorithm for when everything goes smoothly • Try blocks will possibly throw an exception • Syntax try { Code_To_Try } • Example try { average = scoreSum/numGames; }

6. The catch Block • The catch block is used to deal with any exceptions that may occur • This is your error handling code • Syntax catch(Exception_Class_Name Catch_Block_Parameter) { Process_Exception_Of_Type_Exception_Class_Name } Possibly_Other_Catch_Blocks • Example catch(ArithmeticException e) { System.out.println(e.getMessage()); System.out.println("Cannot compute average for 0 games"); }

7. Example • Using Exception Handling (try/catch blocks) int score = keyboard.nextInt(); int scoreSum = 0; int numGames = 0; while(score >= 0) { scoreSum += score; numGames++; score = keyboard.nextInt(); } double average = 0; try { average = scoreSum/numGames; } catch(ArithmeticException e) { System.out.println(e.getMessage()); System.out.println("Cannot compute average for 0 games"); }

8. Files • Your music, pictures, videos, even your Java programs are stored on your computer in files • Files can also be used to store input for a program, or a program’s output

9. Streams • Writing to & reading from files is done using a stream • A stream is a flow of data • This data might be characters, numbers or bytes of binary digits • Data that flows INTO your program is called an input stream • Data that flows OUT of your program is called an output stream

10. Streams Input Stream Output Stream Monitor Keyboard Input Stream Output Stream Hard drive Program CD

11. Stream Class • In Java, streams are objects of special stream class • Scanner objects are input streams • We’ve used the Scanner class to read data from the keyboard • System.out is an output stream • We use it to print data out to screen

12. File I/O • File I/O stands for File Input/Output • Why use files for input/output? • Permanent data storage • Easy to read in large amount of data • We can also read it in repeatedly • Easy to output large amounts of data that can be analyzed later

13. Text Files vs Binary Files • All files are stored as binary digits (bits) • In some cases this data is interpreted as text (text files) • Your Java files • Text files can be easily read/edited by humans • All other files are binary files • Your music & picture files • Binary files cannot be easily read/edited by humans

14. Creating a Text File • The PrintWriter class is provided by Java to aid in creating and writing text files • Need to import from java.io • Before we can write to a text file, we need to connect to an output stream • This is essentially opening the file, which allows us to write to it • All files have a name, such as out.txt, that we use when opening the file

15. Opening a Text File //need to import java.io.PrintWriter //& java.io.FileNotFoundException String fileName = "out.txt"; PrintWriter outputStream = null; try { outputStream = new PrintWriter(fileName); } catch(FileNotFoundException e) { System.out.println("Error opening file " + fileName); System.exit(0); }

16. Opening a Text File outputStream = new PrintWriter(fileName); • Calls the constructor of the PrintWriter class • Opens the text file with the name fileName ("out.txt") • If the file already exists, its contents are overwritten • If the file doesn’t exist, an empty file with that name is created • Since the constructor might throw a FileNotFoundException, we must enclose it in a try block • Also need a corresponding catch block to catch the exception

17. Writing to a Text File • Once the file is open, we can write to it • The PrintWriter class has methods print & println that work just like methods in System.out • Data is written to the file instead of to screen • Calls to these methods do not have to be within a try block outputStream.println("I’m writing to a file!"); outputStream.print("Another message!");

18. Buffering • When you write to a file, the data may not immediately reach its destination • This is because of buffering • The output stream will wait until it has collected a large amount of data to write before it writes anything to the file itself • This is done for efficiency

19. Closing a Text File • Once you’re finished writing to the file you should disconnect the stream from the file itself • This is done using the close method outputStream.close(); //close the file • Calling the close() method ensures that any remaining data is written out to the file

20. Example: Writing to a File import java.io.*; public class TextFileOutput { public static void main(String[] args) { String fileName = "out.txt"; PrintWriter outputStream = null; try { outputStream = new PrintWriter(fileName); } catch(FileNotFoundException e) { System.out.println("Error opening file " + fileName); System.exit(0); } //print the numbers 0-9 to the file one on each line for(int i = 0; i < 10; i++) { outputStream.println(i); } outputStream.close(); //close the file } }

