CP122 CS I. Chapter 11: File I/O and Exceptions

Transcription

1 CP122 CS I Chapter 11: File I/O and Exceptions

2 Waymo full autonomy vehicles Tech News!

3 Tech News! Waymo full autonomy vehicles CMU and Pitt researchers use AI with fmri to detect suicidal thoughts with 90% accuracy

4 Chapter 11: File I/O and Exceptions How can our programs interact with files stored on the computer's hard drive? What should our programs do when encountering errors (exceptions)?

5 Getting Input Scanner myscanner = new Scanner(System.in); String line = myscanner.nextline();

6 Getting Input Scanner myscanner = new Scanner(System.in); String line = myscanner.nextline(); We used System.in to get keyboard input from the user

7 Producing Output System.out.println( Hello, y'all! );

8 Producing Output System.out.println( Hello, y'all! ); Our programs produce output to the screen using System.out.println

9 Using Files Instead of using keyboard input and screen output we can use files instead Reading: the process of getting data from a file Writing: the process of putting data into a file

10 Application 1: Processing Heart Rates

11 File Format

12 File Format Column 1: time in seconds since beginning of recording. Records about every 3 seconds.

13 File Format Column 2: current heart rate in beats per minute.

14 File Input File inputfile = new File( heartrates.txt ); Scanner myscanner = new Scanner(inputFile);

15 File Input File inputfile = new File( heartrates.txt ); Scanner myscanner = new Scanner(inputFile); Create a new File object using a String containing the filename.

16 File Input File inputfile = new File( heartrates.txt ); Scanner myscanner = new Scanner(inputFile); Create a new Scanner using the File object instead of System.in

17 File Input File inputfile = new File( heartrates.txt ); Scanner myscanner = new Scanner(inputFile); while (myscanner.hasnextint()) { int seconds = myscanner.nextint(); int heartrate = myscanner.nextint();... }

18 File Input File inputfile = new File( heartrates.txt ); Scanner myscanner = new Scanner(inputFile); while (myscanner.hasnextint()) { int seconds = myscanner.nextint(); int heartrate = myscanner.nextint();... } Use the Scanner just like you did before!

19 HeartRate.java

20 Handling Exceptions try { File inputfile = new File( heartrates.txt ); Scanner myscanner = new Scanner(inputFile);... } catch (FileNotFoundException e) { System.out.println( ERROR: + e); }

21 Handling Exceptions try { File inputfile = new File( heartrates.txt ); Scanner myscanner = new Scanner(inputFile);... } A new Java keyword: try. catch (FileNotFoundException e) { System.out.println( ERROR: Try to execute a block + of e); code. }

22 Handling Exceptions try { File inputfile = new File( heartrates.txt ); Scanner myscanner = new Scanner(inputFile);... } catch (FileNotFoundException e) { System.out.println( ERROR: + e); } A new Java keyword: catch. If an error (exception) occurred in the try-block, then handle it here.

23 Handling Exceptions try { File inputfile = new File( heartrates.txt ); Scanner myscanner = new Scanner(inputFile);... } catch (FileNotFoundException e) { System.out.println( ERROR: + e); } There are different kinds of exceptions. In this case, watch out for the FileNotFoundException.

24 Handling Exceptions try { File inputfile = new File( heartrates.txt ); Scanner myscanner = new Scanner(inputFile);... } catch (FileNotFoundException e) { System.out.println( ERROR: + e); } If a FileNotFoundException occurred, then create an object (called e) to store its information.

25 Handling Exceptions All the code between these curly braces is executed when there is an try { File inputfile exception. = new File( heartrates.txt ); Scanner myscanner = new Scanner(inputFile);... } catch (FileNotFoundException e) { System.out.println( ERROR: + e); }

26 Handling Exceptions try { File inputfile = new File( heartrates.txt ); Scanner on myscanner the screen = using new Scanner(inputFile); println.... } catch (FileNotFoundException e) { System.out.println( ERROR: + e); } In this case, the error is just displayed

27 Passing Exceptions Along with throws public void readheartrate() throws FileNotFoundException { File inputfile = new File( heartrates.txt ); Scanner myscanner = new Scanner(inputFile);...

28 Passing Exceptions Along with throws public void readheartrate() throws FileNotFoundException { File inputfile = new File( heartrates.txt ); Scanner myscanner = new Scanner(inputFile);... throws is another new Java keyword. Doesn't handle the exception...instead passes the burden up to the method that called this method.

29 Passing Exceptions Along with throws try { readheartrate(); } catch (FileNotFoundException e) { System.out.println( ERROR: + e); } The calling method should then have a try-catch block surrounding the method call in order to handle any possible exception.

30 Software Engineering Tip Always try-catch any exceptions your code may generate unless there is a highly compelling reason not to % of the time use try-catch

31 Generating Exceptions When Things Go Wrong if (heartrate < 0) { throw new IllegalArgumentException( HR can't be negative! ); }

32 Generating Exceptions When Things Go Wrong if (heartrate < 0) { throw new IllegalArgumentException( HR can't be negative! ); } New Java keyword: throw. Generate a new exception because something went wrong in the program.

33 Generating Exceptions When Things Go Wrong if (heartrate < 0) { throw new IllegalArgumentException( HR can't be negative! ); } IllegalArgumentException is used for illegal values that are passed into a method. E.g. a negative heartrate.

34 Generating Exceptions When Things Go Wrong if (heartrate < 0) { throw new IllegalArgumentException( HR can't be negative! ); } This String is just a description of the error.

35 Keyword Soup catch: an exception has occurred; do something about it. Must be paired with a try block

36 Keyword Soup catch: an exception has occurred; do something about it. Must be paired with a try block throws: used by a method to ignore an exception and let the calling method handle it

37 Keyword Soup catch: an exception has occurred; do something about it. Must be paired with a try block throws: used by a method to ignore an exception and let the calling method handle it throw: generate a new exception because something went wrong in the program

38 Writing Data into a New File PrintWriter mywriter = new PrintWriter( output.txt ); mywriter.write( Hello y'all! );

39 Writing Data into a New File PrintWriter mywriter = new PrintWriter( output.txt ); mywriter.write( Hello y'all! ); Use the PrintWriter class to put data into a new file.

40 Writing Data into a New File PrintWriter mywriter = new PrintWriter( output.txt ); mywriter.write( Hello y'all! ); PrintWriter's constructor takes a String filename as input.

41 Writing Data into a New File PrintWriter mywriter = new PrintWriter( output.txt ); mywriter.write( Hello y'all! ); If this file doesn't exist, it is created. If it does exist, the old data is overwritten. Be careful here!

42 Application 2: Word frequency counts in Wikipedia

43 Algorithm Open the text file containing all of Wikipedia Process the file one line at a time Split the line into tokens Check for desired word and increase count if found

44 WikiReader.java