Text-File I/O

1
Text-File I/O
2
Text-File Output with PrintWriter

• The class PrintWriter is the preferred stream
  class for writing a text file.
• PrintWriter contains a method called println,
  like the method of the same name we used as
  System.out.println except it sends output to a
  text file instead of the screen.
• A text file is opened with a statement like
• outputStream
• new PrintWriter(new FileOutputStream(out.txt))
  

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Text-File Output with PrintWriter (contd)

• The above statement connects the stream named
  outputStream to the file named out.txt
• Making this connection is referred to as opening
  the file.
• When you connect a file to a stream in this way,
  your program always starts with an empty file.
• If the file already exists, the old contents are
  lost.

4
Example of File I/O

• import java.io.import java.util.public
  class TextFileOutputDemo public static void
  main(String args) PrintWriter
  outputStream null try
  outputStream new
  PrintWriter(new FileOutputStream("out.txt"))
  catch(FileNotFoundException e)
  System.out.println("Error
  opening the file out.txt.")
  System.exit(0)
  System.out.println("Enter three lines of
  text") String line null
  Scanner keyboard new Scanner(System.in)
  int count for (count 1 count lt 3
  count) line
  keyboard.nextLine( )
  outputStream.println(count " " line)
  outputStream.close( )
  System.out.println("Those lines were written to
  out.txt.")

5
A File Has Two Names

• One name is the real name of the file that is
  used by the operating system.
• The other name is the name of the stream that is
  connected to the file. This is the name your
  program uses when it refers to the file.
• How you can name the real file depends on your
  operating system. The name must be a legal name
  in that system.

6
FileNotFoundException

• This is a poorly named exception.
• When it is thrown when you are trying to open a
  file for output, it doesnt really mean it could
  find the file.
• It means that for some reason it could not open
  the file. It might mean that the name was
  already used for a directory (folder) name, for
  example.

7
Closing a File

• When your program is finished writing to a file,
  it should close the stream connected to it, e.g.,
  outputStream.close()
• The class PrintWriter, and every other class for
  file output or file input streams, has a method
  named close.
• When this method is invoked, the system releases
  any resources used to connect the stream to the
  file.

8
Closing a File (contd)

• If your program does not close a file before the
  program ends, Java will close it for your as long
  as your program ends normally.
• If your program ends abnormally with a file still
  open, the file could be damaged.
• It is best to close files in your program as soon
  as they are no longer needed.

9
Overwriting a File

• When you connect a stream to a text file by using
  the class PrintWriter in the following way, you
  always produce an empty file
• PrintWriter outputStream
• new PrintWriter(new FileOutputStream(out.txt))
  
• If there is no file named out.txt, one will be
  created.
• If out.txt already exists, the existing file will
  be eliminated and a new empty file will be
  created.

10
Appending to a File

• If, however, you want append to an existing file,
  you can connect the file to the output stream as
  follows
• outputStream
• new PrintWriter(new FileOutputStream(out.txt,
  true))
• If the file out.txt does not already exist, Java
  will create an empty file of that name and append
  the output to the end of this empty file.
• If the file out.txt already exists, the old
  contents will remain and the programs output
  will be placed after those contents.

11
Use toString for Text-File Output

• It is common to include a method toString() in
  classes.
• This method should produce a string value that
  intuitively represents the data in an object.
• The method toString has a special property. If
  you do not include it in an invocation of
  System.out.println, it is invoked automatically.

