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Chapter 6 Using Classes: Dates and I/O Streams

Chapter 6 Using Classes: Dates and I/O Streams. Date, DateFormat, GregorianCalendar Classes Reading from Keyboard with Input Stream Using Streams to Read and Write Text Files Database Management Systems. Classes: Uses in Java. Package related group of methods

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Chapter 6 Using Classes: Dates and I/O Streams

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  1. Chapter 6Using Classes: Dates and I/O Streams Date, DateFormat, GregorianCalendar Classes Reading from Keyboard with Input Stream Using Streams to Read and Write Text Files Database Management Systems

  2. Classes: Uses in Java • Package related group of methods • Example: Math class • Use methods with name of the class • int x = Math.abs(y); • Make a new data type • Example: Turtle class • Make an object of the class • Turtle myTurtle = new Turtle(); • Use methods with objects • myTurtle.move(100); Programming and Problem Solving With Java

  3. Classes: Why Use? • Make Java extensible • Can add new features to the language • Can build on existing features, or start from scratch • Example: String class • String not primitive in Java • Works almost like a primitive type • Many standard classes • Dates  User Interface  Exceptions • Files  Networking  Databases • Applets  Data storage  Electronic • Threads  Random numbers commerce Programming and Problem Solving With Java

  4. Classes: Why Use? • Classes form basis of program development • Given problem, Java programmer should • Look for classes appropriate to the problem domain • If there are no existing classes • Extend existing classes, or • Write new classes from scratch • Classes are essence of object-oriented programming Programming and Problem Solving With Java

  5. Date Classes: Date • Three of Java's many Dateclasses • Date • DateFormat • GregorianCalendar • The Date class • Date object represents a particular point in time • To create an object of the class • Date aDate = new Date(); • Date constructor initializes with today's date and time new operator makes object Constructor Programming and Problem Solving With Java

  6. Date Classes: DateFormat • Display Date object • Date aDate = new Date(); • System.out.println(aDate); • Mon Apr 20 16:49:32 CST 1998 • DateFormat: format Date object for display • DateFormat is abstract: can't create objects with new • To create a DateFormat object • DateFormat aDateFormatter = DateFormat.getDateInstance(); • Use format() method to format a Date object • Date aDate = new Date(); • DateFormat aDateFormatter = DateFormat.getDateInstance(); • String aString = aDateFormatter.format(aDate); • 20-Apr-1998 Programming and Problem Solving With Java

  7. Date Classes: DateFormat • Four date formats • Specify format in DateFormat.getDateInstance() • Date aDate = new Date(); • DateFormat aDateFormatter • = DateFormat.getDateInstance(DateFormat.FULL); • String aString = aDateFormatter.format(aDate); • Monday, April 20, 1998 • Can specify locale in DateFormat.getDateInstance() • DateFormat aDateFormatter • = DateFormat.getDateInstance(Locale.FRANCE); Programming and Problem Solving With Java

  8. Date Classes: GregorianCalendar • Use GregorianCalendar object to set day • // Example of GregorianCalendar class. This program initializes a • // GregorianCalendar object to October 15, 1999, converts the object • // to a Date object, then displays the result with DateFormat. • import java.util.GregorianCalendar; • import java.util.Calendar; • import java.text.DateFormat; • import java.util.Date; • class SetDate • { • public static void main(String[] args) • { • // Make a DateFormat object that formats in FULL style • DateFormat aDateFormatter • = DateFormat.getDateInstance(DateFormat.FULL); • // Initialize aGregCal to October 15, 1999 • GregorianCalendar aGregCal • = new GregorianCalendar(1999, Calendar.OCTOBER, 15); • // Convert aGregCal to aDate object • Date aDate = aGregCal.getTime(); • // Display the date with the formatter • System.out.println(aDateFormatter.format(aDate)); • } • } Programming and Problem Solving With Java

  9. Date Classes: Discarding Objects • Create new GregorianCalendar object, assign to variable • GregorianCalendar someDate • = new GregorianCalendar(1999, Calendar.MAY, 12); • Create another object, assign to same variable • someDate = new GregorianCalendar(1998, Calendar.APRIL, 20); • Garbage collection: Clean up discarded objects Programming and Problem Solving With Java

