CS535: Doc 12, Exceptions

CS535 Object-Oriented Programming & Design
Fall Semester, 1996
Doc 12, Exceptions

[To Lecture Notes Index]
San Diego State University -- This page last updated Oct 7, 1996

Contents of Doc 12, Exceptions

Doc 12, Exceptions Slide # 1

References

Core Java, Chapter 10

The Java Programming Language, Arnold and Gosling, Chapter 7

A slipped gear could let your M203 grenade launcher fire when you least expect it. That would make you quite unpopular in what's left of your unit.

The U.S. Army's PS magazine, August 1993

Doc 12, Exceptions Slide # 2

Exceptions

Out-of Bounds
Java checks the bounds of arrays

class  ArrayOutOfBounds 
   {

   public static void main( String args[] ) 
      {
      int  students[] = new int[5];

      students[  10  ]  =  1;
      System.out.println( " The End" );
      }
   }

Output
java.lang.ArrayIndexOutOfBoundsException: 10
at ArrayOutofBounds.main(All.java:8)

Doc 12, Exceptions Slide # 3

Exception Handling
In a program there are events which may be considered exceptional or unusual

Error conditions
On zero divide have the program do some recovery process or give some debugging information
Special conditions If a plane is about to collide midair the navigation program should perform some special operation

Exceptional cases can be handled by explicit tests

   Boolean computeFoobar( ) {
      Boolean  zeroDivideFlag = false;

      if  ( A * A - 2 * B = 0 )
         zeroDivideFlag = true;
      else 
         result = sqrt( C )/ ( A * A - 2 * B )
      return zeroDivideFlag 
   }

   // in main

   flag  =  computeFoobar( )
   if ( flag == true )
      // now what?
Doc 12, Exceptions Slide # 4

Definitions Exception


A condition that is detected by an operation, that cannot be resolved within the local context of the operation, and must be brought to the attention of the operation's invoker

Exception handler or handler

The procedure or code that deals with the exception

Raising the exception

The process of noticing the exception, interrupting the program and calling the exception handler

Propagating the exception

Reporting the problem to a higher authority (the calling program)

Handling the exception

Taking appropriate corrective actions

Languages with Exception Handling
PL1, Clu, ADA, Mesa, C++, Smalltalk, Java

Doc 12, Exceptions Slide # 5

Issues

How is the exception raised?

Where can the Handler be placed - Scope of handler

How is the handler for the exception found?

All languages find the handler dynamically

What is executed after the handler has finished?

What information is passed to the handler?

Doc 12, Exceptions Slide # 6

How is the exception raised?

Implicitly by some error condition


class  ImplicitlyRaisedException
   {
   public static void main( String[] arguments )
      {
      int  students[] = new int[5];
      students[  10  ]  =  1;         // Exception occurs here
      }
   }

Explicitly by the Programmer

class  ExplicitlyRaisedException
   {
   public static void main( String[] arguments )
      {
      throw  new  ArrayIndexOutOfBoundsException()
      }
   }

Doc 12, Exceptions Slide # 7

Where can the Handler be placed?

Out-of Bounds - With try


class  ArrayOutOfBounds 
   {

   public static void main( String args[] ) 
      {
      int  students[] = new int[5];

      try  
         {
         System.out.println(  "Start Try"  );
         students[  10  ]  =  1;
         System.out.println(  "After assignment statement"  );
         }

      //  catch is the handler
      catch (ArrayIndexOutOfBoundsException e) 
         {
         System.err.println( "OutOfBounds: "  +  e.getMessage());
         };

      System.out.println(  " After try " );
      }
   }

Output
Start Try
OutOfBounds: 10
After try

Doc 12, Exceptions Slide # 8

try and catch must be together

try and catch are not separate blocks
try and catch are one construct

class  NeedCatchWithTryBlock 
   {

   public static void main( String args[] ) 
      {
      int  students[] = new int[5];

      try  
         {
         System.out.println(  "Start Try"  );
         students[  10  ]  =  1;
         System.out.println(  "After assignment statement"  );
         }

      System.out.println(  " After try " );      // Compile error
      }
   }

Compile Error

All.java:15: 'try' without 'catch' or 'finally'.
System.out.println( " After try " );

