Streams and File I/O

1
Streams and File I/O

• Chapter 9

2
Objectives

• become familiar with the concept of an I/O stream
• understand the difference between binary files
  and text files
• learn how to save data in a file
• learn how to read data from a file

3
Outline

• Overview of Streams and File I/O
• Text-File I/O
• Using the File Class
• Basic Binary-File I/O
• Object I/O with Object Streams
• (optional) Graphics Supplement

4
Objectives, cont.

• learn how use the classes ObjectOutputStream and
  ObjectInputStream to read and write class objects
  with binary files

5
Overview of Streams and File I/O Outline

• The Concept of a Stream
• Why Use Files for I/O?
• Differences Between Text Files and Binary Files

6
The Concept of a Stream

• Files can be used to store
• Java classes
• Java programs
• output from a program
• input for a program.
• File I/O as well as keyboard and screen I/O are
  handled by streams.

7
The Concept of a Stream, cont.

• A stream is a flow of data (characters, numbers,
  etc.).
• Data flowing into a program is called an input
  stream.
• Data flowing out of a program is called an output
  stream.

8
The Concept of a Stream, cont.

• A stream is implemented as an object.
• It delivers data to a destination such as a file
  or a stream or
• it takes data from a source such as a file or the
  keyboard, and delivers it to a program.
• System.out is the only output stream we have used
  so far.
• Objects of class Scanner, used for keyboard
  input, are streams, too.

9
The Concept of a Stream, cont.

• This chapter discusses streams that connect
  programs to files.

10
Why Use Files for I/O?

• Keyboard input and screen output deal only with
  temporary data, which is lost when the program
  ends.
• Files permit data to be stored permanently (or at
  least until a program changes the file).
• Input files can be used over and over by
  different programs.
• Files also provide convenient storage and
  retrieval of large quantities of data.

11
Text Files and Binary Files

• All data in a file is stored as binary digits.
• Files with contents that must be treated as
  sequences of binary digits are called binary
  files binary files can be read only by machines.

12
Text Files and Binary Files, cont.

• Sometimes, it is more convenient to think of a
  files contents as a sequence of characters.
• Files with streams and methods to make them look
  like sequences of characters are called text
  files text files can be read by people.

13
Text Files and Binary Files, cont.

• Text files usually appear to be the same on all
  computers.
• Binary files usually differ from machine to
  machine and from programming language to
  programming language.
• Normally, the computer and programming language
  used to create the file must be used to read the
  file.

14
Text Files and Binary Files, cont.

• However, binary files are more efficient to
  process than text files.
• In Java, binary files are platform- independent.
• Binary files can be created by one computer and
  read by another, combining portability and
  efficiency.

15
Text Files and Binary Files, cont.

• Though text files can be read and written using
  an editor, binary files must be read and written
  by a program.

16
Text-File I/O Outline

• Text-File Output with PrintWriter
• Text-File Input with BufferedReader
• The StringTokenizer Class
• The Classes FileReader and FileOutputStream

17
Text-File Output with PrintWriter

• Class PrintWriter has a method println that
  behaves like System.out.println
• The java.io package contains the class
  PrintWriter and the other file I/O classes
  discussed in this chapter.

18
Text-File Output with PrintWriter, cont.

• class TextFileOutputDemo

19
Text-File Output with PrintWriter, cont.
20
Text-File Output with PrintWriter, cont.

• File out.text now exists and contains
• 1 A tall tree
• 2 in a short forest is like
• 3 a big fish in a small pond.

21
Text-File Output with PrintWriter, cont.

• A file is opened using something similar to
• outputStream new PrintWriter(
• new FileOutputStream(out.txt))
• An empty file is connected to a stream.
• If the named file (out.txt, for example) exists
  already, its old contents are lost.
• If the named file does not exist, a new empty
  file is created (and named out.txt, for example).

22
Text-File Output with PrintWriter, cont.

• Class Printwriter has no constructor that takes a
  file name as an argument.
• So, we use class FileOutputStream to create a
  stream and can be used as an argument to a
  PrintWriter constructor.
• Syntax
• PrintWriter Output_Stream_Name new
• PrintWriter (new
• FileOutputStream(File_Name))

23
Text-File Output with PrintWriter, cont.

• The FileOutputStream constructor, and thus the
  PrintWriter constructor invocation can throw a
  FileNotFoundException, which means that the file
  could not be created.
• The PrintWriter object is declared outside the
  try block.
• If it were declared inside the try block, it
  would be local to the try block.