  10. Input Streams • Stream is flow of data • Reader at one end • Writer at the other end • Stream generalizes input & output • Keyboard electronics different from disk • Input stream makes keyboard look like a disk Stream Writer Reader Programming and Problem Solving With Java

  11. What's the Keyboard class hiding? The System..in stream! System.in Input Streams: Keyboard Class • Have been using Keyboard class for reading • int num = Keyboard.readInt("Enter number: "); Programming and Problem Solving With Java

  12. Input Streams: System.in • System.in: the standard input stream • By default, reads characters from the keyboard • Can use System.in many ways • Directly (low-level access) • Through layers of abstraction (high-level access) System.in Program Programming and Problem Solving With Java

  13. Input Streams: Read Characters • Can read characters from System.in with read() • // Reads a single character from the keyboard and displays it • class DemonstrateRead • { • public static void main(String[] args) • throws java.io.IOException • { • char character; • // Prompt for a character and read it • System.out.print("Enter a character: "); • System.out.flush(); • character = (char) System.in.read(); • // Display the character typed • System.out.println(); • System.out.println("You typed " + character); • } • } Programming and Problem Solving With Java

  14. Input Streams: Read Characters • System.in.read() returns an integer • Usually cast to char • char character = (char) System.in.read(); • System.in.read returns -1 if EOF detected • EOF = end of file (no more characters in stream) • Signaling EOF at keyboard • Control-Z (Windows), Control-D (Unix) • Example: Read all characters in stream until EOF • int intChar = System.in.read(); • while (intChar != -1) • { • // Convert to character • char character = (char) intChar; • System.out.println("Next character is " + character); • // Get next one • intChar = System.in.read(); • } Programming and Problem Solving With Java

  15. Input Streams: Read Characters • Example: Count digits, letters, other characters • Part 1: Setup • // Reads text from the keyboard and displays the • // number of digits, upper case letters, lower case letters, • // and other characters that the user typed. • class CountCharacters • { • public static void main(String[] args) • throws java.io.IOException • { • int nextValue, numUpperCase = 0, numLowerCase = 0, • numDigits = 0, numOther = 0; • char nextChar; • // Display instructions • System.out.println("Enter some text, terminate with EOF"); Programming and Problem Solving With Java

  16. Input Streams: Read Characters • Part 2: Read and count • // Read from the input stream, count characters until no more • // characters in the input stream • nextValue = System.in.read(); • while (nextValue != -1) • { • nextChar = (char) nextValue; • if (Character.isDigit(nextChar)) • { • numDigits++; • } • else if (Character.isUpperCase(nextChar)) • { • numUpperCase++; • } • else if (Character.isLowerCase(nextChar)) • { • numLowerCase++; • } • else • { • numOther++; • } • nextValue = System.in.read(); • } Programming and Problem Solving With Java

  17. Input Streams: Read Characters • Part 3: Display results • // Display results • System.out.println(); • System.out.println(); • System.out.println("Number of digits: " • + numDigits); • System.out.println("Number of upper case letters: " • + numUpperCase); • System.out.println("Number of lower case letters: " • + numLowerCase); • System.out.println("Number of other characters: " • + numOther); • } • } Programming and Problem Solving With Java

  18. Input Streams: Read Strings • No String-reading methods in System.in • To read strings from keyboard • First wrap System.in inside InputStreamReader object • InputStreamReader anInputStreamReader • = new InputStreamReader(System.in); Programming and Problem Solving With Java

  19. Input Streams: Read Strings • Next, wrap InputStreamReader object in BufferedReader object • InputStreamReader anInputStreamReader • = new InputStreamReader(System.in); • BufferedReader inStream = new BufferedReader(anInputStreamReader); Programming and Problem Solving With Java

