File I/O

1. File I/O Chapter 10

2. Chapter Contents Chapter Objectives 10.1 Introductory Example: Weather Data Analysis 10.2 Java/s I/O System: Readers, Writers, Streams 10.3 Exceptions 10.4 More About I/O Streams 10.5 Example: Scanning for a Virus 10.6 Example: Student Data Retrieval 10.7 Graphical/Internet Java: Student Data Retrieval GUI Part of the Picture: Data Base Systems

3. Chapter Objectives • Study Java's I/O with text and binary files • Examine role of exceptions in I/O • Observe use of files in GUI programs in context of an information retrieval problem • Learn about the role of files in database management systems

4. Need for File Access • Files saved for word processors, spreadsheet programs, etc. • Saved on secondary memory • hard disks, floppy disks, CD-ROMs, etc. • Requirements information stored in secondary memory • can be retrieved in the future • kept separate from other documents, programs, etc.

5. Intro Example: Weather Data Analysis • Problem: large amounts of weather-related data is recorded and processed • year's worth of atmospheric pressure readings • taken every 15 minutes • stored in a text file pressure.dat • Weatherman needs minimum, maximum, and average of the data • Data must be read from file, statistics calculated, results written to a text output file

6. Input file is a static object with readLine()method Solution Overview • Note source code, Figure 10.1 • Names of input and output files are given to the program via the args parameter • input file is args[0] • output file is args[1] • Command to open the input fileBufferedReader in = new BufferedReader ( new FileReader (args[0])); • Reading text from input filevalueString = in.readLine();

7. Solution Overview • Opening text output filePrintWriter out = new PrintWriter ( new BufferedWriter( new Filewriter (args[1]))); • Writing text values to a file out.println ( . . . ); • Closing the filesin.close(); out.close();

8. Handling Exceptions • Many things can go wrong when dealing with files • requested input file does not exist • input file has invalid data • output file unavailable • When such an error occurs, the method in which the problem happened "throws an exception" • Java can "catch" the exception if it happens in a "try block" • Note the try { … } block in the source code

9. Handling Exceptions • A try block is followed by one or more "catch blocks" • Note the source code:catch (Exception anException) { Controller.fatal ( … ) ; } • This catch block will • determine the kind of exception • report the problem • terminate the program

10. Input Stream Output Stream 10.2 Java's I/O System: Readers, Writers, and Streams • All input and output in Java is accomplished by classes called streams • Input streams provide ways to move bytes of data from an input device to a program • Output streams move the data in the opposite direction Executing Program

11. Predefined Streams • System.in • InputStream object, usually for the keyboard • System.out • a buffered PrintStream object, usually the screen or an active window • System.err • an unbuffered PrintStream object usually associated with the screen or console window

12. Wrapping Classes • InputStream class provides methods for reading bytes only • to read at a higher level we must "wrap" System.in with another class • Example: BufferedReader classBufferedReader myReader = new (BufferedReader ( new InputStreamReader( System.in)); • Now we can send myReader either • the .read() message for a single char value or … • the .readLine() message for an entire line of text

13. Using Readers and Writers • Reader and Writer classes provide support for char (16-bit Unicode) I/O • InputStream and OutputStream provide support for byte I/O • The Java language is still growing and "maturing" • extensions of Reader and Writer classes have yet to be fully accepted • author of this text suggests: Use Reader or Writer class whenever possible, use Stream class only when there is no Reader/Writer class that fits

14. Using a Reader • Wrap FileReader in BufferedReader classBufferedReader inFile = new BufferedReader ( new FilReader (input_filename) ); • Now the BufferedReader object (inFile) can be sent a .readLine() message • A BufferedReader is so named because it buffers the input • this improves program performance

15. Using a Reader • How to know when all the data in a file has been read? • the readLine() method returns the value nullvalueString = inFile.readLine():while (valueString != null) { … // process valueString valueString = inFile.readLine(); } • Also possible to check for other things such as an empty line with no textif (valueString.equals("")) continue;

16. Using a Reader Sequence of steps for using text input • Build a BufferedReader by wrapping one around a FileReader • Use an input loop to read lines of text from the BufferedReader • Convert those lines to numeric values if necessary • Close the BufferedReader

17. Using a Writer • When writing values to a text file, author suggests use a Writer class • increase FileWriter capabilities by wrapping it in BufferedWriter and PrintWriter classes PrintWriter outFile = new PrintWriter(new BufferedWriter ( new FileWriter (file_name) ) );

18. 10.3 Exceptions • Easy for things to go wrong in a program • faulty logic • bad input from user, etc. • Java classifies all these as "exceptional occurrences" • provides Exception class • tells something abnormal occurred

19. Exception thrown here Program searches catch blocks looking for type match for exception thrown When match found, that block of statements executed Try-Catch Blocks • To signal an abnormal occurrence • method can throw an exception • throw occurs inside a try block • try block followed by one or more catch blocks Form: try { … statement list … } catch (Exception_Type1 variable_name1){ … } catch (Exception_type2 variable_name2){ … } finally { … }

20. Try-Catch Blocks • After catch block executed • execution continues in finally { } block or … • if no finally { }, continues to next statements following catch { } • If no matching exception type is found • program terminates • If no exception thrown • catch blocks skipped • execution continues in finally { } or in statements following catch block

