Lecture 7. File Processing

Transcription

1 Lecture 7 File Processing 1

2 Data (i.e., numbers and strings) stored in variables, arrays, and objects are temporary. They are lost when the program terminates. To permanently store the data created in a program, you need to save them in a file on a disk. 2

3 Lecture Content 1. Text File 2. Binary File 3. Random-Access File 4. Binary I/O of Class Objects 3

4 1. Text File In general, I/O classes for file processing can be classified as input classes and output classes. An input class contains the methods to read data from a file. - Scanner is an example of an input class. An output class contains the methods to write data to a file. - PrintWriter is an example of an output class. 4

5 1. Text File An input object reads a stream of data from a file. - An input object is called an input stream. An output object writes a stream of data to a file. - An output object is called an output stream. This section presents how to read/write strings and numeric values to/from a text file using the Scanner and PrintWriter classes. 5

6 1. Text File The program receives data through an input object and sends data through an output object. Scanner PrintWriter 6

7 Writing Data Using PrintWriter import java.io.printwriter; import java.io.file; public class Ex1 { public static void main(string[] args) throws Exception { // Create file object in memory // The file named Ex1Out.txt does not // exist on disk File file = new File("Ex1Out.txt"); 7

8 Writing Data Using PrintWriter // Create a file on disk PrintWriter fo = new PrintWriter(file); // we can use PrintWriter fo = // new PrintWriter("Ex1Out.txt); // Write formatted output to the file fo.print("john T Smith "); fo.println(90); // integer number 8

9 Writing Data Using PrintWriter } } fo.print("eric K Jones "); fo.println(85); System.out.println( "File writing is done."); // Close the file fo.close(); 9

10 Writing Data Using PrintWriter Invoking the constructor of PrintWriter will create a new file if the file does not exist. - If the file already exists, the current content in the file will be discarded (empty file exists). if (file.exists() == true) { System.out.println( File already exists."); System.exit(0); } 10

11 Writing Data Using PrintWriter Invoking the constructor of PrintWriter may throw an I/O exception. - Java forces you to write the code to deal with this type of exception. Simply declare throws Exception in the method. The close() method must be used to close the file. If this method is not invoked, the data may not be saved properly in the file. 11

12 Methods of the PrintWriter Class. 12

13 Reading Data Using Scanner import java.util.scanner; import java.util.file; public class Ex2 { public static void main(string[] args) throws Exception { // Create a File instance File file = new File("Ex1Out.txt"); // Create a Scanner for the file Scanner fi = new Scanner(file); 13

14 Reading Data Using Scanner // Read data from a file while (fi.hasnext()) { // while not end of file String firstname = fi.next(); String middlename = fi.next(); String lastname = fi.next(); int score = fi.nextint(); System.out.println( firstname + " " + middlename + " " + lastname + " " + score); } 14

15 Reading Data Using Scanner } } System.out.println( "File reading is done."); // Close the file fi.close(); 15

16 Reading Data Using Scanner Invoking the constructor new Scanner(file) may throw an I/O exception. Thus, the main() method declares throws Exception. To create a Scanner to read data from a file, you must use the java.io.file class to create an instance of the File by using the constructor new File(filename), and use new Scanner(file) to create a Scanner for the file. 16

17 Reading Data Using Scanner Each iteration in the while loop reads first name, middle name, last name, and score from the text file. Finally, the file is closed. It is not necessary to close the input file, but it is a good practice to do so to release the resources occupied by the file. 17

18 Methods of the Scanner Class. 18

19 StringTokenizer import java.util.stringtokenizer;... String inputline; StringTokenizer parser; String name; double score; while ( fi.hasnext() ) { // while not end of file inputline = fi.nextline(); parser = new StringTokenizer(inputLine); try { 19

20 StringTokenizer name = parser.nexttoken() + " " + parser.nexttoken() + " " + parser.nexttoken(); score = Double.parseDouble( parser.nexttoken() ); System.out.printf("%s %.2f\n", name, score); } catch (NoSuchElementException e) { // no token System.out.println(e); } } fi.close(); 20

