Bài giảng điện tử môn tin học: Developing Comprehensive Projects pps

08/13/14 Võ Phương Bình - ITFAC - DLU 1 Part IV: Developing Comprehensive Projects  Chapter 10: Exception Handling  Chapter 11: Multithreading  Chapter 12: Multimedia  Chapter 13: Input and Output  Chapter 14: Networking 08/13/14 Võ Phương Bình - ITFAC - DLU 2 Chapter 10 Exception Handling  What exceptions are for  What exceptions are NOT for  Catching & Throwing exceptions  Exception Specifications  Standard Java Exceptions  Exceptions and Polymorphism  The finally clause  Resource Management  Uncaught Exceptions 08/13/14 Võ Phương Bình - ITFAC - DLU 3 What Exceptions are For  To handle Bad Things  I/O errors, other runtime errors  when a function fails to fulfill its specification  so you can restore program stability (or exit gracefully) 08/13/14 Võ Phương Bình - ITFAC - DLU 4 What Exceptions are For, cont  To force you to handle Bad Things  because return codes can be tedious  and sometimes you’re lazy 08/13/14 Võ Phương Bình - ITFAC - DLU 5 Example File I/O public FileReader(String fileName) throws FileNotFoundException public void close() throws IOException 08/13/14 Võ Phương Bình - ITFAC - DLU 6 import java.io.*; class OpenFile { public static void main(String[] args) { if (args.length > 0) { try { // Open a file: FileReader f = new FileReader(args[0]); System.out.println(args[0] + " opened"); f.close(); } catch (IOException x) { System.out.println(x); } } } } Example File I/O, cont 08/13/14 Võ Phương Bình - ITFAC - DLU 7 What Exceptions are For, cont  To signal errors from constructors  because constructors have no return value 08/13/14 Võ Phương Bình - ITFAC - DLU 8 What Exceptions are NOT For  NOT For Alternate Returns:  e.g., when end-of-file is reached: while ((s = f.readLine()) != null) …  Exceptions are only for the exceptional! 08/13/14 Võ Phương Bình - ITFAC - DLU 9 Catching Exceptions  Wrap code to be checked in a try-block  checking occurs all the way down the execution stack  try-blocks can be nested  control resumes at most enclosed matching handler 08/13/14 Võ Phương Bình - ITFAC - DLU 10 Catching Exceptions, cont  Place one or more catch-clauses after try- block  runtime system looks back up the call stack for a matching handler  subclass types match superclass types  catching Exception catches everything (almost)  handlers are checked in the order they appear  place most derived types first!  execution resumes after last handler  if you let it (could branch or throw) [...]...  Same as logic_error in C++ Program logic errors   e.g., bad cast, using a null handle, array index violation, etc Shouldn’t happen!   Similar in spirit to C’s assert( ) macro   fixed during testing mainly for debugging These are called “unchecked exceptions” 08/13/14 Võ Phương Bình - ITFAC 26 java.lang.RuntimeException Subclasses (sample)        ArithmeticException (e.g., divide by 0) . 08/13/14 Võ Phương Bình - ITFAC - DLU 1 Part IV: Developing Comprehensive Projects  Chapter 10: Exception Handling  Chapter 11: Multithreading  Chapter