1. Lecture 17
Exception Handling
CSE 1322
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2. Exception Handling
An exception is an indication of a problem that occurs during a program’s execution.
Exception handling enables applications to resolve exceptions.
Exception handling enables clear, robust and more fault-tolerant programs.
Exception handling helps improve a program’s fault tolerance.
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3. Exception Handling in Real life
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4. Exception Handling in Real life
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5. Exceptions
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6. Exceptions Illegal operations at run time can generate an exception.
For example, we have seen these exceptions:
Arithmetic Exception
Null Pointer Exception
Input Mismatch Exception
Number Format Exception
Array Index Out Of Bounds Exception
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7. Exception Handling Consider the following pseudocode:
Perform a task
If the preceding task did not execute correctly Perform error processing Perform next task
If the preceding task did not execute correctly Perform error processing …
In this pseudocode, we begin by performing a task; then we test whether that task executed correctly. If not, we perform error processing.
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8. Exception Handling
Exception handling enables programmers to remove error-handling code from the “main line” of the program’s execution.
Programmers can decide to handle all exceptions, all exceptions of a certain type or all exceptions of related types.
Such flexibility reduces the likelihood that errors will be overlooked.
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9. Exception Class Hierarchy Java
The Exception class, RuntimeException class,
and their subclasses are in the java.lang package.
The ExceptionClass parameter to the catch block can be any of these exceptions.
The IOException class and its subclasses are in the java.io package.
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10. Exception Class Hierarchy Java
The Exception class, RuntimeException class,
and their subclasses are in the java.lang package.
The ExceptionClass parameter to the catch block can be any of these exceptions.
The IOException class and its subclasses are in the java.io package.
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11. Exception Class Hierarchy Java
The Exception class, RuntimeException class,
and their subclasses are in the java.lang package.
The ExceptionClass parameter to the catch block can be any of these exceptions.
The IOException class and its subclasses are in the java.io package.
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12. C# Exception Hierarchy
System exceptions are defined in .NET libraries and are used by the framework, while application exceptions are defined by application developers and are used by the application software.
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13. .NET Exception Hierarchy
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14. Handling Exceptions
We don't want invalid user input to terminate our programs! It is better to detect the problem and re-prompt the user for the input.
Java and C# allow us to intercept and handle some of these exceptions using try and catch blocks.
Inside the try block, we put the code that might generate an exception.
Inside catch blocks, we put the code to handle any exceptions that could be generated.
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15. Exception Handling
When a method called in a program detects a problem, the method throws an exception.
The point at which an exception occurs is called the throw point
If an exception occurs in a try block, program control immediately transfers to the first catch block matching the type of the thrown exception.
After the exception is handled, program control resumes after the last catch block.
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16. Exception Handling
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17. Minimum try/catch Syntax{
// code that might generate an exception }
catch( ExceptionClass exceptionObjRef )
// code to recover from the exception
If an exception occurs in the try block, control jumps immediately to the catch block. No further instructions in the try block are executed.
If no exceptions are generated in the try block, the catch block is not executed.
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18. Catch Block
When an exception occurs in a try block, a corresponding catch block catches the exception and handles it.
At least one catch block must immediately follow a try block.
A catch block specifies an exception parameter representing the exception that the catch block can handle.
Optionally, you can include a catch block that does not specify an exception type to catch all exception types.
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19. Catch Block
catch(IO.FileNotFoundException fnfe) {   handle file not found (using fnfe object) } catch(Exception e) {   handle other type of exception (using e object)   }
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20. finally Block
Programs frequently request and release resources dynamically. Operating systems typically prevent more than one program from manipulating a file.
Therefore, the program should close the file (i.e., release the resource) so other programs can use it. If the file is not closed, a resource leak occurs.
The finally block is guaranteed to execute regardless of whether an exception occurs.
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21. finally Block
Local variables in a try block cannot be accessed in the corresponding finally block, so variables that must be accessed in both should be declared before the try block.
