Catching Exceptions

• If an exception is not caught, it will terminate the program.
• Catching an exception requires a try-catch block.  You must wrap the exception-prone code in this block.
• It starts with try {
• It ends with } catch
• All code that might throw an exception, such as calls to exception-throwing methods, must go between these curly braces.
• You can have just one catch, or multiple catches.  Multiple catches only require one try.
• Make sure you go from most general to most specific.
• You can even nest try-catches.
• Over-simplified examples (these are all legal):
try {
    someMethod();
} catch (Exception e) {
    System.out.println(e.getMessage());
    e.printStackTrace();
}

try {
    someMethod1();
    try {
        someMethod2();
        System.out.println("Print Something Meaningless");
    } catch (Exception e) {
        e.printStackTrace();
    }
} catch (FileNotFoundException f) {
    f.printStackTrace();
} catch (Exception e) {
    e.printStackTrace();
}

• When an exception is thrown, it skips all of the code between it and the catch block.  So if someMethod2 threw an exception, the System.out.println() line would not print.
• If you do not have a handler for an exception or one of its superclasses, it acts as if there is no exception handler at all.  It will end the program.
• You can re-throw an exception in a class.  This throws it to the calling method.  Example:


} catch (Exception e) {
    System.out.println("Exception caught.  Re-throwing.");
    throw e;
}
 

• If you don't have a catch block, you can pass the exception along by declaring that exception in the signature of the calling method.  This passes up the exception indefinitely until it finds a catch block.  Sometimes this is the best approach.

Created by:  Brandan Jones December 17, 2001