Doc 12, Exceptions Slide # 9

try and catch must be together

try and catch are not separate blocks
try and catch are one construct


class  NeedTryWithCatch 
   {

   public static void main( String args[] ) 
      {
      int  students[] = new int[5];

      System.out.println(  "Start Try"  );
      students[  10  ]  =  1;
      System.out.println(  "After assignment statement"  );

      //  catch is the handler
      catch (ArrayIndexOutOfBoundsException e) 
         {
         System.err.println( "OutOfBounds: "  +  e.getMessage());
         };

      System.out.println(  " After try " );
      }
   }

Compile Error
'catch' without 'try'.
catch (ArrayIndexOutOfBoundsException e)

Doc 12, Exceptions Slide # 10

try and catch must be together

try and catch are not separate blocks
try and catch are one construct


class  CanNotHaveCodeBetweenTryAndCatch 
   {
   public static void main( String args[] ) 
      {
      int  students[] = new int[5];

      try  
         {
         System.out.println(  "Start Try"  );
         students[  10  ]  =  1;
         System.out.println(  "After assignment statement"  );
         }

      int  DontHaveCodeHere  =  5 / 32;      // Error here

      //  catch is the handler
      catch (ArrayIndexOutOfBoundsException e) 
         {
         System.err.println( "OutOfBounds: "  +  e.getMessage());
         };

      System.out.println(  " After try " );
      }
   }

Compile Error
'try' without 'catch' or 'finally'.
int DontHaveCodeHere = 5 / 32;
'catch' without 'try'.

                catch (ArrayIndexOutOfBoundsException e)
Doc 12, Exceptions Slide # 11

How is the handler for the exception found?

The rules are slightly different for the two types of exceptions: Checked and Unchecked

Checked

Methods are required to state if they throw these exceptions

Unchecked

Standard runtime exceptions and errors
Methods are not required to state if they throw these exceptions

Subclasses of RuntimeException and Error are unchecked

AWTError	ArithmeticException
LinkageError	ArrayStoreException
ClassCircularityError	ClassCastException
ClassFormatError	EmptyStackException
IncompatibleClassChangeError	IllegalArgumentException
AbstractMethodError	IllegalThreadStateException
IllegalAccessError	NumberFormatException
InstantiationError	IllegalMonitorStateException
NoSuchFieldError	IndexOutOfBoundsException
NoSuchMethodError	ArrayIndexOutOfBoundsException
NoClassDefFoundError	StringIndexOutOfBoundsException
UnsatisfiedLinkError	NegativeArraySizeException
VerifyError	NoSuchElementException
ThreadDeath	NullPointerException
VirtualMachineError	SecurityException
InternalError	StackOverflowError
OutOfMemoryError	UnknownError

Doc 12, Exceptions Slide # 12

Finding the handler for Unchecked exception

Unchecked Exceptions: Search the stack for the first enclosing try block

class  FindHandlerSimpleExample  
{
   public  static  void  change(  int[]  course  )  
   {
      System.out.println( "Start Change"  );
      course[ 10 ]  =  1;
      System.out.println( "End Change"  );
   };

   public  static  void  main( String args[] ) 
   {
      int  students[] = new int[5];
      try  
      {
         System.out.println( "Start Try"  );
         change( students );
         System.out.println(  "After change"  );
      }  
      catch  (ArrayIndexOutOfBoundsException e) 
      {
         System.err.println( "OutOfBounds: "  +  e.getMessage());
      }   
      System.out.println(  "After try "  );
   }
}

Output Start Try
Start Change
OutOfBounds: 10
After try

Doc 12, Exceptions Slide # 13

Finding handler for Unchecked Exceptions

class  FindHandlerDoubleExample  {
   public  static  void  doubleChange(  int[]  course  )  {
      System.out.println( "Start Double Change"  );
      course[ 10 ]  =  1;
      System.out.println( "End Double Change"  );
   };

   public  static  void  change(  int[]  course  )  {
      System.out.println( "Start Change"  );
      doubleChange( course );
      System.out.println( "End Change"  );
   };

   public  static  void  main( String args[] ) {
      int  students[] = new int[5];
      try  
      {
         System.out.println( "Start Try"  );
         change( students );
         System.out.println(  "After change"  );
      }  
      catch  (ArrayIndexOutOfBoundsException e) 
      {
         System.err.println( "OutOfBounds: "  +  e.getMessage());
      }   
      System.out.println(  "After try "  );
   }
}

