This site is from a past semester! The current version will be here when the new semester starts.

Week 5 [Mon, Feb 6th] - SE Topics

Detailed Table of Contents

[W5.1] Exception Handling

W5.1a

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

W5.1b

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 runtime.

Exception:

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.)

Examples:

  • 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.

W5.1c

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 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.

Exercises

W5.1d

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'.