21. Summary: Writing to a File • Open the file • Create a PrintWriter object • Pass the name of the file you want to write to the constructor • Use try/catch blocks to catch a possible FileNotFoundException • Write to the file • Use the print/println methods of the PrintWriter object you created • Close the file

22. Reading from a Text File • We can read from a text file using an object of the Scanner class • Recall, we have used the scanner class to read input from the keyboard, as in: Scanner keyboard = new Scanner(System.in); • We can create a scanner object to read from a file in the following way String fileName = "in.txt"; Scanner inputFile = new Scanner(new File(fileName));

23. Opening a File for Reading • The Scanner class constructor can also throw a FileNotFoundException • We must enclose it in a try block String fileName = "in.txt"; Scanner inputFile = null; try { inputFile = new Scanner(new File(fileName)); } catch(FileNotFoundException e) { System.out.println("Error opening file " + fileName); System.exit(0); }

24. Reading from a Text File • All methods of the Scanner class we have used previously can also be used to read from a text file • nextInt(), nextDouble(), nextLine(), etc • The Scanner class also has methods to determine whether more input data remains in the file • hasNext(), hasNextDouble(), hasNextInt(), hasNextLine() etc.

25. Read a File & Print to Screen import java.util.Scanner; import java.io.*; public class TextFileInput { public static void main(String[] args) { String fileName = "in.txt"; //the name of the file we want to open Scanner inputFile = null; try { inputFile = new Scanner(new File(fileName)); //open the file } catch(FileNotFoundException e) { System.out.println("Error opening file " + fileName); System.exit(0); } while(inputStream.hasNextLine()) { String line = inputStream.nextLine(); //read a line of text from the file System.out.println(line); //print the line of text to screen } inputFile.close(); //close the file } }

26. Closing an Input File • One you’re finished reading from a text file, you should close the stream • Allowing you to write to it later etc • This is done using the close method inputFile.close();

27. Summary: Reading from a File • Open the file • Create a Scanner object • Use try/catch blocks to catch a possible FileNotFoundException • Read from the file • Use the methods of the Scanner object you created • Close the file

28. The Class File • Java provides the class File as a way of representing file names • A string such as "out.txt" may be a file name, but Java treats it as any other String object • Passing "out.txt" to the constructor of the class file allows us to treat this as a file name in Java

29. Using the Class File • The class File has a constructor that takes in the name of the file • Example File outFile = new File("out.txt"); File inFile = new File("in.txt");

30. Using the Class File • The class File also defines several useful methods for working w/ files • public boolean canRead() • Tests whether the program can read from the file • public boolean canWrite() • Tests whether the program can write to the file • public boolean delete() • Attempts to delete the file. Returns true on success • public boolean exists() • Tests whether the file currently exists • public String getName() • Returns the name of the file • public String getPath() • Returns the path name of the file • public long length() • Returns the length of the file in bytes

31. Using Path Names • When specifying a file name such as "out.txt", the file is assumed to be in the same directory as your program • We can refer to a file in a different directory using a path name instead of just the file name • Example • "C:\\COMP110\\out.txt"

32. Using Path Names • A full path name is a complete path name starting at the root directory • e.g. "C:\\COMP110\\out.txt" • A relative path name is a path to the file starting at the directory containing your program • e.g. "files\\out.txt"

33. Using Path Names • Why use two backslashes (\\) when specifying file paths in Java? • e.g. "C:\\COMP110\\out.txt" • Recall that backslash is the escape character in Java • '\n' – newline, '\t' – tab, etc • "\\" in a string means a single backslash

34. Using File Paths • To get around having to use two backslashes, we can use UNIX-style file paths • e.g. "C:/COMP110/out.txt" • This works on both Windows and UNIX!

35. File Names • What if we don’t know the name of the file when writing the program? • Ask the user for the name of the file!

36. Programming Demo • Write a program that searches a file of numbers and displays the largest number, smallest number and average of all numbers in the file • Write the statistics out to a separate file • Ask the user for the names of the input/output files

37. Programming Demo • Programming

38. Friday • Recitation • Short lab • Work on Program 5