Exceptions

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Exceptions
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Definition

• Exception something unexpected that can occur
  in the execution of a program
• e.g., divide by zero or attempt to open a file
  that does not exist
• Java provides a way to handle exceptions that are
  thrown
• the try-catch statement

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Try-catch Statement

• Syntax
• try catch(Exception e) ...
• Example
• try
• System.out.println(5/x)
• catch(Exception e)
• System.out.println(/ by zero)

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Breaking out of the try Block

• try
• statement1
• statement2 // if exception occurs here,
• // statement3 will be
  skipped
• statement3
• catch(Exception e)
• statement4 // executed after exception occurs

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Why Use Try-catch ?

• Alternative if-statement
• will be complex and hard to read if there are
  several exceptions
• what if the exception occurs within a loop ?
  (will need to worry about breaking out of the
  loop)
• Using try-catch is a more robust and structured
  way of handling exceptions

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Exception Classes

• Classes that extend the Exception class
• Allows the programmer to be more specific about
  the exception
• try catch (ArithmeticException e)
• Useful in a try-catch chain

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Try-catch Chain

• try
• catch(SomeException se)
• catch(AnotherException ae)
• catch(YetAnotherException yae)

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Built-in Exceptions(Unchecked RuntimeException
Subclasses)
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Built-in Exceptions(Unchecked RuntimeException
Subclasses)
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Files
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File

• Unit of secondary storage
• Stores a sequence of bytes/characters
• Stream
• operations read from stream, write to stream
• Associated with a filename
• Often organized under a directory hierarchy

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Input/Output Classesin Java

• I/O viewed as a stream of bytes
• parent classes InputStream, OutputStream
• As a stream of (Unicode) characters
• parent classes Reader, Writer
• Need to import java.io.
• An application employing files will use a
  subclass of one of the above classes

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Text Files

• To create a text file, use PrintStream
• f new PrintStream(new FileOutputStream(filename
  .txt))
• To write to the text file use print methods
• f.println() // use like System.out
• Make sure to close the file before exiting the
  program
• f.close() // ensures contents are updated

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Text Files, continued

• To read from text files, use either
  DataInputStream or BufferedReader
• f new DataInputStream( FileInputStream(filename
  .txt))
• f new BufferedReader(new FileReader(filename.tx
  t))
• Use read methods to read from file
• s f.readLine() // reads a string

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Exceptions

• File operations throw exceptions so make sure
  statements are enclosed in a try-catch statement
• Exceptions thrown
• IOException
• Common (specific) exception FileNotFoundExceptio
  n

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Reading a File from the Web

• Use URL class from java.net
• To open ,
• wpage new URL(address)
• f new BufferedReader(new InputStreamReader(wpage
  .openStream())))
• address is a String specifying the webpage
  address (e.g., http//www.math.admu.edu.ph)

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Binary Files

• Text files are often sufficient but sometimes we
  want to store objects as they are (not their text
  forms) in a file
• Use ObjectOutputStream and ObjectInputStream
• operations writeObject() and readObject()
• common technique store objects in a Vector and
  then save the Vector in the file

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java.io. Summary

• There is a host of classes under this package
  that serve a variety of purposes
• Hints
• use javap java.io.classname to find out
  available constructors and methods
• you often need to use FileInputStream,
  FileOutputStream, FileReader, and FileWriter to
  associate a name to the file