Learn English – Is “warn from” grammatically wrong

Standing on the train platform today, waiting for Nick, you might say, anxiously:

“If Nick is here we will not miss the train.”

If you narrate this tomorrow, you will backshift the tenses:

“Yesterday I said that if Nick was here we would not miss the train.”

Now suppose that in fact Nick is not on time. Now your condition, your IF clause, is an irrealis or counterfactual. Just before the train leaves you might say, despairingly:

“If Nick were here we would not miss the train.”

And afterwards, as the train pulls out of the station, leaving you on the platform, you might say bitterly:

“If Nick had been here we would not have missed the train.”

What is confusing here is that would does double duty: it may act as either the past indicative form or the present conditional form of will. You have to figure out which is in play from the context.

Your question:

Is it really grammatically correct to say what kind of an animal instead of what kind of animal...?

Yes, it is. It is possible to keep an, though it's fair that you'd expect it dropped because it's usually dropped. Then again, it's quite obvious that his writing style is informal.

This entry of Practical English Usage by Michael Swan should help explain why both alternatives (with or without a/an) are possible.

551 sort of, kind of and type of

1 articles
The article a/an is usually dropped after sort of, kind of and type of, but structures with articles are possible in an informal style.

​　　That's a funny sort of (a) car.
　　What sort of (a) bird is that?

So, if it were:

If you already know what kind of animal Java is and know that you want to use Java, skip Chapter 1 and go straight to Chapter 2. Believe me, I won’t mind.

would the meaning be different?

Not at all (to put it simply), because in his usage, what kind of animal would mean the same thing as his what kind of an animal.