CODEWITHSUNDEEP
javagenerics
NomenNescio
NomenNescio

Reputation: 3030

Insert into generic Number ArrayList Java

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 -_-)

Upvotes: 1

Views: 520

Answers (5)

msi
msi

Reputation: 2639

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);
}

Upvotes: 3

Brian Roach
Brian Roach

Reputation: 76908

Your method makes no sense. You want to add a Double to an ArrayList that is defined as holding any class that extends Number.

What happens if the array you pass in is ArrayList<Integer> and you try and add your Double to it?

If you're only interested in having an ArrayList that holds things that are subclasses of Number, you don't need generics in your method; simply use an ArrayList<Number>

Upvotes: 2

Don Roby
Don Roby

Reputation: 41135

You should change your add to:

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

and then in any concrete instance of your generic class, where E resolves to a specific number class, you'll be able to add numbers of that type.

Upvotes: 2

IllegalArgumentException
IllegalArgumentException

Reputation: 680

I think that what you really want is:

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

Otherwise you can't add anything to the List with adding the cast to E

Upvotes: 1

Sergey Vyacheslavovich Brunov
Sergey Vyacheslavovich Brunov

Reputation: 18116

Do you mean this?

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

Upvotes: 1

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