Output Start Try
Start Change
Start Double Change
OutOfBounds: 10
After try

Doc 12, Exceptions Slide # 14

Finding handler for Unchecked Exceptions

class  FindHandlerTwoTryBlocks  {
   public  static  void  change(  int[]  course  )  {
      try
      {
         System.out.println( "Start Change"  );
         course[ 10 ]  =  1;
         System.out.println( "End Change"  );
      }
      catch  (ArrayIndexOutOfBoundsException e) 
      {
         System.err.println( "Change Catch: "  +  e.getMessage());
      }
   };

   public  static  void  main( String args[] )  {
      int  students[] = new int[5];
      try  
      {
         System.out.println( "Start Try"  );
         change( students );
         System.out.println(  "After change"  );
      }
      catch  (ArrayIndexOutOfBoundsException e) 
      {
         System.err.println( "Main Catch: "  +  e.getMessage());
      }
      System.out.println(  "After try "  );
   }
}

Output Start Try
Start Change
Change Catch: 10
After change
After try

Doc 12, Exceptions Slide # 15

Finding handler for Unchecked Exceptions
What happens if there is no try block?

class  NoHandler  
{
   public  static  void  change(  int[]  course  )  
   {
      System.out.println( "Start Change"  );
      course[ 10 ]  =  1;
      System.out.println( "End Change"  );
   };

   public  static  void  main( String args[] ) 
   {
      int  students[] = new int[5];
      System.out.println( "Start Try"  );
      change( students );
      System.out.println(  "After change"  );
   }
}

Output
Start Try
Start Change
java.lang.ArrayIndexOutOfBoundsException: 10
at test.main(classNotes.java)

Doc 12, Exceptions Slide # 16

Finding the handler for Checked exceptions

Check Exceptions must be either:

handled in the method they are thrown or

the method in which they are thrown must state they will throw the exception

import sdsu.io.ASCIIInputStream;
import java.io.*;

class  CheckedExceptionDeclaredInMethod  {
   public  static  void  readSomething() throws IOException
   {
      System.out.println( "Please type an integer: " );
      int  youTyped;
      ASCIIInputStream  input = new ASCIIInputStream(System.in);

      youTyped = input.readInt();   // throws IOException
   };

   public  static  void  main( String args[] ) {
      try  
      {
         System.out.println( "Start Try"  );
         readSomething();
         System.out.println(  "After readSomething"  );
      }  
      catch  (IOException e) 
      {
         System.err.println( "IO error: "  +  e.getMessage());
      }   
      System.out.println(  "After try "  );
   }
}
Doc 12, Exceptions Slide # 17

Finding the handler for Checked exceptions

class  CheckedExceptionHandledInMethod  
{
   public  static  void  readSomething()
   {
      try
      {
         System.out.println( "Please type an integer: " );
         int  youTyped;
         ASCIIInputStream  input;
         input = new ASCIIInputStream(System.in);

         youTyped = input.readInt();   // throws IOException
      }
      catch  (IOException e) 
      {
         System.err.println( "IO error: "  +  e.getMessage());
      }   
   };

   public  static  void  main( String args[] ) 
{
      System.out.println( "Start Try"  );
      readSomething();
      System.out.println(  "After readSomething"  );
   }
}
Doc 12, Exceptions Slide # 18

Finding the handler for Checked exceptions

class  IfYouDontDeclaredInMethod  
{
   public  static  void  readSomething() 
   {
      System.out.println( "Please type an integer: " );
      int  youTyped;
      ASCIIInputStream  input = new ASCIIInputStream(System.in);

      youTyped = input.readInt();   // throws IOException
   };

   public  static  void  main( String args[] ) {
      try  
      {
         System.out.println( "Start Try"  );
         readSomething();
         System.out.println(  "After readSomething"  );
      }  
      catch  (IOException e) 
      {
         System.err.println( "IO error: "  +  e.getMessage());
      }   
      System.out.println(  "After try "  );
   }
}