  20. Input Streams: Read Strings • Can combine these two statements • BufferedReader inStream • = new BufferedReader(new InputStreamReader(System.in)); • InputStreamReader & BufferedReader in java.io.* • Must import java.io.*; • Skeleton for reading • import java.io.*; • class ClassName • { • public static void main(String[] args) • throws java.io.IOException • { • // Create a buffered input stream and attach it to standard • // input • BufferedReader inStream • = new BufferedReader(new InputStreamReader(System.in)); • ... • } • } Programming and Problem Solving With Java

  21. Input Streams: Read Strings • Methods in BufferedReader • read(): Use same as System.in.read() • readLine(): Returns complete line typed by user • Example: Read user's name • // Reads a user's first name, middle initial, • // and last name. Demonstrates use of InputStreamReader, • // BufferedReader and the readLine() method. • import java.io.*; • class ReadInputAsString • { • public static void main(String[] args) • throws java.io.IOException • { • String firstName, lastName; • char middleInitial; • // Create an input stream and attach it to the standard • // input stream • BufferedReader inStream • = new BufferedReader(new InputStreamReader(System.in)); Programming and Problem Solving With Java

  22. Input Streams: Read Strings • Example: continued • // Read a line from the user as a String • System.out.print("Enter your first name: "); • System.out.flush(); • firstName = inStream.readLine(); • // Read a character from the user • System.out.print("Enter your middle initial and last name: "); • System.out.flush(); • middleInitial = (char) inStream.read(); • // Read a line from the user as a String • lastName = inStream.readLine(); • // Display the strings • System.out.println(); • System.out.println("Your name is " + firstName + " " • + middleInitial + ". " + lastName); • } • } Enter your first name: Linda Enter your middle initial and last name: EJones Your name is Linda E. Jones Programming and Problem Solving With Java

  23. To read a number from keyboard (int, double, ...) Define a NumberFormat object Define BufferedReader object Use BufferedReader object to Read response as a string Use NumberFormat object to parse string into Number object Convert Number object to primitive type Input Streams: Read Numbers Programming and Problem Solving With Java

  24. Input Streams: Read Numbers • Code to read a number from keyboard (int, double, ...) • Define a NumberFormat object • NumberFormat aNumberFormatter = NumberFormat.getInstance(); • Define BufferedReader object • BufferedReader inStream • = new BufferedReader(new InputStreamReader(System.in)); • Read response as a string • System.out.print("Enter an integer: "); • System.out.flush(); • String response = inStream.readLine(); • Use NumberFormat object to parse string into Numberobject • Number aNumberObject = aNumberFormatter.parse(response); • Convert Number object to primitive type • int intNumber = aNumberObject.intValue(); Programming and Problem Solving With Java

  25. Input Streams: Read Numbers • Can combine reading, parsing, conversion steps • import java.io.*; • import java.text.NumberFormat; • class ReadAnInt2 • { • public static void main(String[] args) • throws java.io.IOException, java.text.ParseException • { • // Create an input stream and attach it to the standard • // input stream • BufferedReader inStream • = new BufferedReader(new InputStreamReader(System.in)); • // Create a number formatter object • NumberFormat aNumberFormatter = NumberFormat.getInstance(); • // Prompt for input • System.out.print("Enter an integer: "); • System.out.flush(); • // Read the response from the user, convert to Number, • // then convert to int • int intNumber • = aNumberFormatter.parse(inStream.readLine()).intValue(); • // Display the value • System.out.println("You typed " + intNumber); • } • } Note ParseException! Programming and Problem Solving With Java

  26. Input Streams: Multiple Values • To read severalvalues on one line • Use StringTokenizer object • Breaks onestring into parts • Must still convertnumbers (ifnecessary) Programming and Problem Solving With Java

  27. Input Streams: Multiple Values • How to use StringTokenizer • Get string to break apart somehow • Initialize object with string to break apart • StringTokenizer tokenizer = new StringTokenizer("42 58"); • Use nextToken() method to get parts • String token; • token = tokenizer.nextToken(); • System.out.println(token); // Displays 42 • token = tokenizer.nextToken(); • System.out.println(token); // Displays 58 • Can use hasMoreTokens() method to find last one • StringTokenizer tokenizer = new StringTokenizer("42 58"); • while (tokenizer.hasMoreTokens()) • { • System.out.println(tokenizer.nextToken()); • } Programming and Problem Solving With Java

