Java File IO

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Java File I/O

• N Gattuso 12/2003

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Reading Info (Strings) From a File

• We will use our friend the BufferedReader Class
  in conjunction with a class called FileReader
• So as usual first we need to import the java.io.
  package.
• And we need to define a String .. Lets call it
  record String recordnull

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Reading Info (Strings) From a File

• Next we need to create an object from the
  FileReader class
• FileReader fr new FileReader("C\\mydata.txt")
• Above code creates a FileReader Object called fr
  this is just a pointer to the file!

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Reading Info (Strings) From a File

• Now that the file is open we need to read
  contents of using BufferedReader NOTE if file
  cant be opened IOException is thrown!
• BufferedReader br new BufferedReader(fr)

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Reading Info (Strings) From a File

• Every line in the file is considered a string
  so we use a while loop to read all lines in file
  when file empty we will receive a null passed
  back into our string

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Reading Info (Strings) From a File

• while ( (recordbr.readLine()) ! null )
  System.out.println(record)

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Reading Info (Strings) From a File

• while ( (recordbr.readLine()) ! null ) //
  read all lines from br which// is a
  BufferedReader Object pointing // to the fr
  FileReader object// line by line in file goes
  into record which // is a string when file
  empty, record // string is set to key word null
  (empty)

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Reading Info (Strings) From a File

• When done reading, close the BufferedReader
  object -gt br.close()
• Also, good practice to close the FileReader
  object as well -gt fr.close()
• Note when opening the file we passed
• C\\mydata.txt \ is a special character
  which needs to be escaped with another \ to take
  special meaning away.

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Writing Info to a File

• We need to create an object from the FileWriter
  class
• FileWriter fw new FileWriter ("C\\new.txt,true
  ) -gt Note 2 args, first is file name, 2nd is
  boolean true to append!
• Above code creates a FileWriter Object called fw
  this is just a pointer to the file!

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Writing Info to a File

• Now that the file is open we need to create a
  write output buffer to using BufferedWriter
  NOTE if file cant be opened IOException is
  thrown!
• BufferedWriter wr new BufferedWriter(fw)

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Writing Info to a File

• To Write a line (String) of info to an open file
  using BufferedWriter
• wr.write(record,0,record.length())
• // where record is a string (first arg) with
  info to
• // write to file
• // 0 is the offset to start writing info in
  string // from and record.length() is where to
  // stop in record string so all info in
• // record string is written

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Writing Info to a File

• When done writing, close the BufferedWriter
  object -gt wr.close()
• Also, good practice to close the FileWriter
  object as well -gt fw.close()

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Putting it all together

• File.java reads every line from a file called
  C\mydata.txt and writes to a file called
  C\new.txt -gt so it is the copy command found in
  DOS if you add the args string logic via main
  you can pass command line args make full copy
  command..
• Demo file.java via source code run!