Java Input and Output

1
Java Input and Output

• We will concentrate on IO from and to a file
• Can use files to store persistent data
• For instance, visitor counter
• Java provides the java.io package for io
  operations

2
Reading from and Writing to files

• Java supports two file types text and binary
• Text ? data is stored as characters
• Binary ? data is stored as raw bytes
• The type of the file is determined when the file
  is written and depends on which class was used to
  write to the file
• ? to read from an existing file, we must know the
  file type in order to select the appropriate
  classes

3
Java Reading from text files

• The class FileReader is designed to read
  character files
• It has several constructors, one of them takes a
  String as its argument, representing the name of
  the file
• FileReader does not use buffering to read ahead ?
  it is inefficient to read many lines of text
• The class BufferedReader uses buffering, in
  particular with its readLine method

4
Java Reading from text files

• FileReader Constructor
• FileReader( String fileName ) throws
  FileNotFoundException
• BufferedReader constructor
• BufferedReader( Reader r )
• Class FileReader is a subclass of class Reader ?
  a FileReader object is a Reader object ? can use
  it as the argument of the BufferedReader
  constructor

5
Java Reading from text files

• Methods of class BufferedReader
• String readLine( ) throws IOException
• // reads a line of text returns a null String
  when the end of the file is reached /
• void close( )
• / releases resources allocated to the
  BufferedReader object /

6
Java Reading from text files

• Need to import the following classes
• java.io.FileReader
• java.io.BufferedReader
• java.io.FileNotFoundException
• Java.io.IOException

7
Java Reading from text files

• try
• FileReader fr new FileReader( fileName )
• BufferedReader br new BufferedReader( fr )
• String s br.readLine( )
• // use a while loop to read the whole file if
  needed
• // process s
• br.close( )

8
Java Reading from text files

• catch( FileNotFoundException fnfe )
• // output message ( file not found )
• catch( IOException ioe )
• // output message ( reading problem )

9
Java Writing and appending
10
Java Writing to text files

• Symmetry with reading from text files
• FileWriter class
• BufferedWriter class

11
Java Writing to text files

• // FileWriter constructor
• FileWriter( String fileName, boolean mode)
  throws IOException
• mode false ? write
• mode true ? append
• // BufferedWriter constructor
• BufferedWriter( Writer w )
• Class FileWriter is a subclass of class Writer ?
  a FileWriter object is a Writer object ? can use
  it as the argument of the BufferedWriter
  constructor

12
Java Writing to text files

• Methods of class BufferedWriter
• void write( String s ) throws IOException
• void newLine( )
• // writes a line separator
• void close( )
• / releases resources allocated to the
  BufferedWriter object /

13
Java Writing to text files

• Need to import the following classes
• java.io.FileWriter
• java.io.BufferedWriter
• java.io.IOException

14
Java Writing to text files

• try
• FileWriter fw new FileWriter( fileName, false
  )
• BufferedWriter bw new BufferedWriter( fw )
• bw.write( Hello world )
• bw.newLine( )
• bw.write( hello again )
• bw.close( )
• catch( IOException ioe )
• // feedback on exception that occured

15
Java Writing basic data types text files

• We can write basic data types, such as booleans,
  ints, doubles, chars, .. Directly to files
• We can also write objects to file (and read them
  as objects)

16
Java Writing basic data types to text files

• Class FileOutputStream is designed to write a
  stream of bytes to files
• Class PrintWriter is designed to convert basic
  data types to characters and write them to files

17
Java Writing basic data types to text files

• // FileOutputStream constructor
• FileOutputStream( String fileName, boolean mode)
  throws FileNotFoundException
• mode false ? write
• mode true ? append
• // PrintWriter constructor
• PrintWriter( OutputStream os )
• Class FileOutputStream is a subclass of class
  OutputStream ? a FileOutputStream object is an
  OutputStream object ? can use it as the argument
  of the PrintWriter constructor

18
Java Writing basic data types to text files

• Methods of class PrintWriter
• void print( DataType dt )
• // writes the argument to the file
• void println( DataType dt )
• // same as print newline character
• void close( )

19
Java Writing basic data types to text files

• Need to import the following classes
• java.io.FileOutputStream
• java.io.PrintWriter
• java.io.FileNotFoundException

20
Java Writing basic data types to text files

• try
• FileOutputStream fos new FileOutputStream(
  fileName,
• false )
• PrintWriter pw new PrintWriter( fos )
• pw.println( 95 )
• pw.println( A )
• pw.println( true )
• pw.close( )
• catch( FileNotFoundException ioe )
• // output file not found

21
Java Reading/Writing objects to files

• We can write objects to a file
• The objects must be instances of a class that
  implement the interface Serializable
• Use the ObjectOutputStream class to write to the
  file (writeObject method)
• Use the ObjectInputStream class to read objects
  from a file (readObject method)
• When the end of the file is reached, an
  EOFException is thrown

22
Java Visitor counter

• Store visitor count in a file
• Read file into a variable (use BufferedReader)
• Close file
• Increment variable by 1
• Write back variable to file (use PrintWriter)
• Close file