Introduction

1. Introduction • Files • Long-term storage of large amounts of data • Persistent data exists after termination of program • Files stored on secondary storage devices • Magnetic disks • Optical disks • Magnetic tapes • Sequential and random access files

2. Data Hierarchy • Smallest data item in a computer is a bit • Bit can be either 0 or 1 • Bit short for “binary digit” • Programmers work with higher level data items • Decimal digits: (0-9) • Letters: (A-Z and a-z) • Special symbols: (e.g., $, @, %, &, *, (, ), -, +, “, :, ?, /, etc.) • Java uses Unicode characters composed of 2 bytes • A byte is normally 8 bits long • Fields (Java instance variables) • Composed of characters or bytes • Conveys meaning

3. Data Hierarchy • Data hierarchy • Data items in a computer form a hierarchy • Progresses from bits, to characters, to fields, etc. • Records • Composed of several fields • Implemented as a class in Java • See the figure on the next slide for example • File is a group of related records • One field in each record is a record key • Record key is a unique identifier for a record • Sequential file • Records stored in order by record key

4. The data hierarchy.

5. Files and Streams • Java views a file as a stream of bytes • File ends with end-of-file marker or a specific byte number • File as a stream of bytes associated with an object • Java also associates streams with devices • System.in, System.out, and System.err • Streams can be redirected • File processing with classes in package java.io • FileInputStream for byte-based input from a file • FileOutputStream for byte-based output to a file • FileReader for character-based input from a file • FileWriter for character-based output to a file

6. Java’s view of a file of n bytes.

7. Files and Streams • Buffering • Improves performance of I/O • Copies each output to a region of memory called a buffer • Entire buffer output to disk at once • One long disk access takes less time than many smaller ones • A program can convert a unbuffered stream into a buffered stream using the wrapping idiom • Example inputStream = new BufferedReader(new FileReader("xanadu.txt")); outputStream = new BufferedWriter(new FileWriter("characteroutput.txt"));

8. A portion of the class hierarchy of the java.io package.

9. A portion of the class hierarchy of the java.io package (cont.).

10. import java.util.StringTokenizer; import java.io.*; import java.text.DecimalFormat; class InventoryItem { private String name; private int units; // number of available units of this item private float price; // price per unit of this item private DecimalFormat fmt; public InventoryItem (String itemName, int numUnits, float cost) { name = itemName; units = numUnits; price = cost; fmt = new DecimalFormat ("0.##"); } public String toString() { return name + ":\t" + units + " at " + price + " = " + fmt.format ((units * price)); } } Example: reading a text file

11. public class Inventory{ // Reads data about a store inventory from an input file, // creating an array of InventoryItem objects, then prints them. public static void main (String[] args) { final int MAX = 100; InventoryItem[] items = new InventoryItem[MAX]; StringTokenizer tokenizer; String line, name, file="inventory.txt"; int units, count = 0; float price; try{ FileReader fr = new FileReader (file); BufferedReader inFile = new BufferedReader (fr); line = inFile.readLine(); while (line != null) { tokenizer = new StringTokenizer (line); name = tokenizer.nextToken(); try { units = Integer.parseInt (tokenizer.nextToken()); price = Float.parseFloat (tokenizer.nextToken()); items[count++] = new InventoryItem (name, units, price); } catch (NumberFormatException exception) { System.out.println ("Error in input. Line ignored:"); System.out.println (line); } line = inFile.readLine(); } inFile.close(); Example: reading a text file

12. for (int scan = 0; scan < count; scan++) System.out.println (items[scan]); } catch (FileNotFoundException exception) { System.out.println ("The file " + file + " was not found."); } catch (IOException exception) { System.out.println (exception); } } } Example: reading a text file

13. import java.io.*; public class TestData { // Creates a file of test data that consists of ten lines each // containing ten integer values in the range 0 to 99. public static void main (String[] args) throws IOException { final int MAX = 10; int value; String file = "test.txt"; FileWriter fw = new FileWriter (file); BufferedWriter bw = new BufferedWriter (fw); PrintWriter outFile = new PrintWriter (bw); for (int line=1; line <= MAX; line++) { for (int num=1; num <= MAX; num++) { value = (int) (Math.random() * 100); outFile.print (value + " "); } outFile.println (); } outFile.close(); System.out.println ("Output file has been created: " + file); } } Example: writing a text file

14. Data Streams and Object Streams • Data streams support I/O of primitive data types • Object streams support I/O of objects • Most, but not all, standard classes support serialization of their objects. • Those classes implement the marker interface Serializable • Every instance variable of the class should is a Serializable • Object stream classes • ObjectInputStream and ObjectOutputStream • Output using the writeObject() method

