Question

In this specific scenarios, are asserts more appropriate then exceptions?

It is my understanding that assert should be used when program is FUBAR to a degree where it can not recover and will exit.

I also was told to always throw exceptions for clarity and error message handling.

Is there a fine line between when to use each? Is there an example where assert must be used in place of exception unconditionally?

   public void subscribe(DataConsumer c) throws IllegalArgumentException {
        if (c == null) {
            // Almost certainly FUBAR
            throw new IllegalArgumentException("Can't subscribe null as a DataConsumer. Object not initialized");
        }

        if (dataConsumerList == null) {
            // Definetely FUBAR
            throw new IllegalArgumentException("Nothing to subscribe to. DataConsumerList is null");
        }

        dataConsumerList.add(c);
    }

Answer 1

Personally I’m not keen on using assertions for this sort of thing, simply because they can be turned off. Some places use assertions when running tests, but then disable them for production for the sake of speed. To me this is like taking your driving test as normal, but then removing your seatbelt when you get on the motorway.

An assertion is only going to throw an exception anyway, of course. If you absolutely want to take the JVM down right now, you’d need to use something like Runtime.halt.

So I’m a fan of exceptions here, although I’d typically use a NullPointerException when given a null argument, and if dataConsumerList is part of your state then I would personally use IllegalStateException to differentiate that situation. (It’s a shame that Java doesn’t have the same ArgmentNullException that .NET has, given what a common check it is.)

Guava has the useful Preconditions class which lets you write this more concisely:

public void subscribe(DataConsumer c) throws IllegalArgumentException {
    Preconditions.checkNotNull(c,
        "Can't subscribe null as a DataConsumer. Object not initialized");
    Preconditions.checkState(dataConsumerList != null,
        "Nothing to subscribe to. DataConsumerList is null");
    dataConsumerList.add(c);
}