21 Lecture Content 1. Text File 2. Binary File 3. Random-Access File 4. Binary I/O of Class Objects 21

22 2. Binary File This section introduces the classes for performing binary I/O. Data stored in a text file are represented in humanreadable form. Data stored in a binary file are represented in binary form. You cannot read binary files. They are designed to be read by programs. 22

23 2. Binary File The advantage of binary files is that they are more efficient to process than text files. Example: The decimal integer 199 is stored as the sequence of three characters, 1, 9, 9, in a text file, and the same integer is stored as a byte-type value C7 in a binary file, because decimal 199 equals hexadecimal number C7 ( ). 23

24 2. Binary File Text I/O requires encoding and decoding whereas binary I/O does not, as shown below. 24

25 2. Binary File Text I/O requires encoding and decoding whereas binary I/O does not, as shown below. 25

26 2. Binary File The ASCII code for character 1 is 49 (0x31 in hex) and for character 9 is 57 (0x39 in hex). So to write the characters 199, three bytes 0x31, 0x39, and 0x39 are sent to the output, as shown above. Binary I/O is more efficient than text I/O, because binary I/O does not require encoding and decoding. 26

27 2. Binary File Some of the classes for performing binary I/O are listed below. 27

28 2. Binary File FileOutputStream is for writing bytes to a file (i.e., byte-based output). - If the file does not exist, a new file will be created. If the file already exists, the current content of the file will deleted. - To retain the current content and append new data into the file, use true for the append parameter. 28

29 2. Binary File FileInputStream is for reading bytes from a file (i.e., byte-based input). Almost all the methods in the I/O classes throw java.io.ioexception. Therefore you have to declare java.io.ioexception to throw in the method or place the code in a try-catch block, as shown below: 29

30 2. Binary File. 30

31 FileOutputStream / FileInputStream Example: uses binary I/O (i.e., FileOutputStream and FileInputStream) to write ten byte values from 1 to 10 to a file named Ex5Out.dat and reads them back from the file. 31

32 FileOutputStream / FileInputStream //import java.io.ioexception; //import java.io.fileoutputstream; //import java.io.fileinputstream; import java.io.*; public class Ex5 { public static void main(string[] args) throws IOException { // Create an output stream to the file FileOutputStream fo = new FileOutputStream("Ex5Out.dat"); 32

33 FileOutputStream / FileInputStream // Output values to the file for (int i = 1; i <= 10; i++) fo.write(i); // Close the output stream fo.close(); // Create an input stream for the file FileInputStream fi = new FileInputStream("Ex5Out.dat"); // Read values from the file int value; 33

34 FileOutputStream / FileInputStream } } while ((value = fi.read())!= -1) // while not end of file System.out.print(value + " "); System.out.println(); // Close the output stream fi.close(); 34

35 Methods of FileOutputStream Class. Weakness? - byte-based write 35

36 Methods of FileInputStream Class. Weakness? - byte-based read 36

37 DataInputStream / DataOutputStream DataInputStream reads bytes from the stream and converts them into appropriate primitive type values or strings. DataOutputStream converts primitive type values or strings into bytes and outputs the bytes to the stream. 37

38 DataInputStream / DataOutputStream Primitive values are copied from memory to the output without any conversions. The writeutf(string s) method converts a string into a series of bytes in the UTF-8 format and writes them into a binary stream. The readutf() method reads a string that has been written using the writeutf method. 38

39 DataInputStream / DataOutputStream Two character encoding schemes are ASCII (American Standard Code for Information Interchange, 1 byte) code and UTF (Unicode Transformation Format, UTF-8, UTF-16, UTF-32) code. 39

40 DataInputStream / DataOutputStream Example: writes student names and scores to a file named Ex6Out.dat and reads the data back from the file. 40

41 DataInputStream / DataOutputStream import java.io.*; public class Ex6 { // process primitive numeric types, strings public static void main(string[] args) throws IOException { // Create output stream for Ex6Out.dat DataOutputStream fo = new DataOutputStream( new FileOutputStream("Ex6Out.dat")); 41

42 DataInputStream / DataOutputStream // Write student test scores to the file fo.writeutf("john"); fo.writedouble(85.5); fo.writeutf("jim"); fo.writedouble(185.5); fo.writeutf("george"); fo.writedouble(105.25); // Close output stream fo.close(); 42

