Exceptions, Singleton,

1
Lecture 3

• Exceptions, Singleton,
• Bridge Logging

2
Error handling in procedural languages

• int foo()
• if ( !doSomething1() )
• return 1
• if ( !doSomething2() )
• return 2

3
Exceptions

• public interface Algorithm
• public void run() throws
  ExecutionException, IllegalDataException

4
Example

• int iArray randomSortedIntArray()
• IAlgorithm alg new BinarySearch( iArray, 29 )
• try
• alg.run()
• catch ( IllegalDataException e1 )
• System.err.println( "Illegal Data Exception
  while running Binary Search " e.getMessage()
  )
• e.printStackTrace()
• catch ( ExecutionException e )
• System.err.println( "Execution Exception while
  running Binary Search " e.getMessage() )
• e.printStackTrace()

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How the catch block works

• if ( thrown exception instance of
  IllegalDataException )
• else if ( thrown exception instanceof
  ExecutionException )

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printStackTrace method

• java.io.FileNotFoundException fred.txt
• at java.io.FileInputStream.ltinitgt(FileInpu
  tStream.java)
• at java.io.FileInputStream.ltinitgt(FileInpu
  tStream.java)
• at ExTest.readMyFile(ExTest.java19)
• at ExTest.main(ExTest.java7)

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Heap/Stack
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Swallowing exceptions is is a major NO-NO!
try anObject.method( arg1 ) catch (
Exception e )
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(No Transcript)
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Runtime error 3256 at 0x34AD546
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Exceptions allow for extra info

• public class ParserException extends Exception
• public ParserException( File f, int iLine )
• this.f f
• this.iLine iLine
• public final File getFile() return f
• public final int getLine() return iLine
• private File f
• private int iLine

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Creating your own exception classes

• public class ApplicationException
• extends Exception
• public class ApplicationOpenFailedException
• extends ApplicationException

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Throwing multiple exceptions

• public interface IApplication
• public void open() throws ApplicationOpenFailedE
  xception, IOException
• public void close() throws ApplicationCloseFaile
  dException, IOException

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finally

• public class AlwaysDoThatWithStreams
• public static void main( String args )
• FileWriter fw null
• try
• fw new FileWriter( "args.txt" )
• for ( int i 0 i lt args.length i )
• os.println( "ARG" i " "
• args i "" )
• catch ( IOException e )
• System.out.println( "Error while writing
  args " e.getMessage() )
• finally
• if ( fw ! null ) fw.close()

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public static int parseInt(String s,
int radix) throws NumberFormatException
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Logging

• 01/01/2002 94503 EntityReader ERROR class
  StrangeEntity not found
• 01/01/2002 94505 EntityReader DEBUG created
  instance of BasicEntity
• Log.error( this, "class StrangeEntity not found"
  )
• Log.debug( this, "created instance of
  BasicEntity" )

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Singleton

• public class Singleton
• public static Singleton getInstance()
• return instance
• private Singleton()
• // create instance
• private static Singleton instance new
  Singleton()
• public static void main( String args )
• Singleton handle1 Singleton.getInstanc
  e()
• Singleton handle2 Singleton.getInstance
  ()
• System.out.println( "Handle1 " handle1
  "" )
• System.out.println( "Handle2 "
  handle2 "" )
• Handle1 Singleton_at_1c9f7e
• Handle2 Singleton_at_1c9f7e

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Log class

• public final class Log
• public static void debug( String msg )
• public static void debug( Object this, String
  msg )
• public static void error( String msg )
• public static void error( Object this, String
  msg )
• public static void warning( String msg )
• public static void warning( Object this,
  String msg )
• public static void setOptionsOn( int options
  )
• public static void setOptionsOff( int options
  )
• public static void setPrinter( ILogPrinter lp
  )
• public static ILogPrinter getPrinter()
• public static void registerClass( Class c )
• public static void unregisterClass( Class c
  )

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Log as utility class

• Log.getInstance().error( "Something is wrong" )
• Log.error( "Something is wrong" )

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Basic Log methods

• public final class Log
• public static void error( String msg )
• public static void error( Object o, String msg
  )
• public static void debug( String msg )
• public static void debug( Object o, String msg
  )

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Passing the object whichreports the message

• public static void error( Object o, String msg )
  
• Log( "ERROR " o.getClass().getName()
• "" msg )

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Factoring out the implementation

• public class ILogImpl
• public void print( String sMessage )

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Factoring out the implementation

• public class Log
• public static void setLogImpl( ILogImpl lm )
• logImpl lm
• private void print( String s )
• logImpl.print( s )
• private static ILogImpl logImpl

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StdOutLogImpl

• public final class StdOutLogImpl implements
  ILogImpl
• public StdOutLogImpl ()
• public void print( String s )
• System.out.println( s )

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Complete Log class

• public class Log
• /
• _at_param Object o - the object, which
  signals the error
• _at_param String sMsg - the error message
• _at_param Throwable - the exception which
  occurred
• /
• public static void error( Object o, String
  sMsg, Throwable e )
• public static void error( String sMsg,
  Throwable e )
• public static void error( Object o, String
  sMsg )
• public static void error( String sMsg )
• public static void debug( Object o, String
  sMsg )
• public static void debug( String sMsg )
• public static void warning( Object o, String
  sMsg )
• public static void warning( String sMsg )
• /
• _at_param Boolean bDebug - indicates if the
  debugging messages

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Assignment

• Please read
• Arnold Gosling Java Programming language
  Chapters on Exception and Garbage collection
• Chapters 16 and 25 from Agile Software
  Development Book
• Assignment 1
• Define implement Log, ILogImpl, StdOutLogImpl,
  FileLogImpl, MultiLogImpl