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22. Full try/catch/finally Syntax
try { // code that might generate an exception } catch( Exception1Class e1 ) // code to handle an Exception1Class exception } … catch( ExceptionNClass eN ) // code to handle an ExceptionNClass exception finally // code to execute whether or not an exception occurs
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23. Exception Handling in Real life
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24. Reminder
Do not place try blocks around every statement that might throw an exception.
It’s better to place one try block around a significant portion of code, and follow this try block with catch blocks that handle each possible exception.
Then follow the catch blocks with a single finally block.
Separate try blocks should be used when it is important to distinguish between multiple statements that can throw the same exception type.
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25. Checked and Unchecked Exceptions
Java distinguishes between two types of exceptions:
Unchecked exceptions are those that are subclasses of Error or RuntimeException
It is not mandatory to use try and catch blocks to handle these exceptions.
Checked exceptions are any other exceptions.
Code that might generate a checked exception must be put inside a try block or the method must acknowledge that the exception may occur by using a throws clause in the method header. Otherwise, the compiler will generate an error.
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26. Checked vs Unchecked Exceptions
Unchecked exceptions are those that are subclasses of Error or RuntimeException
Checked exceptions are any other exceptions.
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27. Checked vs Unchecked Exceptions
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28. Checked vs Unchecked Exceptions
Unchecked exceptions are those that are subclasses of Error or RuntimeException
Checked exceptions are any other exceptions.
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29. Checked and Unchecked Exceptions
Java distinguishes between two types of exceptions:
Unchecked exceptions are those that are subclasses of Error or RuntimeException
It is not mandatory to use try and catch blocks to handle these exceptions.
Checked exceptions are any other exceptions.
Code that might generate a checked exception must be put inside a try block or the method must acknowledge that the exception may occur by using a throws clause in the method header. Otherwise, the compiler will generate an error.
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30. SOFTWARE ENGINEERING TIP
Write code to catch and handle exceptions generated by invalid user input.
Always try to write code that is user-friendly.
Although the methods of the Exception class are good debugging tools, they are not necessarily appropriate to use in the final version of a program.
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31. Designing Your Own Execution Types
public class InsufficientFundsException extends RuntimeException { public InsufficientFundsException() {} public InsufficientFundsException(String message) { super(message); } }
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32. Throwing an Exception
The pattern for a method that throws a user-defined exception is:
  accessModifier returnType methodName( parameters ) throws ExceptionName {
if ( parameter list is legal )
process the parameter list
else
throw new ExceptionName( "Message here" ); }
The message passed to the constructor identifies the error detected. In a client's catch block, the getMessage method retrieves that message.
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33. Designing Your Own Execution Types
public class _1322_Exception : Exception { public _1322_Exception(string message) : base(message) }
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34. Throwing an Exception
The pattern for a method that throws a user-defined exception is:
  accessModifier returnType methodName( parameters ) {
if ( parameter list is legal )
process the parameter list
else
throw new ExceptionName( "Message here" ); }
The message passed to the constructor identifies the error detected. In a client's catch block, the Message property retrieves that message.
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35. Throwing an Exception { CreateExc(); } catch (_1302_Exception myE)
try {
CreateExc(); }
catch (_1302_Exception myE)
Console.WriteLine("caught it " + myE.Message);
private static void CreateExc()
Console.Write("Enter 1 to see our own exception work: ");
int s = int.Parse(Console.ReadLine());
if (s == 1)
_1302_Exception ex = new _1302_Exception("value == 1 for testing purposes");
throw ex;
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36. The throw vs throws Keywords in Java
An Interview Question
The throw vs throws Keywords in Java
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37. The throw vs throws Keywords in Java
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38. The final, finally, and finalize keywords in Java
Other Interview Questions
The final, finally, and finalize keywords in Java
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39. The final, finally, and finalize keywords in Java
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40. The final, finally, and finalize keywords in Java
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41. Exception Handling in Real life
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