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Example using a toString Method

• import java.io.import java.util./
  Class for data on endangered species. This class
  is serialized./public class Species implements
  Serializable private String name
  private int population private double
  growthRate public Species( )
  name null population 0
  growthRate 0 public Species(String
  initialName, int initialPopulation,
  double initialGrowthRate)
  name initialName if
  (initialPopulation gt 0) population
  initialPopulation else
  System.out.println("ERROR Negative
  population.") System.exit(0)
  growthRate initialGrowthRate
  

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Example using a toString Method (contd)

• public String toString()
  return ("Name " name "\n"
  "Population " population "\n"
  "Growth rate " growthRate "")
  public void readInput( ) Scanner
  keyboard new Scanner(System.in)
  System.out.println("What is the species'
  name?") name keyboard.nextLine( )
  System.out.println(
  "What is the population of the species?")
  population keyboard.nextInt( ) while
  (population lt 0)
  System.out.println("Population cannot be
  negative.") System.out.println("Reent
  er population") population
  keyboard.nextInt( )
  System.out.println( "Enter growth
  rate (percent increase per year)")
  growthRate keyboard.nextDouble( )

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Example using a toString Method (contd)

• public void writeOutput( )
  System.out.println("Name " name)
  System.out.println("Population "
  population) System.out.println("Growth
  rate " growthRate "") /
  Precondition years is a nonnegative number.
  Returns the projected population of the calling
  object after the specified number of years.
  / public int projectedPopulation(int
  years) double populationAmount
  population int count years
  while ((count gt 0) (populationAmount gt 0))
  populationAmount
  (populationAmount
  (growthRate/100) populationAmount)
  count-- if (populationAmount gt
  0) return (int)populationAmount
  else return 0

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Example using a toString Method (contd)

• public void set(String newName, int
  newPopulation,
  double newGrowthRate) name
  newName if (newPopulation gt 0)
  population newPopulation else
  System.out.println("ERROR using
  a negative population.")
  System.exit(0) growthRate
  newGrowthRate public String getName(
  ) return name public
  int getPopulation( ) return
  population public double
  getGrowthRate( ) return
  growthRate public boolean
  equals(Species otherObject) return
  ((name.equalsIgnoreCase(otherObject.name))
  (population otherObject.population)
  (growthRate
  otherObject.growthRate))

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Text-File Input with BufferedReader

• The class BufferedReader is the preferred stream
  class for reading from a text file.
• To open a file for reading use the statement
• BufferedReader Stream_Name
• new BufferedReader(new FileReader(File_Name))
• If your program attempts to open a file for
  reading, but there is no such file, then a
  FileNotFoundException is thrown

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Text-File Input with BufferedReader (contd)

• BufferedReader has no methods that, like nextInt,
  can read a number.
• The only way that you can use the class
  BufferedReader to read a number from a text file
  is to read it as a string and then convert the
  string to a number.
• The class BufferedReader does have a method,
  named simply read, that will read a single
  character, however, it read it as an integer. To
  get the character you should write
• char next (char)(inputStream.read())

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Example Using the Class BufferedReader

• import java.io.public class
  TextFileInputDemo public static void
  main(String args) try
  BufferedReader inputStream
  new BufferedReader(new FileReader("data.txt"))
  String line null
  line inputStream.readLine( )
  System.out.println("The first line in data.txt
  is") System.out.println(
  line) line inputStream.readLine(
  ) System.out.println("The second line
  in data.txt is") System.out.println(l
  ine) inputStream.close( )
  catch(FileNotFoundException e)
  System.out.println("File data.txt was not
  found") System.out.println("or could
  not be opened.")
  catch(IOException e)
  System.out.println("Error reading from file
  data.txt.")

19
Reading a File Name from the Keyboard

• import java.io.import java.util.public
  class TextFileInputDemo2 public static void
  main(String args)
  System.out.println("Enter file name")
  Scanner keyboard new Scanner(System.in)
  String fileName keyboard.next( ) try
  BufferedReader inputStream
  new BufferedReader(new
  FileReader(fileName)) String line
  null line inputStream.readLine( )
  System.out.println("The first line in
  " fileName "
  is") System.out.println(line)
  line inputStream.readLine( )
  System.out.println("The second line in "
  fileName " is")
  System.out.println(line)
  inputStream.close( )
  catch(FileNotFoundException e)
  System.out.println("File " fileName " not
  found.") catch(IOException e)
  System.out.println("Error
  reading from file " fileName)