24
Some Methods in Class PrintWriter

• constructor
• PrintWriter(OutputStream streamObject)
• to create a new file
• new PrintWriter(new
• FileOutputStream(File_Name))
• to append new text to an old file
• new PrintWriter(new
• FileOutputStream(File_Name, true))

25
Some Methods in Class PrintWriter, cont.

• to output to the file connected to the stream
• public final void
• println(Almost_Anything)
• public final void
• print(Almost_Anything)
• To close a streams connection to a file
• public void close()
• To flush the output stream
• public void flush()

26
Closing Text Files

• When a program finishes writing to or reading
  from a file, it should close the file.
• examples
• outputStream.close()
• inputStream.close()
• If a program does not close a file before the
  program ends, Java will will close it when the
  program ends, provided the program ends normally.

27
Closing Text Files, cont.

• The sooner a file is closed, the less likely it
  is to be damaged by being left open when a
  program ends abnormally.
• If a program writes a file, it must close the
  file before it attempts to read from it.

28
Spelling File Names

• The rules for spelling file names depend upon the
  operating system, not upon Java.
• Operating systems typically allow you to use
  letters, digits, and the dot symbol to spell file
  names.
• A suffix such as .txt indicates a text file, but
  that is just a common convention.

29
Use toString for Text-File Output

• Classes typically include a method toString.
• The methods println and print in class
  PrintWriter behave like System.out.println and
  System.out.print, respectively.

30
Use toString for Text-File Output, cont.

• class Species

31
Use toString for Text-File Output, cont.

• class TextFileObjectOutputDemo

32
Use toString for Text-File Output, cont.

• File species.records now exists and contains
• Name Calif. Condor
• Population 27
• Growth rate 0.02
• Name Calif. Condor
• Population 27
• Growth rate 0.02

33
Text-file Input with BufferedReader

• Class BufferedReader is the preferred stream
  class for reading from a text file.
• Class BufferedReader has no constructor that
  takes a filename as its argument.
• Class FileReader accepts a file name as a
  constructor argument and produces a stream that
  is a Reader object.
• The constructor for class BufferedReader accepts
  a Reader object as an argument.

34
Text-file Input with BufferedReader, cont.

• syntax
• BufferedReader Stream_Name new
• BufferedReader(new
• FileReader(File_Name))
• Methods readln and read are used to read from the
  file.
• The FileReader constructor, and thus the
  BufferedReader constructor invocation can throw a
  FileNotFoundException.

35
Text-file Input with BufferedReader, cont.

• class TextFileInputDemo

36
Text-file Input with BufferedReader, cont.
37
Text-file Input with BufferedReader, cont.

• File data.txt existed previously and contained
• 1 2
• buckle my shoe.
• 3 4
• shut the door.

38
Some Methods in Class BufferedReader

• constructor
• BufferedReader(Reader, readerObject)
• to create a stream
• new BufferedReader(new
• FileReader(File_Name))
• to read a line of input from the file
• public String readLine() throws IOException
• If the read operation goes beyond the end of the
  file, null is returned.

39
Some Methods in Class BufferedReader, cont.

• to read a single character from the file and
  return it as an int value
• public int read() throws IOException
• If the read operation goes beyond the end of the
  file, -1 is returned.
• to read a single character from the file and to
  treat it as a character
• char next (char)(inputStream.read())

40
Some Methods in Class BufferedReader, cont.

• To read a number from a text file, the number
  must be read in as a string and the string must
  be converted to a number.
• to close a streams connection to a file
• public void close()

41
Programming Example Reading a File Name from the
Keyboard

• A user may need to enter a file name at the
  keyboard at the time a program is run.

42
Programming Example Reading a File Name from the
Keyboard, cont.

• class TextFileInputDemo2

43
Programming Example, cont.
44
Programming Example, cont.

• File data.txt
• 1 2
• buckle my shoe.
• 3 4
• shut the door.

45
Java Tip Using Path Names

• When providing a file name as an argument for
  opening a file, a simple file name may be used if
  the file is in the same directory as the program
  being run.
• A full or relative path name also can be used.
• A full path name is the complete path name,
  starting from the root directory.

46
Java Tip Using Path Names, cont.

• A relative path name is the path name starting
  from the directory containing the program.
• The way to specify path names depends upon the
  operating system.

