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TE3201 2020
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  • Week 5 [Feb 10] - SE Topics

    • [W5.1] Exception Handling
    • [W5.1a] Implementation → Error Handling → Introduction → What

    • [W5.1b] Implementation → Error Handling → Exceptions → What

    • [W5.1c] Implementation → Error Handling → Exceptions → How

    • [W5.1d] Implementation → Error Handling → Exceptions → When

    [W5.1] Exception Handling


    Implementation → Error Handling → Introduction → What

    Can explain error handling

    Well-written applications include error-handling code that allows them to recover gracefully from unexpected errors. When an error occurs, the application may need to request user intervention, or it may be able to recover on its own. In extreme cases, the application may log the user off or shut down the system. --Microsoft


    Implementation → Error Handling → Exceptions → What

    Can explain exceptions

    Exceptions are used to deal with 'unusual' but not entirely unexpected situations that the program might encounter at run time.


    The term exception is shorthand for the phrase "exceptional event." An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions. –- Java Tutorial (Oracle Inc.)


    • A network connection encounters a timeout due to a slow server.
    • The code tries to read a file from the hard disk but the file is corrupted and cannot be read.


    Implementation → Error Handling → Exceptions → How

    Can explain how exception handling is done typically

    Most languages allow code that encountered an "exceptional" situation to encapsulate details of the situation in an Exception object and throw/raise that object so that another piece of code can catch it and deal with it. This is especially useful when the code that encountered the unusual situation does not know how to deal with it.

    The extract below from the -- Java Tutorial (with slight adaptations) explains how exceptions are typically handled.

    When an error occurs at some point in the execution, the code being executed creates an exception object and hands it off to the runtime system. The exception object contains information about the error, including its type and the state of the program when the error occurred. Creating an exception object and handing it to the runtime system is called throwing an exception.

    After a method throws an exception, the runtime system attempts to find something to handle it in the the ordered list of methods that had been called to get to the method where the error occurredcall stack. The runtime system searches the call stack for a method that contains a block of code that can handle the exception. This block of code is called an exception handler. The search begins with the method in which the error occurred and proceeds through the call stack in the reverse order in which the methods were called. When an appropriate handler is found, the runtime system passes the exception to the handler. An exception handler is considered appropriate if the type of the exception object thrown matches the type that can be handled by the handler.

    The exception handler chosen is said to catch the exception. If the runtime system exhaustively searches all the methods on the call stack without finding an appropriate exception handler, the program terminates.

    Advantages of exception handling in this way:

    • The ability to propagate error information through the call stack.
    • The separation of code that deals with 'unusual' situations from the code that does the 'usual' work.

    Which are benefits of exceptions?

    • a. Exceptions allow us to separate normal code from error handling code.
    • b. Exceptions can prevent problems that happen in the environment.
    • c. Exceptions allow us to handle in one location an error raised in another location.

    (a) (c)

    Explanation: Exceptions cannot prevent problems in the environment. They can only be used to handle and recover from such problems.


    Implementation → Error Handling → Exceptions → When

    Can avoid using exceptions to control normal workflow

    In general, use exceptions only for 'unusual' conditions. Use normal return statements to pass control to the caller for conditions that are 'normal'.