  28. Text Files • Text file • File that is human-readable with simple tools (type, more, edit, ...) • Variable-length lines • Each line terminated by end-of-line marker • Example: Java source file • Easy to read and write text files • Use same classes and methods as System.in and System.in • Advantage of "streams" approach to I/O Programming and Problem Solving With Java

  29. Text Files: The File Class • Need File object for each file program uses • To define • File inFile = new File("myFile.dat"); • Purpose • Contains informationabout the file • A "placeholder" forthe file • Not the same asthe file Programming and Problem Solving With Java

  30. Text Files: The File Class • Methods in the File class • canRead(): Tells if program can read the file • canWrite(): Tells if program can write to the file • delete(): Deletes the file • exists(): Tells if the file is there • isDirectory(): Tells if the file is really a directory name • isFile(): Tells if the file is a file (not a directory) • length(): Tells the length of the file, in bytes Programming and Problem Solving With Java

  31. Text Files: The File Class • Example of File class • import java.io.*; • class TellIfExists • { • public static void main(String[] args) • throws java.io.IOException • { • File myFile = new File("myFile.dat"); • if (myFile.exists()) • { • System.out.println("myFile.dat exists"); • } • else • { • System.out.println("myFile.dat does not exist"); • } • } • } Programming and Problem Solving With Java

  32. Text Files: Writing to a File • Before writing, make sure either • File doesn't exist • !outFile.exists() • File exists, and is writeable • outFile.exists() && outFile.canWrite() • Combine conditions • !outFile.exists() || outFile.canWrite() • Attach file to a stream • FileWriter object: knows how to write stream to a file • Wrap FileWriter object in BufferedWriter object for efficiency • Wrap BufferedWriter object in PrintWriter object Programming and Problem Solving With Java

  33. Text Files: Writing to a File • Example code • // Initialize the file variable • File outFile = new File("myFile.dat"); • // Make sure that the file either doesn't exist or • // we can write to it • if (!outFile.exists() || outFile.canWrite()) • { • // Create an output stream and attach it to the file • PrintWriter fileOutStream • = new PrintWriter(new BufferedWriter( • new FileWriter(outFile))); • // Write to the file • ... • } • else • { • // Error: Can't write to the file • } Programming and Problem Solving With Java

  34. Text Files: Writing to a File • Use PrintWriter object like System.out • fileOutStream.print("This goes "); • fileOutStream.println("to the file"); • System.out and fileOutStream are both output streams • System.out is PrintStream object • fileOutStream is PrintWriter object • PrintStream and PrintWriter are almost identical (PrintWriter is newer) • print(), println(), flush() work the same • Once done writing to the file, close it • fileOutStream.close(); • Program does this when it end • Good idea to do explicitly, though Programming and Problem Solving With Java

  35. Text Files: Writing to a File • Example • // This program writes a table of squares and • // square roots to the file myfile.dat. • import java.io.*; • class WriteSquaresAndSquareRoots • { • public static void main(String[] args) • throws java.io.IOException • { • // Initialize the output file variable • File outFile = new File("myFile.dat"); • if (!outFile.exists() || outFile.canWrite()) • { • // Create a buffered output stream, wrapped by • // PrintWriter, and attach it to the file • PrintWriter fileOutStream • = new PrintWriter(new BufferedWriter • (new FileWriter(outFile))); • // Write the output to the file • for (int number = 1; number <= 10; number++) • fileOutStream.println(number + "\t" + Math.sqrt(number) • + "\t\t" + number * number); Programming and Problem Solving With Java

  36. Text Files: Writing to a File • Example (continued) • // Close the output stream • fileOutStream.close(); • } • else • { • System.out.println("Problem creating output file "); • } • System.out.println("Done"); • } • } • Contents of file • 1 1.0 1 • 2 1.4142135623730951 4 • 3 1.7320508075688772 9 • 4 2.0 16 • 5 2.23606797749979 25 • 6 2.449489742783178 36 • 7 2.6457513110645907 49 • 8 2.8284271247461903 64 • 9 3.0 81 • 10 3.1622776601683795 100 Programming and Problem Solving With Java

