Defensive programming vs exception handling

Question

Hi
im new to java programming and having problems getting my head around defensive programming vs exception handling.
i've read in my uni books about design by contract and the idea of defensive programming but im having problems with it.
so say i write the code.
String container = //some value
if (container.equals.("")) // so if i am looking for a value and find a empty String
   throw new // a custom exception i have made for this error.
else
   //do whatever i had planned with a non-empty String
}is this Defensive programming or exception handling. Or am i complete of base.
thanks

Answer 1

jverd wrote:
JustSomeGuy wrote:
Don't fall into the trap of saying "Meh, the exception will handle it". If you can forsee something easily being a problem, and it can be controlled, then control it. Save the exception handling for those things you don't forsee and those you can't control. I disagree. Or at least, I disagree with what I think you're saying.
For instance, if null is not a valid argument, then document it, and don't waste code defending against it. Either throw an explicit IllegalArgumentException, or let the NPE be thrown.
And absolutely under no circumstances should you ever write code that will deliberately lead to an exception, Eh? If an exceptional condition occurs, you should delibrately throw an exception. Consider if you will the following code snippet
import java.io.*;
class ExceptionExample {
    public static void main(String[] args)
        throws IOException {
        if (args.length == 0) {
            System.out.println("Must give filename as first arg.");
            return;
        FileInputStream in;
        try {
            in = new FileInputStream(args[0]);
        catch (FileNotFoundException e) {
            System.out.println("Can't find file: " + args[0]);
            return;
        int ch;
        while ((ch = in.read()) != -1) {
            System.out.print((char) ch);
        System.out.println();
        in.close();
} What I was saying in the first part is shown with the following part of the above code
if (args.length == 0) {
   System.out.println("Must give filename as first arg.");
   return;
} Would the exception handler that follows this peice of code have taken care of that? Sure! Would it be the best choice to just let the exception handler do it? No way no how!
It is easy to forsee someone forgetting or not realizing they need to add the filename as an arg and it can be controlled, so in this and most cases just like it the best choice is to either inform the user with a more specific statement than could be used in the exceptionhandler and do nothing or fix it yourself like for instance by using a default filename.
What I meant by deliberately throwing an exception can be illustrated with this small portion of my code above
int ch;
while ((ch = in.read()) != -1) {
    System.out.print((char) ch);
} A -1 to mark the EOF is expected and reaching the EOF is expected. There is absolutely nothing abnormal about this, so it should not be handled by an exception. But far to often I have seen novice and especially "think they know it all novices" use an exception in place of exactly this. They know the file will end, they know they must do something about that, but instead of doing it right they just set it up so an exception will be thrown and they can just handle it in the exceptionhandler. This is what I meant by deliberately throwing exceptions, in that an exception must be thrown in order for their program to work as intended. Bad, very bad.
Now that I have provided an example and further clarified my position do you still disagree? I can't imagine you would, so if you do I would be very interested in getting some further insight into your reasoning.
JSG