Object-Oriented Programming Design Topic : Exception Programming

Transcription

1 Electrical and Computer Engineering Object-Oriented Topic : Exception Maj Joel Young Joel.Young@afit.edu 18-Sep-03 Maj Joel Young

2 Error Handling General error handling options Notify the user, and Return to a safe state and resume program, or Terminate program gracefully Exception handling Transfer control from where error occurs to an error handler that can handle the situation Alternate execution path Types of errors User input errors usually handled in GUI Device errors handle with exception Physical limitations handle with exception Code errors best to test and eliminate 2

3 Java Exceptions Throwing exceptions Find or create an appropriate exception class Advertise that method can throw an exception Make an instance of exception class Throw the exception String readdata(bufferedreader in) throws EOFException... while(...) if (ch == -1) // EOF Encountered if (n < len) throw new EOFException();... return s; 3

4 Creating Exception Classes If available exception classes do not handle your problem then, create your own exception class FileFormatException extends IOException public FileFormatException() public FileFormatException(String gripe) super(gripe); 4

5 Catching Exceptions try/catch block try code more code... catch (ExceptionType e) handler for this type If any code in try block throws an exception of the specified type, then The program skips the remainder of the code in the try block The program executes the handler code inside the catch clause The method exits Otherwise, if none of the code in try block throws an exception, the method skips the catch clause If the code throws an exception of a different type, the method exists immediately 5

6 Catching Exceptions public void read(bufferedreader reader) try boolean done = false; while (!done) String line = reader.readline(); if (line == null) // end of file done = true; else process line catch (IOException exception) exception.printstacktrace(); 6

7 Passing Exceptions If your methods is not equipped to handle a method, it can be passed up to the caller public void read(bufferedreader reader) throws IOException boolean done = false; while (!done) String line = reader.readline(); if (line == null) // end of file done = true; else process line 7

8 More on exceptions Multiple exceptions Rethrowing exceptions The finally clause Graphics g = image.getgraphics(); try code that might throw exceptions catch (MalformedURLException e1) // action for handling malformed URLs throw e1; // rethrow exception pass to caller catch (IOException e2) // action for handling IO exceptions finally g.dispose(); 8

9 Hints for using exceptions Exception handling costs time, so use only where appropriate Hints Exception handling is not supposed to replace a simple test E.g. check if stack is empty before popping rather than throwing an exception when trying to pop and empty stack Do not micromanage exceptions No need to put every statement in try block Do not squelch exceptions As a minimum print some sort of error message Propagating exceptions is not a sign of shame Sometimes calling methods are better equipped to handle the exception 9