Handling Files with Java

1
Handling Files with Java

• java.io

2
File

• File is a collection of data stored on disk that
  has a specific name
• Operating systems are usually responsible of
  keeping track and managing all the files on a
  system
• Java as programming language, provides constructs
  to manipulate files

3
Java File Manipulation

• java.io.File is the class used for accessing
  files on your disk
• File object does not represent a file
• Rather it represents a pathname or reference
• This path may or may not exist

4
Creating File Object

• File f new File ( C\\temp\\new.txt)
• f represents a file named C\temp\new.txt
• File f2 new File(C\\temp)
• Here f2 represents not a file, but the directory
  C\temp
• Note File object may refer to a path or file
  that does not exist

5
Different ways of creating File object

• File f new File(C\\temp\\new.txt)
• File f new File(C/temp/new.txt)
• File f new File(C/temp, new.txt)

6
Relative Path

• File f new File(something.doc)
• Something.doc is placed in the folder where your
  program (class files) are placed

7
Getting information about File

• getName( ) method returns the file name , without
  Path
• File f new File(C\\temp\\no.txt)
• String s f.getName( )
• Here s has no.txt

8
Getting information about File

• getPath( ) method returns the Path including the
  file name
• File f new File(C\\temp\\no.txt)
• String s f.getName( )
• Here s has C\\temp\\no.txt

9
Getting information about File

• getParent( ) method returns the string containing
  path of parent directory
• File f new File(C\\temp\\child)
• String s f.getParent( )
• Here s has C\\temp

10
Getting information about File

• exists( ) checks whether the specified path or
  file exists or not
• File f new File(C\\temp\\test.doc)
• Here f.exists( ) returns true if above file
  exists, otherwise returns false.

11
Getting information about File

• isDirectory( )
• isFile( )
• canRead( )
• canWrite( )
• list( )
• listFiles( )

12
Creating Directories

• File f new File (C\newDir)
• f.mkdir()

13
Creating Empty File

• File f new File( C\newFile.txt)
• if( ! f.exists( ) )
• f.createNewFile( )

14
Input and Output

• Java handles I/O using a set of classes found in
  java.io.
• Input of all types is handled through InputStream
• Output of all types is handled through
  OutputStream

15
InputStream and OutputStream

• A stream is a sequential flow of bytes
• Input Stream is a flow of data from some external
  source into our program
• Output stream is a flow of data from out program
  to some external target

16
Reading data from InputStream

• We instantiate an InputStream object
• Associate it with some source of data
• Read data in some memory location

17
Reading data from File

• We use a special type of Input Stream for reading
  data from file named FileInputStream
• FileInputStream is the sub-class of InputStream

18
Reading data from file

• // Creating a file object
• File f new File("doc.txt")
• //Creating FileInputStream object and associating
  it to the above file
• FileInputStream in new FileInputStream (f)

19
Reading data from file

• byte b new byte20
• for(int i0 iltb.length i)
• bi(byte)in.read()
• String s new String (b)
• System.out.println( s)
• This program prints first 20 characters from the
  file into the byte array

20
Detecting End of File

• If read( ) method returns -1, this means that the
  stream has ended i.e. we have reached the end of
  file

21
Another way of reading data from file

• If you know the total number of bytes you need to
  read from file, you can use this code
• File f new File("doc.txt")
• FileInputStream in new FileInputStream (f)
• byte b new byte20
• in.read(b)
• Sytem.out.println( new String(b))

22
Reading strings and numbers

• Above program reads a specified number of data
• We now write a program that reads a program TOKEN
  by TOKEN
• Prints every token on a new line
• We need a class called StreamTokenizer for this
  purpose

23
Reading Tokens from file

• File f new File("doc.txt")
• StreamTokenizer st new StreamTokenizer( new
  BufferedReader( new FileReader(f)))
• int tokenType
• while ( true )
• tokenType st.nextToken()
• if(tokenType st.TT_EOF)
• break
• if(tokenType st.TT_NUMBER)
• System.out.println(st.nval)
• if(tokenType st.TT_WORD)
• System.out.println(st.sval)

24
How it works

• StreamTokenizer st new StreamTokenizer( new
  BufferedReader( new FileReader(f)))
• File reader is a utility class used for reading
  character files
• Buffered Reader buffers the data read from the
  stream till the buffer is full
• StreamTokenizer is used to read tokens out of
  buffered reader

25
StreamTokenizertokenType st.nextToken()

• Method nextToken returns an integer of one of the
  following types
• nval If the current token is a number, this
  field contains the value of that number. 
• Val If the current token is a word token, this
  field contains a that string
• TT_EOF A constant indicating that the end of the
  stream has been read.
• TT_EOL A constant indicating that the end of the
  line has been read
• TT_NUMBER A constant indicating that a number
  token has been read.
• TT_WORD A constant indicating that a word token
  has been read. 
• Ttype   After a call to the nextToken method,
  this field contains the type of the token just
  read

26
Reading Tokens from file

• File f new File("doc.txt")
• StreamTokenizer st new StreamTokenizer( new
  BufferedReader( new FileReader(f)))
• int tokenType
• while ( true )
• tokenType st.nextToken()
• if(tokenType st.TT_EOF)
• break
• if(tokenType st.TT_NUMBER)
• System.out.println(st.nval)
• if(tokenType st.TT_WORD)
• System.out.println(st.sval)

27
Writing Data to Streams

• Create an OutputStream
• Associate it with a data destination
• Write data to it

28
Writing Data to File

• File f new File("docz.txt")
• FileOutputStream of new FileOutputStream(f)
• String s " This data is to be written to the
  file "
• of.write(s.getBytes())
• of.close()

29
Further Reading

• Chapter 8, 9, 10, 11 of Ivor-Horton
• Lets try things out now..