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

Programming in Java

Exception Handling again

蔡文能交通大學資訊工程學系[email protected]

http://www.csie.nctu.edu.tw/~tsaiwn/java/

交通大學資訊工程學系蔡文能 7-第 2頁

Java More Java Examples

Agenda

Exception handling

Exception handling in Java

Exception handling in C++



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



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”?



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



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");

}



Exception Handling: Some Options

Print something

Throw a new exception

Re-Throw the exception

Fix the problem

Exit



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).



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



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()



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.



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).



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}



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.



Exception Handling: exiting

• Sometimes the error is fatal, and you want to stop the program immediately.

System.exit();


Java More Java ExamplesWait.java

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"); }}



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



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.



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.



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.



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}



ThrowableThrowable

ExceptionException ErrorError

RunTimeExceptionRunTimeException

NullPointerExceptionNullPointerException

ArithmeticExceptionArithmeticException

IOExceptionIOException

Exception Type Hierarchy (partial)

EOFExceptionEOFException

VirtualMachineErrorVirtualMachineError



Some Exception Methods

These are actually inherited from throwable

printStackTrace()

fillInStackTrack()

getMessage()



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).



User-defined Exception Type

class FooException extends Exception {}

// ..

class BlahException extends Exception {

BlahException(){}

BlahException(String s) { super(s); }

}

// ..

throw new BlahException("Invalid blah");



using finally

try { statements . . .} catch (ExceptionType1 ename1) { error handling statements . . .} catch (ExceptionType2 ename2) { error handling statements . . .} finally { … this code always executed …}



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.



Exception Handling again

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交通大學資訊工程學系 programming in java exception handling again 蔡文能...

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