Compile Error

Error   : Exception java.io.IOException must be caught, or it must be declared in the throws clause of this method.
line 12         youTyped = input.readInt();

Error   : Exception java.io.IOException is never thrown in the body of the corresponding try statement.
line 22         catch  (IOException e)

Doc 12, Exceptions Slide # 19

Finding the handler for Checked exceptions

class  SometimesItMakesSenseToDoBoth  
{
   public  static  void  readSomething() throws
   {
      try
      {
         System.out.println( "Please type an integer: " );
         int  youTyped;
         ASCIIInputStream  input;
         input = new ASCIIInputStream(System.in);

         youTyped = input.readInt();   // throws IOException
      }
      catch  (IOException e) 
      {
         System.err.println( "IO error: "  +  e.getMessage());
         throw e;
      }   
   };

   public  static  void  main( String args[] ) {
      try  
      {
         readSomething();
      }  
      catch  (IOException e) 
      {
         System.err.println( "Error: "  +  e.getMessage());
      }   
      System.out.println(  "After try "  );
   }
}
Doc 12, Exceptions Slide # 20

Multiple Exceptions


class  MultipleExceptions  
   {

   public static void main( String args[] ) 
      {
      int  students[] = new int[5];
      int  classSize  =  0;

      try  
         {
         System.out.println( "Start Try"  );
         int  classAverage  =  10 / classSize;
         System.out.println( "After class average"  );
         students[  10  ]  =  1;
         System.out.println(  "After array statement"  );
         } 

      catch  (ArrayIndexOutOfBoundsException e) 
         {
         System.err.println( "OutOfBounds: "  +  e.getMessage());
         }

      catch  (  ArithmeticException  e  )  
         {
         System.err.println( " Math Error"  +  e.getMessage() );
         }
      System.out.println(  " After try " );
      }
   }

Output Start Try
Math Error/ by zero
After try

Doc 12, Exceptions Slide # 21

One Size Fits All Exception


class  MultipleExceptions  
   {
   public static void main( String args[] ) 
      {
      int  students[] = new int[5];
      int  classSize  =  0;

      try  
         {
         System.out.println( "Start Try"  );
         students[  10  ]  =  1;
         System.out.println(  "After array statement"  );
         
         int  classAverage  =  10 / classSize;
         } 

      catch  ( Exception  e )    // Not a good idea
         {
         System.err.println( "Exception:"  +  e.getMessage());
         e.printStackTrace();
         }
 
      System.out.println(  " After try " );
      }
   }

Output
Start Try
Exception:10
java.lang.ArrayIndexOutOfBoundsException: 10
at MultipleExceptions.main(All.java:11)
After try

Doc 12, Exceptions Slide # 22

Beware of Order

class  MultipleExceptions  
{
   public static void main( String args[] ) 
      {
      int  students[] = new int[5];
      int  classSize  =  0;

      try  
      {
         System.out.println( "Start Try"  );
         students[  10  ]  =  1;
         System.out.println(  "After array statement"  );
         
         int  classAverage  =  10 / classSize;
      }  
      catch  ( Exception  e ) 
      {
         System.err.println( "Exception:"  +  e.getMessage());
         e.printStackTrace();
      }  
      catch  (ArrayIndexOutOfBoundsException e) 
      {
         System.err.println( "OutOfBounds: "  +  e.getMessage());
      }
      System.out.println(  " After try " );
   }
}

Compile Error All.java:21: catch not reached.
} catch (ArrayIndexOutOfBoundsException e) {
^
1 error

Doc 12, Exceptions Slide # 23

Select the First Exception That Applies

class  MultipleExceptions  
{
   public static void main( String args[] ) 
      {
      int  students[] = new int[5];
      int  classSize  =  0;

      try  
      {
         System.out.println( "Start Try"  );
         students[  10  ]  =  1;
         System.out.println(  "After array statement"  );
         
         int  classAverage  =  10 / classSize;
      }  

      catch  (ArrayIndexOutOfBoundsException e) 
      {
         System.err.println( "OutOfBounds: "  +  
                                 e.getMessage());
      }   

      catch  ( Exception  e ) 
      {
         System.err.println( "Exception:"  +  e.getMessage());
         e.printStackTrace();
      }
      System.out.println(  " After try " );
   }
}

Output Start Try
OutOfBounds: 10
After try

Doc 12, Exceptions Slide # 24

What is executed after the handler has finished?

