Overview

1
Chapter 8

• Overview
• Learn to use try catch blocks
• Learn to use streams
• Learn to use text files

2
Exceptions

• We have used if statements to Prevent errors
• if (count ! 0)
• average sum / count
• else
• average 0
• System.out.println(Error count is zero)
• Protects against runtime error

3
Exceptions classes

• There are a series of classes used to trap
  exceptions.
• See table top of page 517 for some typical run
  time exceptions

4
Try-Catch statement

• Some errors can not be protected against using if
  statements
• Try-catch lets the error occur
• if occurs it catches and handles the error
• readInt uses this
• used to catch bad user input
• See example bottom of page 517

5
New Statement!!! Way Cool ?

• Syntax
• try
• //statements that may throw exception
• catch(exceptionType parameterName)
• // statements for this exception type
• .
• finally
• //statements to execute after try/catch

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Multiple catch blocks

• In the previous code you can have multiple catch
  blocks
• allows you to catch multiple exception types
• finally block is optional executes after
  try-catch blocks
• see example page 520
• btw only one catch

7
Catch block order

• This is important if having multiple catch blocks
• Need to keep in mind class hierarchy
• Only first catch block that matches exception is
  executed.
• Class Exception is the superclass of all
  exceptions classes
• if it is first it will always be executed since
  all exceptions are of this type
• It should be last to catch anything that may have
  been missed
• See example page 522

8
Other helpful methods

• ex.getMessage()
• retrieves message indicating exception type
• ex.printStackTrace()
• shows which step throw error
• See output bottom of page 523

9
Throwing exceptions

• We can throw exceptions out of a method, back to
  the calling method.
• Use throw clause in header of method
• Tells compiler that this method can throw and
  exception
• The calling program must then catch the exception
• See example on page 525

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Checked exceptions

• Special category of exceptions called checked
  exceptions
• You must either
• catch these exceptions
• or handle by throwing to calling method
• If you do not you will get syntax error.
• Subclasses of IOException are checked
• Arithmetic are not

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Throwing your own

• You can also throw exceptions using the throw
  statement
• Would allow you to customize the error message
  thrown
• See bottom page 526 example

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8.2 Streams and Text Files

• We have been using a stream off and on all
  semester
• System.out is a stream to the system console
• print
• println
• Streams are just a continuous stream of
  characters, both printable and non-printable.

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Writing to Output Text File

• Create Stream object and associate it with disk
  file
• Give Stream object desired functionality
• Read information from an input file or write
  information to an output file
• Close the file

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Write to file

• See example page 531
• FileWriter outStream ..
• creates stream object
• PrintWriter outs new.
• Wraps stream object in new object
• Gives desired functionality
• Program then writes the data to text file

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Input/Output Exceptions

• IOException is the most general class for I/O
• Not only can you output to system console
• You can also read from the system console
• System.in
• readLine
• Most useful when running from command prompt.

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8.3 Using Text Files

• This is what we need to learn to do to complete
  Major assignment number 4.
• FileReader inStream new FileReader(myData.txt)
  
• BufferedReader ins new BufferedReader(inStream)
  
• Methods we then use are
• ins.readLine()
• ins.ready()

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How to read text file

• DVD GUI
• If we look on page 544 the DVD example
• The method readDVDs in the middle of the page
• basically does what we need to complete the
  project
• title going to null signifies the end of the file
• can also use ready() to find out when file ends
• We need to read our input file for Major 4,
  although we do not need to store the records in
  an array

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Chapter 8 in closing

• This is what we need to know out of Chapter 8
• Section 8.5 Binary files
• These files are just files that are storing
  binary data, not text data.
• Binary data is stored in native format.
• Our Major 4 uses a text file that can actually be
  opened in notePad