  37. Text Files: Reading from a File • Define File object • File inFile = new File("myFile.dat"); • Before reading, make sure both • File exists and is readable • inFile.exists() && inFile.canRead() • Attach file to a stream • FileReader object: knows how to read stream from a file • Wrap FileReader object in BufferedReader object for efficiency • BufferedReader works on files just like System.in • read(): read a single character, or -1 if EOF • readLine(): read a line, or null if EOF Programming and Problem Solving With Java

  38. Text Files: Reading from a File • Example code • // Initialize the file variable • File inFile = new File("myFile.dat"); • // Make sure the file can be read from • if (inFile.exists() && inFile.canRead()) • { • // Create an input stream and attach it to the file • BufferedReader fileInStream • = new BufferedReader(new FileReader(inFile)); • // Read from the file stream the same way as reading • // from the keyboard • ... • } • else • { • // Error: Can't read from file • } Programming and Problem Solving With Java

  39. Text Files: Reading from a File • Complete example: read() method • // Reads a file and displays the number of digits, • // upper case letters, lower case letters, and other characters • // in the file. • import java.io.*; • class CountLettersDigits • { • public static void main(String[] args) • throws java.io.IOException • { • int nextValue, numUpperCase = 0, numLowerCase = 0, • numDigits = 0, numOther = 0; • char nextChar; • String inputFileName; • // Create a buffered input stream and attach it to standard • // input • BufferedReader inStream • = new BufferedReader(new InputStreamReader(System.in)); • // Get input file name from user • System.out.print("Enter input file name: "); • System.out.flush(); • inputFileName = inStream.readLine(); Programming and Problem Solving With Java

  40. Text Files: Reading from a File • Complete example (continued) • // Initialize the file variable • File inFile = new File(inputFileName); • // Make sure the file can be read from • if (inFile.exists() && inFile.canRead()) • { • // Create an input stream and attach it to the file • BufferedReader fileInStream • = new BufferedReader(new FileReader(inFile)); Programming and Problem Solving With Java

  41. Text Files: Reading from a File • Complete example (continued) • // Read from the file • nextValue = fileInStream.read(); • while (nextValue != -1) • { • nextChar = (char) nextValue; • if (Character.isDigit(nextChar)) • { • numDigits++; • } • else if (Character.isUpperCase(nextChar)) • { • numUpperCase++; • } • else if (Character.isLowerCase(nextChar)) • { • numLowerCase++; • } • else • { • numOther++; • } • nextValue = fileInStream.read(); • } Programming and Problem Solving With Java

  42. Text Files: Reading from a File • Complete example (continued) • // Close the stream • fileInStream.close(); • // Display results • System.out.println("Number of digits: " • + numDigits); • System.out.println("Number of upper case letters: " • + numUpperCase); • System.out.println("Number of lower case letters: " • + numLowerCase); • System.out.println("Number of other characters: " • + numOther); • } • else • { • System.out.println("Can't read " + inputFileName); • } • } • } Input file: CountCharacters.java Number of digits: 7 Number of upper case letters: 96 Number of lower case letters: 1001 Number of other characters: 832 Programming and Problem Solving With Java

  43. Text Files: Reading from a File • Complete example: readLine() method • // This program reads a file and displays the file • // on the screen, with a line number before each line. • import java.io.*; • class DisplayLinesWithLineNumbers • { • public static void main(String[] args) • throws java.io.IOException • { • String inputFileName; • String line; • int lineNumber = 0; • // Create a buffered input stream and attach it to standard • // input • BufferedReader inStream • = new BufferedReader(new InputStreamReader(System.in)); • System.out.println("--- File Lister ---"); • System.out.println(); Programming and Problem Solving With Java

  44. Text Files: Reading from a File • Complete example (continued) • // Get file name from user • System.out.print("Enter input file name: "); • System.out.flush(); • inputFileName = inStream.readLine(); • // Initialize the input file variable • File inFile = new File(inputFileName); • if (inFile.exists() && inFile.canRead()) • { • // Create an input stream and attach it to the file • BufferedReader fileInStream • = new BufferedReader(new FileReader(inFile)); • System.out.println(); • System.out.println("--- Contents of file " • + inputFileName + "---"); Programming and Problem Solving With Java