43 DataInputStream / DataOutputStream // Create input stream for Ex6Out.dat DataInputStream fi = new DataInputStream( new FileInputStream("Ex6Out.dat")); 43

44 DataInputStream / DataOutputStream } } // Read student test scores from file System.out.println(fi.readUTF() + " " + fi.readdouble()); System.out.println(fi.readUTF() + " " + fi.readdouble()); System.out.println(fi.readUTF() + " " + fi.readdouble()); // Close input stream fi.close(); 44

45 DataInputStream / DataOutputStream // Ex6_2 // Read student test scores from file boolean stop = false; while (!stop) try { System.out.println(fi.readUTF() + " " + fi.readdouble()); } catch (IOException ex) { stop = true; } 45

46 Methods of DataOutputStream Class. 46

47 Methods of DataInputStream Class. 47

48 Object I/O DataInputStream and DataOutputStream enable you to perform I/O for primitive type values and strings. ObjectInputStream and ObjectOutputStream enable you to perform I/O for objects in addition to primitive type values and strings. 48

49 ObjectOutputStream import java.io.*; public class Ex7 { // Test ObjectOutputStream and writeobject() public static void main(string[] args) throws IOException { // Create output stream for Ex7Out.dat ObjectOutputStream fo = new ObjectOutputStream( new FileOutputStream("Ex7Out.dat")); 49

50 ObjectOutputStream } } // Write a string, double value, and // object to the file fo.writeutf("john"); fo.writedouble(85.5); fo.writeobject(new java.util.date()); // write Date object, // Mon Feb 04 22:22:54 GMT-04: System.out.println( "File writing is done."); fo.close(); // Close output stream 50

51 ObjectInputStream import java.io.*; public class Ex8 { // Test ObjectInputStream and readobject() public static void main(string[] args) throws ClassNotFoundException, IOException { // Create input stream for Ex7Out.dat ObjectInputStream fi = new ObjectInputStream( new FileInputStream("Ex7Out.dat")); 51

52 ObjectInputStream // Read a string, double value, and // object to the file String name = fi.readutf(); double score = fi.readdouble(); java.util.date date = (java.util.date)(fi.readobject()); System.out.println(name + " " + score + " " + date); // read Date object // Mon Feb 04 22:22:54 GMT-04: System.out.println( "File reading is done."); 52

53 ObjectInputStream } } // Close output stream fi.close(); 53

54 Object I/O The readobject() method may throw java.lang.classnotfoundexception if the class for the object has not been loaded. Since readobject() returns an Object, it is cast into Date and assigned to a date variable. 54

55 Serialization / Deserialization Object serialization is implemented in ObjectOutputStream so that an object can be written to an output stream (called serializable object). Object deserialization is implemented in ObjectInputStream so that an object can be read from an input stream. 55

56 Write/Read int and String Arrays Example: Write and read entire int array and String array 56

57 Write/Read int and String Arrays public class Ex9 { // Test ObjectStream for int and String // arrays public static void main(string[] args) throws ClassNotFoundException, IOException { int[] numbers = {1, 2, 3, 4, 5}; String[] strings = {"John", "Jim", "Jake"}; 57

58 Write/Read int and String Arrays // Create output stream for Ex9Out.dat ObjectOutputStream fo = new ObjectOutputStream( new FileOutputStream( "Ex9Out.dat", true)); // Write arrays to object output stream fo.writeobject(numbers); // int array fo.writeobject(strings); // String array // Close the stream fo.close(); 58

59 Write/Read int and String Arrays // Create input stream for Ex9Out.dat ObjectInputStream fi = new ObjectInputStream( new FileInputStream( "Ex9Out.dat")); int[] newnumbers = (int[])(fi.readobject()); String[] newstrings = (String[])(fi.readObject()); 59

60 Write/Read int and String Arrays } } // Display int and String arrays for (int i = 0; i < newnumbers.length; i++) System.out.print(newNumbers[i] + " "); System.out.println(); for (int i = 0; i < newstrings.length; i++) System.out.print(newStrings[i] + " "); System.out.println(); fi.close(); // Close the stream 60