47
Java Tip Using Path Names, cont.

• example - UNIX
• /user/smith/home.work1/data.txt
• new FileReader (/user/smith/home.work1/data.txt)
  
• example - Windows
• D\homework\hw1\data.txt
• new FileReader (D\\homework\\hw1\\data.txt)

48
Java Tip Using Path Names, cont.

• The \\ would be used if the path name is
  hardcoded, but the \ would be used if the path
  name is entered from the keyboard.
• A Java program will accept a path name written in
  either Windows or UNIX format, even if a computer
  is using an operating system that does not match
  the syntax.
• ...(D/homework/hw1/data.txt)...

49
The StringTokenizer Class

• Class BufferedReader can read entire lines or
  single characters, but not single words.
• Class StringTokenizer can take an entire line of
  text and break it into individual words.
• The class StringTokenizer is in the java.util
  package.
• Individual words are called tokens.

50
The StringTokenizer Class, cont.

• Tokens are nonwhitespace characters.
• example
• StringTokenizer tokenizer new
• StringTokenizer(Read my lips!)
• while (tokenizer.hasMoreTokens())
• System.out.println
• (tokenizer.nextToken())

51
The StringTokenizer Class, cont.

• This will produce
• Read
• my
• lips!

52
The StringTokenizer Class, cont.

• Separators are whitespace characters unless
  otherwise specified.
• To specify a set of separators, a string
  consisting of all the separator characters is
  given as a second argument to the constructor.
• example
• new StringTokenizer(Read my lips!, \n.,!)

53
Some Methods in Class StringTokenizer

• constructors
• public StringTokenizer(String
• theString)
• public StringTokenizer(String
• theString, String delimiters)
• more tokens?
• public boolean hasMoreTokens()
• next token
• public String nextToken()

54
Some Methods in Class StringTokenizer, cont.

• remaining tokens
• public int countTokens()

55
Java Tip Testing for the End of a Text File

• When method readLine in class BufferedReader
  attempts to read beyond the end of a file, the
  method returns the value null.
• When method read attempts to read beyond the end
  of a file, the method returns the value -1.

56
Java Tip Testing for the End of a Text File

• class EOFDemo

57
The Classes FileReader and FileOutputStream

• Class FileReader is used with class
  BufferedReader class FileOutputStream is used
  with class Printwriter.
• Class FileReader and class FileOutputStream
  accept a file name as a constructor argument.

58
The Classes FileReader and FileOutputStream, cont.

• Connecting a BufferedReader object to a file
  using a string name requires two steps.
• First, create an object of the class FileReader.
• Then use this object to create an object of class
  BufferedReader.

59
The Classes FileReader and FileOutputStream, cont.

• example
• BufferedReader inputStream
• new BufferedReader
• (new FileReader(story.txt)

60
The Classes FileReader and FileOutputStream, cont.

• Producing a PrintWriter output stream from a file
  using FileOutputStream requires two steps.
• First, create an object of the class
  FileOutputStream.
• Then use this object to create an object of class
  PrintWriter.

61
The Classes FileReader and FileOutputStream, cont.

• example
• PrintWriter OutputStream
• new PrintWriter
• (new FileOutputStream
• (stuff.txt)

62
Using the File Class

• The methods of the class File can check the
  properties of files.
• Does the named file exist?
• Is the file readable?
• Typically, the operating systems lets you
  designate files as not readable or as readable
  only by certain users.

63
Using the File Class, cont.

• The File class is like a wrapper class for
  strings which are file names.
• example
• new File(treasure.txt)

64
Using the File Class, cont.
class FileClassDemo
65
Using the File Class, cont.

• Method canWrite determines if the operating
  system will let you write to the file.
• Typically, the operating systems lets you
  designate files as not writeable or as writeable
  only by certain users.

66
Some Methods in the Class File

• public boolean exists()
• public boolean canRead()
• public boolean canWrite()
• public boolean delete()
• public boolean length()
• public String getName()
• public String getPath()

67
Basic Binary-File I/O Outline

• Output to Binary Files Using ObjectOutputStream
• (optional) Some Details About writeUTF
• Reading Input from a Binary File Using
  ObjectInputStream
• The EOFException Class
• The Classes FileInputStream and FileOutputStream

68
Binary Files

• Binary files store data in the same format used
  for main memory.
• Bytes in main memory and bytes in binary files
  are read similarly, which leads to efficiency.
• Binary files created by a Java program on one
  computer can read by a Java program on a
  different computer.

69
Binary Files, cont.

• Class ObjectInputStream and class
  ObjectOutputStream are used to process binary
  files.
• Data is read or written, one byte at a time.
• Numbers and characters are converted
  automatically to bytes for storage in a binary
  file.
• Data in files can be treated as Java primitive
  data types, as strings, or as other objects.