The line of code after the try block

Finally blocks change this slightly

class  ArrayOutOfBounds 
   {

   public static void main( String args[] ) 
      {
      int  students[] = new int[5];

      try  
         {
         System.out.println(  "Start Try"  );
         students[  10  ]  =  1;
         System.out.println(  "After assignment statement"  );
         }

      //  catch is the handler
      catch (ArrayIndexOutOfBoundsException e) 
         {
         System.err.println( "OutOfBounds: "  +  e.getMessage());
         };

      System.out.println(  " After try " );
      }
   }

Output
Start Try
OutOfBounds: 10
After try

Doc 12, Exceptions Slide # 25

Exceptions inside Exceptions

class  ExceptionFunction  {
   public  static  boolean  change(  int[]  course  )  {
      try  {
         System.out.println( "Start change"  );
         course[ 10 ]  =  1;
      }  
      catch  (ArrayIndexOutOfBoundsException e) {
         course[ 10 ]  =  1;
         System.err.println( "OutOfBounds: " );
         return  true;
      }   
      finally   {
         System.out.println(  "In Final Block"  );
      }
      return  false;
   };

   public  static  void  main( String args[] ) {
      try  {
         int  students[] = new int[5];
         System.out.println( "Main"  );
         System.out.println(  change( students )  );
         System.out.println(  "After Change"  );
      }
      catch  (ArrayIndexOutOfBoundsException e) {
         System.out.println( "Over Here" );
      }
   }
}

Output

Main
Start change
In Final Block
Over Here
Doc 12, Exceptions Slide # 26

Final Block - For Clean Up


class  FinalBlockWithExceptionCalled  
{
   public static void main( String args[] ) 
   {
      int  students[] = new int[5];

      try  
      {
         System.out.println( "Start Try"  );
         students[  10  ]  =  1;
         System.out.println(  "After array statement"  );
      }

      catch  (ArrayIndexOutOfBoundsException e) 
      {
         System.err.println( "OutOfBounds: "  +  e.getMessage());
      }

      finally
      {
         System.out.println(  "In Final Block"  );
      }

      System.out.println(  " After try " );
   }
}

Output OutOfBounds: 10
In Final Block
After try

Doc 12, Exceptions Slide # 27

Final Block - Always Called


class  FinalBlockExceptionNotCalled  
{
   public static void main( String args[] ) 
   {
      int  students[] = new int[5];

      try  
      {
         System.out.println( "Start Try"  );
         students[  2  ]  =  1;
         System.out.println(  "After array statement"  );
      }  

      catch  (ArrayIndexOutOfBoundsException e) 
      {
         System.err.println( "OutOfBounds: "  +  e.getMessage());
      }   

      finally   
      {
         System.out.println(  "In Final Block"  );
      }

      System.out.println(  " After try "  );
   }
}

Output Start Try
After array statement
In Final Block
After try

Doc 12, Exceptions Slide # 28

Can't Escape Finally


class  ExceptionFunction  
{
   public  static  boolean  change(  int[]  course  )  
   {
      try  
      {
         System.out.println( "Start change"  );
         course[ 10 ]  =  1;
         System.out.println( "End Change"  );
      }  
      catch  (ArrayIndexOutOfBoundsException e) 
      {
         System.err.println( "OutOfBounds: " );
         return  true;
      }   
      finally   
      {
         System.out.println(  "In Final Block"  );
      }
      return  false;
   };

   public  static  void  main( String args[] ) 
   {
      int  students[] = new int[5];
      System.out.println( "Main"  );
      System.out.println(  change( students )  );
      System.out.println(  "After Change"  );
   }
}

Output

Main   In Final Block
Start Change   true
OutOfBounds:   After Change
Doc 12, Exceptions Slide # 29

What information is passed to the handler?