  45. Text Files: Reading from a File • Complete example (continued) • // Read lines from the input stream, display on screen • line = fileInStream.readLine(); • while (line != null) • { • lineNumber++; • System.out.println(lineNumber + ":\t" + line); • line = fileInStream.readLine(); • } • System.out.println("--- End of file ---"); • // Close the stream • fileInStream.close(); • } • else • { • System.out.println("Couldn't open input file " • + inputFileName); • } • } • } --- File Lister --- Enter input file name: JavaTemplate.java --- Contents of file JavaTemplate.java--- 1: class JavaTemplate 2: { ... --- End of file --- Programming and Problem Solving With Java

  46. Text Files: Reading Numbers • Same techniques as reading from keyboard • Get string value from a file • Use NumberFormat object to convert to Number object • Convert Number object primitive type • Example • // Reads a file of numbers, square roots, and • // squares from myfile.dat and displays the contents on the • // screen. Each line of the file should have an integer, the • // square root of that integer, and the square of that integer. • // (Use the program in Listing 6.15 to create the file.) • import java.io.*; • import java.util.StringTokenizer; • import java.text.NumberFormat; Programming and Problem Solving With Java

  47. Text Files: Reading Numbers • Example (continued) • class ReadSquaresAndSquareRoots • { • public static void main(String[] args) • throws java.io.IOException, java.text.ParseException • { • String line; • int number, lineNumber = 0; • double squareOfNumber, squareRootOfNumber; • // Create a number formatter object • NumberFormat aNumberFormatter = NumberFormat.getInstance(); • // Initialize the input file variable • File inFile = new File("myfile.dat"); • if (inFile.exists() && inFile.canRead()) • { • // Create an input stream and attach it to the file • BufferedReader fileInStream • = new BufferedReader(new FileReader(inFile)); Programming and Problem Solving With Java

  48. Text Files: Reading Numbers • Example (continued) • // For each line in the input stream, read an integer and • // two doubles (the square of the number and the square • // root) and display each value. • line = fileInStream.readLine(); • while (line != null) • { • lineNumber++; • // Initialize the tokenizer with the last read line • StringTokenizer tokenizer = new StringTokenizer(line); • // Break the line into an int and two doubles • number • = aNumberFormatter.parse(tokenizer.nextToken()).intValue(); • squareRootOfNumber • = aNumberFormatter.parse(tokenizer.nextToken()).doubleValue(); • squareOfNumber • = aNumberFormatter.parse(tokenizer.nextToken()).doubleValue(); • // Dislay the line's contents • System.out.println("Line " + lineNumber + " contents:"); • System.out.println(" number is " + number); • System.out.println(" square root of " + number + " is " • + squareRootOfNumber); • System.out.println(" square of " + number + " is " • + squareOfNumber); • // Read the next line • line = fileInStream.readLine(); • } Programming and Problem Solving With Java

  49. Text Files: Reading from a File • Example (continued) • // Close the stream • fileInStream.close(); • } • else • { • System.out.println("Couldn't open input file"); • } • } • } Line 1 contents: number is 1 square root of 1 is 1.0 square of 1 is 1.0 Line 2 contents: number is 2 square root of 2 is 1.4142135623730951 square of 2 is 4.0 Line 3 contents: number is 3 square root of 3 is 1.7320508075688772 square of 3 is 9.0 ... Line 10 contents: number is 10 square root of 10 is 3.1622776601683795 square of 10 is 100.0 Programming and Problem Solving With Java

  50. Text Files: Reading from a File Characters Lines Words • Example: Count characters, lines, words • To count characters: increment for each BufferedRead.read() • To count lines: increment for each '\n' • Counting words is trickier • Incremental design and development • Get easy parts written and tested first • Add other features one at a time to the working framework • Advantage: bug is most likely in most recently added part of program Programming and Problem Solving With Java

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