UMass Lowell Computer Science 91'460 Java and Distributed Computing Prof' Karen Daniels Fall, 2000

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UMass Lowell Computer Science 91.460 Java and
Distributed Computing Prof. Karen Daniels Fall,
2000

• Lecture 13
• Java Fundamentals
• File I/O
• Serializing an Object
• Wed. 10/4/00

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Homework 3
HW Assigned Due Content

• 1 Fri, 9/8 Fri, 9/15 Part 1
• Mon, 9/18 Part 2
• 2 Fri, 9/15 Fri, 9/22 Part 1 Part 2
• 3 Fri, 9/22 Fri, 9/29 Part 1 Part 2

Graded
Homework 4 will be assigned on Fri.,
10/6 (skipping one week due to test)
Homework is due at the start of lecture on the
due date.
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Exam 1 Closed Book
Topics not covered Jini, Exception Handling,
Files/Streams Format Multiple choice
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Java File I/O

• Focusing on
• Serialization and Sequential Files
• Sources Deitel, Java 2 Certification, Java 2
  The Complete Reference

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Java I/O Philosophy

• Console I/O is limited
• Text-based console programs are not part of the
  main Java design focus
• Applets have limited access to resources
• Device-independent abstraction

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Java Streams

• Motivated by C stream concept
• remember cin gtgt and cout ltlt ?
• Stream
• Sequence of bytes or characters that travel from
  a source to a destination over a communications
  path
• Relieves programmer of burden of communication
  path details
• Abstraction that is device-independent
• 2 types byte and character (Unicode)

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java.io package

• Major players
• InputStream
• OutputStream
• Reader
• Writer

• Others
• File
• RandomAccessFile
• FileDescriptor
• ObjectStreamClass
• ObjectStreamField
• SerializablePermission
• SteamTokenizer

byte stream
character stream (Unicode)
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java.io packageFile class

• Used to access file and directory objects
• Uses file-naming conventions of host OS
• Some methods
• getAbsolutePath( )
• getParent( )
• listFiles( )
• isDirectory( )
• isFile( )
• Example Using a File constructor
• File file new File(test.dat)

• exists( )
• getName( )
• length( )
• lastModified( )
• canRead( )

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java.io packageInputStream OutputStream classes

• InputStream
• FilterInputStream
• BufferedInputStream
• DataInputStream
• LineNumberInputStream
• PushBackInputStream
• ByteArrayInputStream
• FileInputStream
• ObjectInputStream
• PipedInputStream
• SequenceInputStream
• StringBufferInputStream

• OutputStream
• FilterOutputStream
• BufferedOutputStream
• DataOutputStream
• PrintStream
• ByteArrayOutputStream
• FileOutputStream
• ObjectOutputStream
• PipedOutputStream

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Chaining Streams
The DataOutputStream class is an output filter
that allows Strings and primitive data types to
be written to a file stream.
The FileOutputStream class enables output (stream
of bytes) to be written to a file stream.
Input Stream
Output Stream
File file new File("test.txt") FileOutputStream
outFile new FileOutputStream(file) DataOutputS
tream outData new DataOutputStream(outFile)
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Combining Filters
Input Stream
Output Stream
Filter 1
Filter 2
Filter 3
Filter 4
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File I/O Exception Handling

• Try, catch, throw paradigm (remember from C?)
• Some types of exceptions
• IOException when operating on file
• ClassNotFoundException during reading/deserializat
  ion
• Example
• try output.close( )
• catch (IOException ioex)
• JOptionPane.showMessageDialog(null, Error
  closing file, Error, JOptionPane.ERROR_MESSAGE)
  

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File I/O Example
(see FileTest.java code on class Web site)
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Serializing a Java Object

• Serializing Converting object to a format
  suitable for stream I/O
• Deserializing Converting serialized object back
  into object instance.
• To serialize, class must implement java.io.
  Serializable interface
• Serialized object contains part of state of an
  instance
• identifying info for class
• instance variables
• all reachable non-transient and non-static
  objects
• not the methods!

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Serializing a Java Object (continued)

• Why?
• File I/O Object data persistence
• JavaBean reusable software component
• Remote Method Invocation (RMI)
• Jini

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Serializing a Java Object (continued)

• How?
• To serialize an object, its class must implement
  the Serializable interface
• Example
• public class Point2D extends Object implements
  Serializable
• ...
• To write/read an object, chain ObjectOutputStream

Input Stream
Output Stream
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Serializing Example

(see FileTest2.java code on class Web site)
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Web/Networking Example
(see Deitel Figure 20.2 of Chapter 20)