Exceptions, Singleton, Eworld

1
Lecture 4

• Exceptions, Singleton, Eworld XML

2
Error handling in procedural languages

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

3
Exceptions

• public interface IGraphAlgorithm
• public void apply( IGraph ) throws
  ExecutionException

4
Example

• IGraph graph BasicGraph()
• IGraphAlgorithm alg
• new DepthFirstTraversal()
• try
• alg.apply( graph )
• catch ( ExecutionException e )
• System.err.println( "Exception while running
  DepthFirstTraversal e.getMessage() )
• e.printStackTrace()

5
Creating your own exception class

• public class DatabaseException extends Exception
  
• public DatabaseException(String message )
• super( message )

6
Using exceptions throughout your package

• public class TableDescriptor
• public Enumeration getColumnDescriptors()
• throws DatabaseException
• try
• // do whatever
• catch ( Exception e )
• // Just rethrow it
• throw new DatabaseException(e, while getting
  column descriptors )

7
Wrapping exceptions

• public interface IExceptionWrapper
• public Exception getException()
• public class DatabaseException
• extends Exception implements
  IExeptionWrapper
• public DatabaseException( String message )
  super( message )
• public DatabaseException( String message,
  Exception eWrapped )
• super( message )
• this.eWrapped eWrapped
• public Exception getException() return
  eWrapped
• private Exception eWrapped

8
Catching different exceptions

• try
• // Block of code
• catch ( ExceptionTypeOne e1 )
• // do something
• catch ( ExceptionTypeOne e2 )
• // do something
• catch ( ExceptionTypeN eN )
• // do something
• catch ( Exception e ) // Should always be
  last
• // do something

9
Finally clause

• public class StreamTest
• public static void main( String args )
• OutputStream os null
• try
• os new DataOutputStream( args.txt )
• for ( int i 0 i lt args.length i )
• os.println( ARG i args i
  )
• catch ( IOException e )
• System.err.println( Exception while writing
  to stream e.getMessage() )
• finally
• if ( os ! null )
• try os.close() catch ( Exception e )
  

10
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

11
Static initialization blocks

• static
• Singleton mySingleton
• new Singleton()
• mySingleton.init( "Hello" )
• static
• int iArray new int 100
• for ( int i 0 i lt iArray.length i )
• iArray i i

12
EWorld
Fighting










Lunch
E
E
r
E
E
Trading
r
E
E
E
E
Clone
Mating
E
r
r
13
XML

• XML is a meta language
• Every tag may have attributes and children
• ltemailgt
• ltfromgtAlex Iskoldlt/fromgt
• lttogtStudents of SE Java, 2001lt/fromgt
• ltmessagegt
• XML does not mix syntax semantics!
• lt/messagegt
• lt/emailgt

14
But in HTML

• lthtmlgt
• ltbodygt
• lth1gtFrom Alex Iskoldlt/h1gt
• lth1gtTo Students of SE Java, 2001lt/h1gt
• ltbgtXML does not mix syntax
  semantics!lt/bgt
• lt/bodygt
• lt/htmlgt

15
Assignment

• Please read
• Arnold Gosling Java Programming language
  Chapter 7, 12
• Singleton, page 127 Design Patterns book