70
Opening a Binary File

• syntax
• ObjectOutputStream Output_Stream_Name
• new ObjectOutputStream
• (new FileOutputStream(File_Name))
• example
• ObjectOutputStream myOutputStream
• new ObjectOutputStream
• (new FileOutputStream
• (myfile.dat))

71
Output to Binary Files Using ObjectOutputStream

• class BinaryOutputDemo

72
Output to Binary Files Using ObjectOutputStream,
cont.

• The numbers are not in human-readable form
  because there are no lines or other separators.

73
Some Methods in Class ObjectOutputStream

• to create
• public ObjectOutputStream(OutputStream
• streamObject)
• to create a stream
• new ObjectOutputStream
• (new FileOutputStream
• (File_Name_or_File_Object))
• to write a primitive type
• public void writeInt(int n) throws
• IOException

74
Some Methods in Class ObjectOutputStream, cont.

• to write a primitive type, cont.
• public void writeLong(long n) throws
• IOException
• public void writeDouble(double x)
• throws IOException
• public void writeFloat(float x)
• throws IOException

75
Some Methods in Class ObjectOutputStream, cont.

• public void writeChar(int n)
• throws IOException
• public void writeBoolean(boolean b)
• throws IOException
• to write a String
• public void writeUTF(String aString)
• throws IOException

76
Some Methods in Class ObjectOutputStream, cont.

• To write an object
• public void writeObject(Object
• anObject)
• throws IOException,
• NotSerializableException,
• InvalidClassException
• to close
• public void close() throws IOException

77
Some Methods in Class ObjectOutputStream, cont.

• to flush
• public void flush() throws IOException

78
Some Methods in Class ObjectOutputStream, cont.

• There is no method writeString.
• Instead, use method writeUTF.
• UTF stands for Unicode Text Format.
• UTF provides short, efficient codes for ASCII
  characters.

79
Different Types in the Same File

• Different types can be written to the same file.
• However, the different types must be read from
  the file just as they were written to the file.

80
(optional) Some Details About writeUTF

• Method writeUTF uses different numbers of bytes
  to store strings of different lengths in a file.
• However, there are no separators between data
  items in a binary file.
• Java resolves this problem by writing extra
  information at the start of each string (i.e. the
  number of bytes used to write the string).

81
Reading Input from a Binary File Using
ObjectInputStream

• A file written using ObjectOutputStream can be
  read using ObjectInputStream.
• The methods in class ObjectInputStream correspond
  to the methods in class ObjectOutputStream.

82
Reading Input from a Binary File Using
ObjectInputStream, cont.
83
Some Methods in Class ObjectInputStream

• to create
• ObjectInputStream
• (InputStream streamObject)
• to create a stream
• new ObjectInputStream (new
• FileInputStream
• (File_Name_or_File_Object)
• to read a primitive type
• public int readInt() throws IOException

84
Some Methods in Class ObjectInputStream, cont.

• to read a primitive type, cont.
• public long readLong()
• throws IOException
• public double readDouble()
• throws IOException
• public float readFloat()
• throws IOException
• public char readChar()
• throws IOException
• public boolean ReadBoolean()
• throws IOException

85
Some Methods in Class ObjectInputStream, cont.

• to read a String
• public String readUTF()
• throws IOException
• to read an object
• public Object readObject()
• throws ClassNotFoundException,
• InvalidClassException,
• OptionalDataException, IOException
• to close
• public void close() throws IOException

86
Opening an Input File

• syntax
• ObjectInputStream Input_Stream_Name
• new ObjectInputStream(new
• FileInputStream(File_Name))
• example
• ObjectInputStream myInputStream new
• ObjectInputStream(new
• FileInputStream(myfile.dat))

87
Reading Binary Files and Text Files

• Do not attempt to read a binary file as if it
  were a text file (using BufferedReader) or a text
  file as it if were a binary file (using
  ObjectInputStream).

88
Defining a Method to Open a Stream

• public static ObjectOutputStream
• openFile() throws IOException
• ObjectOutputStream tempStreamName
• System.out.println(Enter file name )
• Scanner keyboard new
• Scanner(System.in)
• String fileName keyboard.next()
• tempStreamName new
• ObjectOutputStream(new
• FileOutputStream(fileName))
• return tempStreamName

89
The EOFException Class

• ObjectInputStream methods that read from a binary
  file throw an EOFException when they try to read
  beyond the end of the file.
• When using class ObjectInputStream, the class
  EOFException can test for the end of a file.