61 ObjectOutputStream ObjectOutputStream can write objects, primitive type values, and strings. 61

62 ObjectInputStream ObjectInputStream can read objects, primitive type values, and strings. 62

63 Lecture Content 1. Text File 2. Binary File 3. Random-Access File 4. Binary I/O of Class Objects 63

64 3. Random-Access File Text file (human-readable) and binary file (machine-readable) discussed above are sequential files. Random-access file (machine-readable) can be written to and read from at random locations. RandomAccessFile class allows a file to be read from and written to at random locations. 64

65 3. Random-Access File The RandomAccessFile class implements the DataInput and DataOutput interfaces, as shown in Figure below. 65

66 Methods of RandomAccessFile Class. 66

67 3. Random-Access File The DataInput interface defines the methods (e.g., readint(), readdouble(), readchar(), readutf()) for reading primitive type values and strings. The DataOutput interface defines the methods (e.g., writeint(), writedouble(), writechar(), writeutf()) for writing primitive type values and strings. 67

68 3. Random-Access File Random-access file can be created with two modes as follows. RandomAccessFile fio = new RandomAccessFile( "Test.dat", "rw"); RandomAccessFile fio = new RandomAccessFile( "Test.dat", "r"); 68

69 File Pointer A random-access file consisting of a sequence of bytes has a special marker called a file pointer positioned at one of these bytes. - A read or write operation takes place at the location of the file pointer. - When a file is opened, the file pointer is set at the beginning of the file (i.e., byte 0). 69

70 File Pointer - When you read or write data to the file, the file pointer moves forward to the next data item (not the next byte). - For example, if you read an int value using readint(), the JVM reads 4 bytes from the file pointer, and now the file pointer is 4 bytes ahead of the previous location, as shown in Figure below. 70

71 File Pointer (a) Before readint() (b) After readint() After an int value is read, the file pointer is moved 4 bytes ahead. 71

72 3. Random-Access File //import java.io.randomaccessfile; import java.io.*; public class Ex10 { // Test RandomAccessFile public static void main(string[] args) throws IOException { int n = 9, i = 0; 72

73 3. Random-Access File // Create a random access file RandomAccessFile fio = new RandomAccessFile( "Ex10InOut.dat", "rw"); // Clear the file to destroy old // contents if exists fio.setlength(0); // Write integers to file for (int j = 1; j <= n; j++) fio.writeint(j); 73

74 3. Random-Access File // Display current length of file System.out.println( "Current file length is " + fio.length()); // Retrieve the first number // Move file pointer to the beginning fio.seek(0); System.out.println( "The first number is " + fio.readint()); 74

75 3. Random-Access File // Retrieve the second number // Move fp to the second number fio.seek(1 * 4); System.out.println( "The second number is " + fio.readint()); // Retrieve the sixth number // Move file pointer to the sixth number fio.seek(5 * 4); 75

76 3. Random-Access File System.out.println( "The sixth number is " + fio.readint()); // Modify the seventh number fio.writeint(55); // Append a new number // Move file pointer to the end fio.seek(fio.length()); fio.writeint(99); 76

77 3. Random-Access File } } // Display the new length System.out.println( "The new length is " + fio.length()); // Retrieve the new seventh number // Move fp to the seventh number fio.seek(6 * 4); System.out.println( "The seventh number is " + fio.readint()); fio.close(); 77

78 Lecture Content 1. Text File 2. Binary File 3. Random-Access File 4. Binary I/O of Class Objects 78

79 4. Binary I/O of Class Objects A class object can be written to and read from a binary file by making the class implement the Serializable interface as follows. public class SomeClass implements Serializable { }... 79

80 4. Binary I/O of Class Objects To write an object of a class such as SomeClass to a binary file, you simply use the writeobject() method of the stream class ObjectOutputStream. If an object is written to a binary file with the writeobject() method, then the object can be read from the file with the readobject() method of the stream class ObjectInputStream. 80

81 4. Binary I/O of Class Objects The readobject() method returns an object of type Object. Thus, if you want to obtain an object of type SomeClass, you must do a type cast as follows. SomeClass u = (SomeClass)fi.readObject(); Example : Write and read class objects to/from a binary file using writeobject() and readobject() methods. 81

82 4. Binary I/O of Class Objects import java.io.ioexception; import java.io.objectoutputstream; import java.io.fileoutputstream; import java.io.objectinputstream; import java.io.fileinputstream; import java.io.filenotfoundexception; import java.io.serializable; 82

