Comments on: Class as a generic type

By: links for 2007-05-03 | Urubatan`s Weblog

links for 2007-05-03 | Urubatan`s Weblog — Thu, 03 May 2007 10:21:54 +0000

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By: Alex Miller

Alex Miller — Wed, 02 May 2007 14:39:11 +0000

Regarding Class and its generic type, I find it most helpful to think of Class as a factory for instances of T (via newInstance() or even getConstructor()).

By: Tim Vernum

Tim Vernum — Mon, 30 Apr 2007 05:32:03 +0000

Yes, you are correct, I got my comment backwards.

By: Sam

Sam — Sat, 28 Apr 2007 16:56:34 +0000

Thanks Tim and Ricky, I think that covers the bad use example just fine.

I had forgotten that extends in generic types would mean that List.add() can’t take String parameters anymore, so in fact Ricky’s correction (setting the generic type of Arrays.asList) is a much better solution. (Tim, I think you meant to say List can’t take the Boolean.TRUE, not List)

By: Tim Vernum

Tim Vernum — Sat, 28 Apr 2007 15:04:21 +0000

The case you are looking for is (almost) exactly the one you have shown.

List<String> strList = new ArrayList<String>() // Valid
List<Object> objList = strList; // NOT valid 
objList.add("Bob"); // This would be safe
objList.add(Boolean.TRUE); // This is OK for a string list,
                           // but not for an object list

That last line if the reason why the 2nd line is not allowed. And that’s the same reason why your examples aren’t allowed.

By: Ricky Clarkson

Ricky Clarkson — Sat, 28 Apr 2007 14:40:08 +0000

List<Object> stooges = Arrays.<Object>asList("Larry", "Moe", "Curly");

Set your IDE to show use of a ‘raw type’ as a warning/error.

“Does anyone know of a valid case where it would be bad to allow a sub-type to be assigned in place of a type?”

List<Number> numbers=new ArrayList<Double>(); //not allowed, but imagine
List<Double> doubles=numbers;
numbers.add(5);
double d=doubles.get(0); //ClassCastException, we don't want this.