15. Updating Sequential-Access File • Difficult to update a sequential-access file • Entire file must be rewritten to change one field • Only acceptable if many records being updated at once

16. Random-Access File • “Instant-access” applications • Record must be located immediately • Transaction-processing systems require rapid access • Random-access files • behave like a large array of bytes stored in the file system • Access individual records directly and quickly • Use fixed length for every record • Easy to calculate record locations • Insert records without destroying other data in file • Figure on the next slide shows random-access file

17. Java’s view of a random-access file

18. Random-Access File • RandomAccessFile objects • Like DataInputStream and DataOutputstream • Reads or writes data in spot specified by file-position pointer • Manipulates all data as primitive types • Normally writes one object at a time to file

19. // Record class for the RandomAccessFile programs. import java.io.*; public class Record { private int account; private String lastName; private String firstName; private double balance; public Record(int a, String f, String l, double b) { account=a; firstName=f; lastName=l; balance=b; } public Record() { account=0; firstName=null; lastName=null; balance=0; } // Read a record from the specified RandomAccessFile public void read( RandomAccessFile file ) throws IOException { account = file.readInt(); byte b1[] = new byte[ 15 ]; file.readFully( b1 ); firstName = new String( b1 ); firstName=firstName.trim(); byte b2[] = new byte[ 15 ]; file.readFully( b2 ); lastName = new String( b2 ); lastName=lastName.trim(); balance = file.readDouble(); } Defining a class to be used for reading and writing records to a random access file

20. // Write a record to the specified RandomAccessFile public void write( RandomAccessFile file ) throws IOException { file.writeInt( account ); byte b1[] = new byte[ 15 ]; if ( firstName != null ) b1 = firstName.getBytes(); file.write( b1 ); byte b2[] = new byte[ 15 ]; if ( lastName != null ) b2 = lastName.getBytes(); file.write( b2 ); file.writeDouble( balance ); } public int getAccount(){ return account;} public String getlName(){ return lastName;} public String getfName(){ return firstName;} public double getBalance(){ return balance;} // NOTE: This method contains a hard coded value for the size of a // record of information. public int size() { return 42; } }

21. import java.io.*; public class CreateRandFile { private Record blank; RandomAccessFile file; public CreateRandFile() { blank = new Record(); try { file = new RandomAccessFile( "credit.dat", "rw" ); } catch( IOException e ) { System.err.println( "File not opened properly\n" + e.toString() ); System.exit( 1 ); } } public void create() { try { for ( int i = 0; i < 100; i++ ) blank.write( file ); } catch ( IOException e ) { System.err.println( e.toString() ); } } public static void main( String args[] ) { CreateRandFile accounts = new CreateRandFile(); accounts.create(); } } This program creates a random access file sequentially by writing 100 empty records to disk.

22. import java.io.*; import java.awt.*; import javax.swing.*; import java.awt.event.*; public class WriteRandFile extends JFrame { // Application window components JTextField acct, // where user enters account number fName, // where user enters first name lName, // where user enters last name bal; // where user enters balance JButton enter, // send record to file done; // quit program JLabel acctLabel, // account label fNameLabel, // first name label lNameLabel, // last name label balLabel; // balance label RandomAccessFile output; // file for output Record data; public WriteRandFile() { super( "Write to random access file" ); data = new Record(); try { output = new RandomAccessFile( "credit.dat", "rw" ); } catch ( IOException e ) { System.err.println( e.toString() ); System.exit( 1 ); } This program uses TextFields to get information from theuser at the keyboard and writes the information to a random access file.

23. Container c = getContentPane(); c.setLayout( new GridLayout( 5, 2 ) ); acct = new JTextField( 20 ); acctLabel = new JLabel( "Account Number" ); fName = new JTextField( 20 ); fNameLabel = new JLabel( "First Name" ); lName = new JTextField( 20 ); lNameLabel = new JLabel( "Last Name" ); bal = new JTextField( 20 ); balLabel = new JLabel( "Balance" ); enter = new JButton( "Enter" ); done = new JButton( "Done" ); c.add( acctLabel ); // add label c.add( acct ); // add TextField c.add( fNameLabel ); // add label c.add( fName ); // add TextField c.add( lNameLabel ); // add label c.add( lName ); // add TextField c.add( balLabel ); // add label c.add( bal ); // add TextField c.add( enter ); // add button c.add( done ); // add button done.addActionListener( new ActionListener () { public void actionPerformed( ActionEvent event ){ cleanup(); // write data, close file, etc. setVisible(false); dispose(); // release system resources System.exit( 0 ); } } ); This program uses TextFields to get information from theuser at the keyboard and writes the information to a random access file.

