Lecture 13 Streams

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Lecture 13Streams

• Jaeki Song

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Introduction

• Store information from one execution of a project
  until the next execution
• Use an object, store it, and then recreate the
  object exactly as it existed at a later time

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Stream

• The most basic of input/output with Java is done
  with stream
• The stream can flow from the program to the
  screen, from a keyboard to the program, to/from a
  disk file or other storage media, to a printer,
  or even to a network or the Web
• The stream commands enable you to write to a disk
  file or to the output screen

Java program
Input stream
Output stream
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Output to the Screen

• Send an output stream to the standard output
  device
• Read an input stream from the standard input
  device
• Java has three standard IO objects in, out, and
  err
• System.out.println(Welcome)

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Saving an Object in a File

• Persistence
• How can you make data persist from one run of the
  program to the next?
• Write all data into a disk file and read it again
  for the next program execution
• Object serialization
• Java added a tool for persisting an entire object
  as a single unit
• Using object serialization, you can use an output
  stream for writing objects
• Creating a serializable class
• Declare class variables and write get methods to
  retrieve the variables value

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Writing an Object

• Create a FileOuputStream object
• Create an ObjectOutputStream object
• Obtain the data from the text fields (after the
  user enters them)
• Create an event for storing the data for one
  object
• After all objects have been stored, close the
  FileOutputStream and ObjectOutputStream objects

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The FileOuputStream Object

• Establishes a link from the program to an actual
  disk file
• FileOutputStream outputEmployee
• Instantiate the object and assign the file name
  in a method
• outputEmployee new FileOutputStream(Employee.tx
  t)

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The ObjectOutputStream Object

• To save an object using the stream, you must also
  set up an ObjectOutputStream class
• ObjectOutputStream objSaveEmployee
• objSaveEmployees new ObjectOutputStream(outputEm
  ployee)

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Event for Storing Data

• The writeObject method of the ObjectOutputStream
  class saves the contentes of the named object
• Employee empCurrent new Employee(
  txtEmployeeName.getText(), txtHireDate.getText())
  
• objSaveEmployee.writeObject(empCurrent)

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Closing the Stream

• When you are finished with FileOutputStream and
  ObjectOutputStream, you need to close both
• objSaveEmployee.close() //close the object
  output stream
• outputEmployee.close() // close the file output
  stream
• Example EmployeeWrite

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Files and Strem

• Binary data I/O
• DataInputStreams
• Read binary data from InputStream
• Methods read, readByte, readChar, readDouble...
• DataOutputStreams
• Write binary data to OutputStream
• Methods write, writeChar, writeInt...

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Reading an Object

• Create a FileInputStream object
• Obtain the data for one object from the file
• Create an event to display the objects data in
  text fields

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The FileInputStream and File Objects

• The FileInputStream object associates the project
  with a file
• The File object enables you to read the entire
  object with one read command
• Creates a new File instance by converting the
  given pathname string into an abstract pathname
• e.x) File inFile new File("employee.txt")

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Example
//Declare stream objects FileInputStream
fis FileOutputStream fos public void
openStream() try File inFile
new File("employee.txt") File outFile new
File("employee_out.txt") fis new
FileInputStream(inFile) fos new
FileOutputStream(outFile)
catch(Exception error) System.err.println("
Error opening file")
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read() and write() method

• Methods in class FileInputStream and
  FileOutputStream
• read( )
• Reads a byte of data from this input stream. This
  method blocks if no input is yet available
• Returns the next byte of data, or -1 if the end
  of the file is reached.
• Write(int c)
• Writes the specified byte to this file output
  stream.
• c - the byte to be written