Question

I have the following method:

  private <E extends Number> void AddOne(ArrayList<E> al, Double num) {
      al.add(num);
  }

al.add(num) doesn’t work. What is the best way I can insert a number into a generic Number Array?

So basically, I have a generic

ArrayList< E > al

And I want to insert a numeric value in it, how can I do this? (I remember C++ being a lot easier -_-)

Answer 1

You can’t add anythig to this list, because you don’t know what type it really is.

Consider inheritance tree like this:

Number
|
|- BigInteger
            |
            |- NonNegativeInteger
                                |
                                |-PositiveInteger

You say that your list is list of elements which inherits from Number. Ok, so suppose you want to add Number to a list, but wait you could have List<BigInteger> and you cannot put Number into it.

Ok, so you could put BigInteger, right? No! Because your list could be List<NonNegativeInteger>. And the story goes on and on…

Declaring list as List<T extends SomeObject> ensures you that when you get something from list it’s of type SomeObject. Nothing else is known.

So how to avoid the problem? Just remove the template.

List<Number>

And now you can put anything what inherites from Number.

Or, what is even better in your case, change Double to E:

private <E extends Number> void AddOne(ArrayList<E> al, E num) {
  al.add(num);
}