21. Most general class Exceptions become more specialized Exception Hierarchy

22. Try/Catch Strategy • Wrap all calls to methods that throw exceptions in single try block • followed by single catch block .. or … • followed by sequence of catch blocks, one for each type of exception • Wrap each call to a method that throws an exception in its own try block • a catch block exists for each particular exception thrown

23. Throwing Exceptions • Method that throws an exception must explicitly state this in its heading public void read(BufferedReader aReader) throws EOFException • Now calling method must invoke this method using try-catch blocks to handle the potential exception

24. 10.4 More About I/O Streams • Data can be stored in a file as text or in binary format • same format used to store values in memory • takes less space to store an integer in binary format than in text format • Java Stream classes provide byte level I/O • FileOutputStream • FileInputStream • Also BufferedOutputStream and BufferedInputStream

25. Why does contents of numbers.dat appear to be gibberish? Writing Primitive types • Java provides DataOutputStream and DataInputStream classes • Note methods provided, Table 10.1 • note number of bytes used to store the data • Consider Data Stream Demonstration, Figure 10.3 in text – note the features … • wrapping of Stream I/O classes • use of try-catch blocks • specific methods used to write, then read • sample run

26. Writing Sequence of Values • Consider a group of values of different types that are written to a file • they must be read in the same order that they were written • Possible to create a class • that group of values is the private data of the class • specific methods are provided for an object of the class to write/read those values to/from a file • the read and write methods receive the file handles as parameters

27. 10.5 Example: Scanning for a Virus • What is a virus? • software that hides itself within other executable programs • that executable program is the "host" for the virus • the virus program tries to "infect" other programs • the virus can cause problems that range from annoying to malicious • A virus that prints an annoying message stores that string value within itself • we will write a program that searches through a file to find that text, or signature

28. Virus-Detection Algorithm • Repeat the following: • read a line from a file • if no line read due to EOF, terminate • otherwise scan the line for specified string of text • if that text found, stop reading from file • Display message whether or not target string was found – signifying virus found/not found • Note source code, Figure 10.4

29. Simulated Virus Scanning • Note use of command line arguments • first argument is the name of the file to be read • second argument is the target string or virus signature • Observe other features studied in this chapter • use of try/catch • wrapping of classes for file read

30. 10.6 Example: Retrieving Student Information • Problem: Information Retrieval Records of information about students consist of • studentID, a 9 digit integer • first name, last name as strings • studentYear as a string • credits and GPA as reals • We seek a program which will • read a sequence of students from students.txt • prompt for studentID, read from keyboard • search for match, display if found

31. Objects

32. Student Class • Operations needed include • initialize itself with default values • initialize itself with explicitly supplied values • read its attributes from a BufferedReader & store them in itself • convert itself to a String (for output) • access each of its attributes • mutate any of its attributes • Note methods provided for these capabilities in source code of Figure 10.5

33. Program Algorithm • Build students as a StudentSequence • initialized from an input file • name of file received in args[0] • Repeatedly do the following: • prompt for and read studentID • search students for target studentID, return its position • if search successful display student at positionotherwise display error message

34. Class StudentSequence • Note source code, Figure 10.6 • Reads numberOfStudents as first value in the file • Constructor declares myStudents, an array of Student • Use numberOfStudents as limiting value in for loop • stores incoming values in myStudents array • Provides linear search method, find()

35. Student Information Retrieval • Note source code, Figure 10.7 • Program declares students, object of type StudentSequence • constructor loads values from file • Forever loop used to • prompt for, receive as input student id • call .find method to determine position of that student in the array • print results (found or not)

36. GUIStudentRetriever Bottom portion not visible until id entered and processed Enter Student ID : Bill Board Alternatively, a "not found" message would appear here Credits GPA Freshman 16.0 3.15 10.7 Graphical/Internet Java:Student Data Retrieval GUI • Implement student data retrieval problem internet GUI context • Much of work already done can be reused • Behavior 111223333

37. Implementation • Methods for GUIStudentRetreiver • constructor • action handler • enter initial state • enter found an ID state • enter not found state • Attributes • labels • entry fields • panes for panels

38. Event-Handler Method public void actionPerformed(ActionEvent event) { String idString = myIdField.getText(); if (idString.equals("")) enterInitialState(); else{ int id = Integer.parseInt(idString); int position = myStudents.find(id); if (position >= 0) enterGoodIDState(myStudents.getStudent(position)); else enterErrorState("No student found with that ID"); } } Note source code, Figure 10.8

39. Part of the Picture:Database Systems • Businesses must make decisions • Based on data • accuracy and detail help make better decisions • large data sets stored on computers • Data sets maintained by Database Management Systems (DBMS)

40. DBMS Facilities • High level views of the data • Access routines • query language • Support for large databases • as large as 1000 gigabytes (one terabyte) • Security • data is sensitive or proprietary • Data sharing • protect data when more than one person wishes to access/ modify same piece of data • Data integrity

41. Name of Relation Retrieve All records Condition Relational Model • Database viewed as a set of tables, called relations • one row for each entry • column for each attribute or field of the database • SQL, "Structured Query Language" • provides commands for manipulate data • ExampleSELECT * FROM Employee WHERE Rate = 10.5