24. enter.addActionListener( new ActionListener () { public void actionPerformed( ActionEvent event ){ addRecord(); } } ); setSize( 300, 150 ); setVisible( true ); } This program uses TextFields to get information from theuser at the keyboard and writes the information to a random access file.

25. public void addRecord() { int acctNum = 0; Double d; acctNum = ( new Integer( acct.getText() ) ).intValue(); // output the values to the file try { if ( acctNum > 0 && acctNum <= 100 ) { String f=fName.getText(); String l=lName.getText(); double b=Double.parseDouble(bal.getText()); data = new Record(acctNum,f,l,b); output.seek( (long) ( acctNum-1 ) * data.size() ); data.write( output ); // clear the TextFields acct.setText( "" ); fName.setText( "" ); lName.setText( "" ); bal.setText( "" ); } else { acct.setText( "Enter valid account (1-100)" ); acct.selectAll(); } } catch ( IOException e ) { System.err.println( "Error during write to file\n" + e.toString() ); System.exit( 1 ); } } This program uses TextFields to get information from theuser at the keyboard and writes the information to a random access file.seek method of RandomAccessFile:sets file-position pointer to a specific point in file

26. public void cleanup() { if ( !acct.getText().equals("") ) addRecord(); try { output.close(); } catch ( IOException e ) { System.err.println( "File not closed properly\n" + e.toString() ); System.exit( 1 ); } } public static void main( String args[] ) { WriteRandFile accounts = new WriteRandFile(); } } This program uses TextFields to get information from theuser at the keyboard and writes the information to a random access file.

27. import java.io.*; import java.awt.*; import javax.swing.*; import java.awt.event.*; public class ReadRandFile extends JFrame { // Application window components JTextField acct, // where user enters account number fName, // where user enters first name lName, // where user enters last name bal; // where user enters balance JButton next, // send record to file done; // quit program JLabel acctLabel, // account label fNameLabel, // first name label lNameLabel, // last name label balLabel; // balance label RandomAccessFile input; // file for output Record data; boolean moreRecords=true; public ReadRandFile() { super( "Write to random access file" ); data = new Record(); try { input = new RandomAccessFile( "credit.dat", "r" ); } catch ( IOException e ) { System.err.println( e.toString() ); System.exit( 1 ); } This program opens a random-access file and displays only the records with data.

28. Container c = getContentPane(); c.setLayout( new GridLayout( 5, 2 ) ); acct = new JTextField( 20 ); acctLabel = new JLabel( "Account Number" ); fName = new JTextField( 20 ); fNameLabel = new JLabel( "First Name" ); lName = new JTextField( 20 ); lNameLabel = new JLabel( "Last Name" ); bal = new JTextField( 20 ); balLabel = new JLabel( "Balance" ); next = new JButton( "Next" ); done = new JButton( "Done" ); c.add( acctLabel ); // add label c.add( acct ); // add TextField c.add( fNameLabel ); // add label c.add( fName ); // add TextField c.add( lNameLabel ); // add label c.add( lName ); // add TextField c.add( balLabel ); // add label c.add( bal ); // add TextField c.add( next ); // add button c.add( done ); // add button This program opens a random-access file and displays only the records with data.

29. done.addActionListener( new ActionListener () { public void actionPerformed( ActionEvent event ){ cleanup(); // write data, close file, etc. setVisible(false)(); dispose(); // release system resources System.exit( 0 ); } } ); next.addActionListener( new ActionListener () { public void actionPerformed( ActionEvent event ){ readRecord(); } } ); setSize( 300, 150 ); setVisible( true ); } This program opens a random-access file and displays only the records with data.

30. public void readRecord() { try { do { // loop over empty records data.read( input ); } while ( input.getFilePointer() < input.length() && data.getAccount() == 0 ); } catch( IOException e ) { moreRecords = false; } // transfer full record data into textfields if ( data.getAccount() != 0 ) { acct.setText( String.valueOf( data.getAccount() ) ); String fN=data.getfName(); fName.setText( fN); String lN=data.getlName(); lName.setText( lN ); bal.setText( String.valueOf( data.getBalance() ) ); } } This program opens a random-access file and displays only the records with data.

31. public void cleanup() { try { input.close(); } catch ( IOException e ) { System.err.println( e.toString() ); System.exit( 1 ); } } public static void main( String args[] ) { ReadRandFile accounts = new ReadRandFile(); } } This program opens a random-access file and displays only the records with data.

32. Application: Transaction Processing • Substantial transaction-processing system • Uses random-access file • Updates, adds and deletes accounts