Post on 20-Dec-2015
交通大學資訊工程學系
Programming in Java
Exception Handling again
蔡文能交通大學資訊工程學系tsaiwn@csie.nctu.edu.tw
http://www.csie.nctu.edu.tw/~tsaiwn/java/
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Agenda
Exception handling
Exception handling in Java
Exception handling in C++
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Exception? An exceptionexception is any unusual event, either erroneous or
not, detectable by either hardware or software, that may require special processing
Without exception handlingexception handling When an exception occurs, control goes to the operating system,
where typically an error message is displayed
the program is terminated
With exception handling Programs are allowed to trap exceptions There is a possibility to fix the problem and continuing execution
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What to do when Exception occurs?There are several ways a function may react when encountering an unexpected/erroneous situation (i.e., Exception ) Terminate the programTerminate the program
Cannot be used in a program that cannot afford to crash Return a value representing “error”Return a value representing “error”
Raise a flagRaise a flagMany standard C library functions used global variable errno
Need to check the flag after every operation Losses information
Call a function supplied to be called in case of errorCall a function supplied to be called in case of errorRaise a signal / EventHow will it handle the error?
char* String::getString(); //ok: return NUKK on errorint atoi(const char *); //what to return for “bad-str”?
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When To Use Exceptions
Required by compiler when using some statements (Java)
Make code safer (catch fatal errors)
Fix the problem and try again
Patch the problem and continue
Make a complex error mechanism simpler
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Exception example
try {
readFromFile("datafile");
} catch (FileNotFoundException e) {
System.err.println("Error: File not found");
}
try {
readFromFile("datafile");
} catch (FileNotFoundException e) {
System.err.println("Error: File not found");
}
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Exception Handling: Some Options
Print something
Throw a new exception
Re-Throw the exception
Fix the problem
Exit
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Exception Handling: Printing
You can print a stack trace by calling the exception method printStackTrace()
Sometimes it's better to send error messages to stderr: System.err.println("Error: invalid thingy");
Some applications log error messages file logging service (syslog).
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Throwing Objects
Why throw an object? Additional data encapsulated
An object is created upon invoking the throw statement
Can be created with additional data, passed to Ctor
Object is destroyed after the catch clause ends
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Exception Handling: throw
• You can throw an exception (object) from an exception handler (a catch block).• Allows you to change exception type and/or
error message.• You can also alter the base of the stack trace
•fillInStackTrace()
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Exception Handling: Re-throw
• You can throw an exception from an exception handler (a catch block) without changing anything:• called rethrowing• The caller needs to deal with the exception.• This also happens if you don't catch the
exception!• sometimes you need to take some action and
then rethrow the exception.
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Another way to re-throw
You can allow selected types of exceptions to be propogated to the caller of your method:
void blah() throws IOException {
Within blah() you don't need to catch these exceptions (to be able to compile).
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Exception examplepublic class ThrowUp {
public static void main(String[ ] args) { String tmp; try { // generate an ArrayIndexOutOfBoundsExceptions ..
// .. (on purpose!). for (int i=0 ; i<args.length+10 ; i++) { tmp = args[i]; } System.out.println("I made it!"); } catch (ArrayIndexOutOfBoundsException up) { throw new SecurityException("Trouble"); } } // main}
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Exception Handling: Fix the problem.
• You can't fix things and then resume execution automatically• you can do this in C++.
• You can have a loop the retries the code again.
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Exception Handling: exiting
• Sometimes the error is fatal, and you want to stop the program immediately.
System.exit();
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public class Wait { public static void main(String[ ] args) { boolean done=false; int seconds=0; try { seconds = Integer.parseInt(args[0]); } catch (Exception e) { System.out.println("Please specify the number of seconds"); System.exit(1); } long initialtime = System.currentTimeMillis( ); while (!done) { try { if (System.currentTimeMillis()-initialtime < seconds*1000) throw new Exception("Not Yet - keep trying"); done=true; } catch (Exception e) { System.out.println(e); } } System.out.println("Done!\n"); }}
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How/when do you generate exceptions?
Use throw:
throw new Exception("broken!");
You can use throw anywhere. you detect some error that means the following code should not be executed.
In some cases, you can think of throw as a alternate return
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Exception Enforcement
In general, you do the following: specify what exceptions each method can generate. write code to catch all exceptions that can be generated by a
method call.
The compiler (usually) enforces this it is a compilation error to call a method without catching it's
declared exception types.
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RunTime Exceptions
There are exceptions that are generated by the system (that are usually caused by programming mistakes): NullPointerException (null references) ArrayIndexOutOfBoundsException
If you don't catch these, a stack trace will be generated and the program will terminate.
The compiler does not force you to catch these exceptions.
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Exception Types
Exceptions are objects!
Exception types are classes. A (quite large!) hierarchy of classes.
All exception types are derived from the class Exception there are some methods defined in this base class.
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NullPointer.javaimport java.util.*;public class NullPointer { public static void main(String[ ] args) { Date f = new Date(); f=null; PrintSomething2(f); // comment out to test PrintSomething PrintSomething(f); } // no checks, if x is null runtime exception default behavior static void PrintSomething(Object x) { System.out.println(x.getClass( ).getName()); } // we explicitly check for runtime exception! static void PrintSomething2(Object x) { try { System.out.println(x.getClass().getName()); } catch (RuntimeException re) { System.out.println("Fatal Error!"); System.exit(1); } } // main}
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ThrowableThrowable
ExceptionException ErrorError
RunTimeExceptionRunTimeException
NullPointerExceptionNullPointerException
ArithmeticExceptionArithmeticException
IOExceptionIOException
Exception Type Hierarchy (partial)
EOFExceptionEOFException
VirtualMachineErrorVirtualMachineError
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Some Exception Methods
These are actually inherited from throwable
printStackTrace()
fillInStackTrack()
getMessage()
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Creating Your Own Exception Types
It is often useful to create your own type of exception. generally all you create is a name. you can get fancy and add new methods to your exception
class(es).
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User-defined Exception Type
class FooException extends Exception {}
// ..
class BlahException extends Exception {
BlahException(){}
BlahException(String s) { super(s); }
}
// ..
throw new BlahException("Invalid blah");
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using finally
try { statements . . .} catch (ExceptionType1 ename1) { error handling statements . . .} catch (ExceptionType2 ename2) { error handling statements . . .} finally { … this code always executed …}
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Why finally?
What is there to clean up? No memory cleanup required in Java! No destructors to call!
Sometimes you need to set the state of things (fields) to some stable (acceptable) state.
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Exception Handling again
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