Question

I’ve got this method I’m working on (I think that’s the name) and it essentially tries to match a part of a string and then return the proceeding part of the string – that part I’ve got, easy stuff. The method is of type String.

When my method fails to find the pattern in the string I want it to return an empty string. I also want to send something along with the empty string to go “hey, I didn’t find your key” but I want this to be optional.

This is essentially what I want the method to do:

public static String getKey(String key) throws KeyNotFoundException {
    if (key.equals("something")) {
        return "great";
    } else {
        throw new KeyNotFoundException();
        return "";
    }
}

But the problem with this code is that the return ""; is obviously unreachable due to throw new KeyNotFoundException();.

If I was going to call this method I’d have to use the try {} catch(KeyNotFoundException knf) {} block. It’s this block that I want to make optional however.

If I choose to be oblivious to the fact the key was not found (e.g. don’t use try catch) then I just want to be given the empty string on return.

But I also want to be able to surround it with a try-catch block if I need to know whether the key was found for my operation to complete correctly.

I think I’m going about this the wrong way but I can’t seem to figure out an alternative (new to Java), could anyone shred some light on this please?

Answer 1

The usual way to do this would be to write two methods, one which returns a default value, and one which throws an exception. You can have one of these call the other, to avoid duplication:

public static String getKey(String key) throws KeyNotFoundException {
    String value = getOptionalKey(key);
    if (value.equals("")) throw new KeyNotFoundException(key);
    return value;
}

public static String getOptionalKey(String key) {
    if (key.equals("something")) {
        return "great";
    } else {
        return "";
    }
}

The caller can then choose which one to call, based on their needs:

String value = getOptionalKey("uma"); // oblivious

try {
    String value = getKey("uma"); // cognisant
}
catch (KeyNotFoundException e) {
    // panic
}