Question

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List versus ArrayList variable type?

I’m intialising a HashSet like so in my program:

Set<String> namesFilter = new HashSet<String>();

Is this functionally any different if I initilise like so?

HashSet<String> namesFilter = new HashSet<String>();

I’ve read this about the collections interface, and I understand interfaces (well, except their use here). I’ve read this excerpt from Effective Java, and I’ve read this SO question, but I feel none the wiser.

Is there a best practice in Java, and if so, why? My intuition is that it makes casting to a different type of Set easier in my first example. But then again, you’d only be casting to something that was a collection, and you could convert it by re-constructing it.

Answer 1

I think it’s better practice to use the interface over the concrete type, i.e.

Set<String> strings = new HashSet<String>();

By doing this, you commit yourself to only using the functionality provided by the Set interface. This makes it easier to swap out the kind of set you’re using at a later date if you decide that you want different performance characteristics (e.g. if you wanted to use a TreeSet instead) without worrying about breaking your code.