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Programming in Java
Exception Handling
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Exception example
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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Exception Handling: throw
• You can throw an exception 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 example
public 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: 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 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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Wait.java
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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 an alternate
return
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Exception Enforcement
In general, you do the following:
 specify what exceptions each method can generate.
(throw)
 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.java
import 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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Exception Type Hierarchy (partial)
Throwable
Exception
Error
VirtualMachineError
RunTimeException
IOException
NullPointerException
EOFException
ArithmeticException
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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).
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
謝謝捧場
http://www.csie.nctu.edu.tw/~tsaiwn/oop/
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