Casting of Exception necessity

Question

Quick question guys, I've see a lot of code regarding try/catch blocks and I've seen it a lot of times, that people use specific Exceptions for stuff. I mean, what kind of reason would you have to do that? I usually just put an Exception e inside the catch block and just read it out from the Log... is it a performance issue or just habit?

Answer 1

Having different exception classes allows you to have specific exception code without having to determine the type.

For example, rather than writing:

try {
   ...do some stuff
} catch( Exception e ) {
    if( e instanceof NullpointerException ) {
        ... do something
    } else if( e instanceof IOExceptio ) {
        ... do something else 
    }
}

you can just write:

try {
    ... do something ...
} catch ( NullpointerException epe ) {
    ... handle NPE
} catch ( IOException ioe ) {
    ... handle IO Exception
}

etc.

Further, you can also choose to catch only some exceptions, and allow others to be thrown by the method:

public void doSomeIO() throws IOException {

    try {
        ... do something ...
    } catch( NullpointerException ) {
        ... handle NPE
    }

}

And then you'd call it:

try {
    doSomeIO();
except ( IOException ) {
    handle the IO exception
}

This way you can have specific exception handling where it belongs, and allow others to bubble up where they can be handled better.

Answer 2

The error is not the same if you catch a ClassCastException than if you catch a ArrayIndexOutOfBounds exceptions. You should handle them accordingly.