Two pieces of information are standard

Message

Stack Info

When you define your own exceptions you can add more data if needed

class  FindHandlerSimpleExample  {
   public  static  void  change(  int[]  course  )  {
      course[ 10 ]  =  1;
   };

   public  static  void  main( String args[] ) {
      int  students[] = new int[5];
      try  {
         change( students );
      }  
      catch  (ArrayIndexOutOfBoundsException e)  {
         System.err.println( "Message: "  +  e.getMessage()  );
         System.err.println( "Stack: "  +  e.printStackTrace()  );
      }   
   }
}

Output Message: 10
java.lang.ArrayIndexOutOfBoundsException: 10
at test.change(All.java:7)
at test.main(All.java:13)

Doc 12, Exceptions Slide # 30

User Defined Exceptions

class  BoringLectureException  extends  Exception  {

   public  BoringLectureException()  {
      super();
   }

   public  BoringLectureException( String  errorMessage )  {
      super(  errorMessage  );
   }
}


class  NewExceptionTest  {

   public static void dullSpeaker() throws BoringLectureException {
      // Code can go here
      throw  new  BoringLectureException(  "Change Topics"  );
   }

   public  static  void  main(  String  args[]  )   {

      try  {
         dullSpeaker();

      } catch  (  BoringLectureException  e  )  {

         System.err.println( "Error: Time to "  +  e.getMessage() );
      }
   }
}

Output Error: Time to Change Topics

Doc 12, Exceptions Slide # 31

Tips on Using Exceptions

1 Use exceptions for exceptional conditions, not simple tests
Good

Stack accountHistory;

if ( accountHistory.isEmpty() == false )
   accountHistory.pop();

Not so Good

try
   {
   accountHistory.pop();
   }
catch ( EmptyStackException doNothing )
   {
   }

Doc 12, Exceptions Slide # 32

A try for every line, and every line for a Try?
2. Don't Micro manage Exceptions
bad

try
   {
   amount = accountHistory.pop();
   }
catch ( EmptyStackException doNothing )
   {
   amount = -1;
   }
try
   {
   out.writeFloat( amount );
   }
catch ( IOException ioError )
   {
   System.err.out( "problem writing to file" );
   }

Better

try
   {
   amount = accountHistory.pop();
   out.writeFloat( amount );
   }
catch ( EmptyStackException doNothing )
   {
   }
catch ( IOException ioError )
   {
   System.err.out( "problem writing to file" );
   }
Doc 12, Exceptions Slide # 33

Free Speech for Exceptions!
3 Don't Squelch Exceptions
Bad

try
   {
   amount = accountHistory.pop();
   out.writeFloat( amount );
   
   // a bunch more code that can throw various exceptions

   }
catch ( Exception dontBotherMeWithExceptions )
   {
   }

4 Catch Exceptions

public static void main( String[] args ) throws Exception
   {
   // Lots of stuff here
   }
Doc 12, Exceptions Slide # 34

List of All Built-in Exceptions

AWTException	InstantiationException
ArithmeticException	InterruptedException
ArrayIndexOutOfBoundsException	InterruptedIOException
ArrayStoreException	MalformedURLException
ClassCastException	NegativeArraysizeException
ClassNotFoundException	NoSuchElementException
CloneNotSupportedException	NoSuchMethodException
EOFException	NullPointerException
EmptyStackException	NumberFormatException
Exception	ProtocolException
FileNotFoundException	RuntimeException
IOException	SecurityException
IllegalAccessException	SocketException
IllegalArgumentException	StringIndexOutOfBoundsException
IllegalMonitorStateException	UTFDataFormatException
IllegalThreadStateException	UnknownHostException
IndexOutOfBoundsException	UnknownServiceException

Errors

AWTError	LinkageErrorNoClassDefFoundError
AbstractMethodError	NoSuchFieldErrorNoSuchMethodError
ClassCircularityError	OutOfMemoryError
ClassFormatErrorError	StackOverflowError
IllegalAccessError	UnknownError
IncompatibleClassChangeError	UnsatisfiedLinkErrorVerifyError
InstantiationErrorInternalError	VirtualMachineError

CS535 Object-Oriented Programming & Design Fall Semester, 1996 Doc 12, Exceptions

Contents of Doc 12, Exceptions

CS535 Object-Oriented Programming & Design
Fall Semester, 1996
Doc 12, Exceptions