get the size of each array in this list

Question

I inserted these two ArraysList into this List:

    ArrayList<String> category = new ArrayList<>();
    ArrayList<Integer> number = new ArrayList<>();
    List list = new ArrayList<>();

    category.add("cat");
    category.add("dog");
    category.add("fish");
    category.add("Hamster");

    number.add(1);
    number.add(2);
    number.add(3);
    number.add(4);

    list.add(category);
    list.add(number);

Now how do I get the size of each array in this list?

`"how do I get the size of each array in this list?"`: what kind of result are you expecting? — ernest_k, Sep 26 '18 at 05:10
@ernest_k This list is then referred as a parameter to another `class` and does not have access to arrays `category` and `number` — MOHS3N, Sep 26 '18 at 06:00

ArockiaRaj · Answer 1 · 2018-09-26T09:54:37.670

5

Try like this

 int sizeOfCategory = category.size();
 int sizeOfNumber = number.size();

OR

    for(Object li : list) {
        System.out.println(((List) li).size());
    }

edited Sep 26 '18 at 09:54

answered Sep 26 '18 at 05:12

ArockiaRaj

588
4
17

The second option will not work (actually it gives a compile-time error) with the code from the question since `list` is declared using the raw type `List` (see my [answer](https://stackoverflow.com/a/52511930/2838289) for more). – LuCio Sep 26 '18 at 07:59
The compile-time error is not present anymore. Now the compiler issues the warning: "References to generic type List should be parameterized". But this also applies to the accepted answer. I wonder why the answers here (given by users with >1k reputation) are using raw type since the [documentation](https://docs.oracle.com/javase/tutorial/java/generics/rawTypes.html) says: _you should avoid using raw types._. Doing one cast isn't even shorter than providing the generic type at declaration of `list`. – LuCio Sep 26 '18 at 10:55

score 1 · Accepted Answer · answered Sep 26 '18 at 05:18

1

Cast the result object of get method from the list with ArrayList.

for(int i=0; i<list.size(); i++) {
   System.out.println(((ArrayList)list.get(i)).size());
}

answered Sep 26 '18 at 05:18

Shahid

2,288
1
14
24

score 0 · Answer 3 · answered Sep 26 '18 at 05:12

Check object instanceOf, if it is type of ArrayList which you added, then down-cast it to ArrayList and get size.

list.add(category);
list.add(number);
for (Object o : list) {
    if (o instanceof ArrayList) {
        System.out.println(((ArrayList) o).size());
    } else {
         // unknown type
    }
}

If you want you can get List item type too.

System.out.println("Object is type of " + o.getClass());

LuCio · Answer 4 · 2018-09-26T07:15:11.720

The problem is that you are using the raw typ of List. You would get it on your own, if you had declared list using a generic type T for List<T> which provides a size() method. Using the raw type the interface List will return elements of type Object - hence there is no size() method.

As list contains two instances of ArrayList there are some candidates for T available. For example:

ArrayList<?>
List<?>
Collection<?>

It depends on what you need to do with the inner lists (category, number). If you need only the size of them then Collectiion<?> will be sufficient.

System.out.println(list.get(0).size()); // 4
System.out.println(list.get(1).size()); // 4

Since the inner lists are using Integer and String as their types, there is no much they have in common in order to replace the wildcard <?>. String and Integer are both implementing Comparable but are only comparable to instance of their own type. Thus declaring List<Collection<? extends Comparable<?>>> list will not give any additional value. Hence having list which contains both inner lists of different types without a useful common type limits the usability of list to cases where you don't need the concrete type of the elements of the inner lists.

Further reading: What is a raw type and why shouldn't we use it?

get the size of each array in this list

4 Answers4