90
The EOFException Class

• class EOFExceptionDemo

91
The EOFException Class, cont.
92
Checking for the End of File

• Different classes with file reading methods check
  for the end of a file in different ways.
• Binary files throw an exception in the class
  EOFException.
• A text file returns a special value, such as
  null.
• Be sure to test for the end of the file in the
  correct way.

93
The Classes FileInputStream and FileOutputStream

• We used stream class FileInputStream when we
  created a stream of class ObjectInputStream.
• We used stream class FileOutputStream when we
  created a stream of class ObjectOutputStream.

94
The Classes FileInputStream and FileOutputStream,
cont.

• FileOutputStream and FileInputStream accept a
  file name as a constructor argument.
• Neither ObjectInputStream nor ObjectOutputStream
  accepts a file name as an argument.

95
The Classes FileInputStream and FileOutputStream,
cont.

• You connect an ObjectInputStream to a file using
  a string name in two steps.
• Create an object of class FileOutputStream
• Use this object of class FileOutputStream to
  create an object of class ObjectOutputStream

96
The Classes FileInputStream and FileOutputStream,
cont.

• example
• ObjectOutputStream outputStream new
• ObjectOutputStream(new
• FileOutputStream(numbers.dat))

97
Programming Example Processing a File of Binary
Data

• Ask the user for two file names.
• Read the numbers in one file, multiply each
  number by two, and write the results to other
  file.

98
Programming Example Processing a File of Binary
Data, cont.

• class Doubler

99
Programming Example Processing a File of Binary
Data, cont.

• class Doubler, contd.

100
Object I/O with Object Streams Outline

• Binary I/O of Class Objects
• The Serializable Interface
• Array Objects in Binary Files

101
Binary I/O of Class Objects

• Using method writeObject of class
  ObjectOutputStream you can output class objects
  to a binary file, and then read objects from the
  file using method readObject of class
  ObjectInputStream.
• However, the class being written and read must be
  serializable.

102
Binary I/O of Class Objects, cont.

• To make a class serializable, add implements
  Serializable to the class heading.
• example
• public class SomeClass implements
• Serializable
• The Serializable interface is available after
  importing java.io.

103
Binary I/O of Class Objects, cont.

• class Species

104
Binary I/O of Class Objects, cont.

• class IODemo

105
Binary I/O of Class Objects, cont.
106
Files and toString

• Method toString provides convenient output to the
  screen and to a text file.
• However, method toString is not needed for object
  I/O to a binary file.

107
The Serializable Interface

• A class which is serializable affects how Java
  performs file I/O with objects of the class.
• Java assigns a serial number to each object of
  the class that it writes to a stream of type
  ObjectOutputStream.
• If the object is written more than once, Java
  writes only the serial number for the object.

108
The Serializable Interface, cont.

• This makes file I/O more efficient and makes
  files smaller.
• When read with a stream of type
  ObjectInputStream, duplicate serial numbers are
  returned as references to the same object.
• When a serializable class has instance variables
  of a class type, the class for the instance
  variables should be serializable.

109
Dont Mix Class Types

• Its good programming practice to store data of
  only one class type on any one file.

110
Array Objects in Binary Files

• An entire array can be saved to a binary file
  using objectWrite, and can be read later using
  objectRead.
• If the base type of the array is a class, the
  class should be serializable.
• All the data in an array can be outputted to a
  binary file using a single invocation of
  objectWrite.

111
Array Objects in Binary Files, cont.

• class ArrayIODemo

112
Array Objects in Binary Files, cont.
113
(optional) Graphics Supplement

• Programming Example A JFrame GUI for
  Manipulating Files
• Accept a file and display its first line.
• Provide an explanatory message if the file does
  not exist or is unreadable.
• Delete a selected file.
• Provide an explanatory message if the file does
  not exist or is unwriteable (and hence cannot be
  deleted).

114
Graphics Supplement

• class FileOrganizer

115
Graphics Supplement, cont.

• class FileOrganizer, cont.

116
Graphics Supplement, cont.

• class FileOrganizer, cont.

117
Graphics Supplement, cont.
118
Summary

• You have become familiar with the concept of an
  I/O stream.
• You now understand the difference between binary
  files and text files.
• You have learned how to save data in a file.
• You have learned how to read data from a file.

119
Summary, cont.

• You have learned how use the classes
  ObjectOutputStream and ObjectInputStream to read
  and write class objects with binary files.