83 4. Binary I/O of Class Objects class SomeClass implements Serializable { // Serializable class private int number; private char letter; public SomeClass() { number = 0; letter = 'A'; } 83

84 4. Binary I/O of Class Objects public SomeClass(int thenumber, char theletter) { number = thenumber; letter = theletter; } 84

85 4. Binary I/O of Class Objects } public String tostring() { // if we omit this method, // the memory address of class // object will be output return "Number = " + number + " Letter = " + letter; } 85

86 4. Binary I/O of Class Objects public class Ex11 { // Test Binary I/O of class objects public static void main(string[] args) throws IOException, ClassNotFoundException { ObjectOutputStream fo = new ObjectOutputStream( new FileOutputStream( "Ex11Out.dat")); 86

87 4. Binary I/O of Class Objects SomeClass x = new SomeClass(1, 'A'); SomeClass y = new SomeClass(42, 'Z'); fo.writeobject(x); fo.writeobject(y); fo.close(); System.out.println( "class objects were sent to file."); 87

88 4. Binary I/O of Class Objects System.out.println( "Now let's reopen the file and display the data."); ObjectInputStream fi = new ObjectInputStream( new FileInputStream( "Ex11Out.dat")); // You must perform type cast SomeClass u = (SomeClass)fi.readObject(); SomeClass v = (SomeClass)fi.readObject(); 88

89 4. Binary I/O of Class Objects } } System.out.println( "The following objects were read from the file:"); System.out.println(u); System.out.println(v); System.out.println("End of program."); 89

90 Binary I/O of Array Object An array is an object. Thus, an entire array can be saved to a binary file using writeobject() and later read using readobject(). If the array has a base type that is a class (e.g., SomeClass), then the class must be serializable. Note that the type cast (SomeClass[]) must be used to obtain the correct data read by readobject(). 90

91 Binary I/O of Array of Objects Example: Write and read an array of objects to/from a binary file using writeobject() and (SomeClass[])fi.readObject(). 91

92 Binary I/O of Array of Objects import java.io.*; class SomeClass implements Serializable { // Serializable class... } public class Ex12 { public static void main(string[] args) throws IOException, ClassNotFoundException { 92

93 Binary I/O of Array of Objects SomeClass[] a = new SomeClass[2]; a[0] = new SomeClass(1, 'A'); a[1] = new SomeClass(2, 'B'); ObjectOutputStream fo = new ObjectOutputStream( new FileOutputStream( "Ex12Out.dat")); fo.writeobject(a); fo.close(); System.out.println( "Array object was written to file."); 93

94 Binary I/O of Array of Objects System.out.println( "Now let's reopen the file and display the array."); SomeClass[] b = null; ObjectInputStream fi = new ObjectInputStream( new FileInputStream( "Ex12Out.dat")); b = (SomeClass[])fi.readObject(); fi.close(); 94

95 Binary I/O of Array of Objects } } System.out.println( "The following array elements were read from the file:"); for (int i = 0; i < b.length; i++) System.out.println(b[i]); System.out.println("End of program."); 95

96 Programming Project Use topics taught in the class to write a simple program of student management system using an array of objects. You need to read/write the student list from/to text and binary files. 96

97 Programming Project Required functionalities are as follows. 1. Input student list 2. Output student list 3. Add a new student 4. Search a student a. Search on given last name b. Search on given student ID 97

98 Programming Project Required functionalities are as follows. 5. Delete a student with a given student ID 6. Update a student with a given student ID 7. Sort on last name or average mark c. Sort on average mark d. Sort on last name 8. Exit 98

99 Programming Project The student data are as follows class Student { public String sid; // student ID public String sname; // student name (First Middle Last) int[] m; // marks of 3 courses, belong to [0..100] double avg; // average mark with 2 digits after decimal point // constructors and methods } // end class Student 99

100 References 1. Y. Daniel Liang Introduction to Java Programming, Comprehensive. 10 th Ed. Pearson. ISBN: Paul Deitel, Harvey Deitel Java How to Program. 10 th Ed. Pearson. ISBN: Water Savitch, Kenrich Mock Absolute Java. 6 th Ed. Pearson. ISBN: