How to convert java ArrayList to an array?

Question

I have an ArrayList created like this:

public class Main {

    public static void main(String[] args) {
        // write your code here
        ArrayList list1 = new ArrayList();

        list1.add(0, 5);
        list1.add(1, 3.5);
        list1.add(2, 10);

    }
}

I am trying to create an array from it, using the toArray method:

public class Main {

    public static void main(String[] args) {
        // write your code here
        ArrayList list1 = new ArrayList();

        list1.add(0, 5);
        list1.add(1, 3.5);
        list1.add(2, 10);

        Double[] list2 = list1.toArray(new Double[list1.size()]);
    }
}

However, I am getting an error:

(Error:(16, 39) java: incompatible types: java.lang.Object[] ).

So I tried to cast the right side to double:

Double[] list2 = (Double[]) list1.toArray(new Double[list1.size()])

This time i am getting Exception in thread "main". I also tried to declare my ArrayList as double from beginning:

ArrayList<double> list1 = new ArrayList()<double>

With no success. How to do it properly? I know that my problem is probably something very basic.

The correct syntax is ArrayList<Double> list = new ArrayList<>(); — Minn
– Minn, Commented Dec 7, 2019 at 11:18
You can`t put primitive type into List or any Collection. Generic classes only work with objects and don't support primitives. Use wrapper class instead. — Vlad Zaichyk
– Vlad Zaichyk, Commented Dec 7, 2019 at 11:21
@StephenC Actually it will solve the problem, I just tested this. Ideally, you would call toArray with new Double[0] but this should work fine in this case. — Minn
– Minn, Commented Dec 7, 2019 at 11:28
@Minn Only that change wouldn't let above code to compile. You still need to change list1.add(0, 5); to list1.add(0, 5.0); otherwise to place 5 to internal Object[] array it would be wrapped in Intger object which can't be placed in Double[] array. — Pshemo
– Pshemo, Commented Dec 7, 2019 at 11:31

Stephen C · Accepted Answer · 2019-12-07 11:56:22Z

The problem is that you are doing a number of things wrong:

ArrayList list1 = new ArrayList(); is incorrect because you are using a raw type. You should have gotten a compiler warning for that.
Given the previous list1.add(0, 5) is incorrect. The 5 will be boxed as an Integer because that compiler doesn't know that the list is only supposed to contain Double values.
You were getting this:
```
(Error:(16, 39) java: incompatible types: java.lang.Object[] ).
```
because you must have done something like this:
```
Double[] list2 = list1.toArray();
```
You appear to have corrected that in the code that you posted. But the no-args toArray method returns an Object[] containing the list content.
ArrayList<double> list1 = new ArrayList()<double> is incorrect because you cannot use a primitive type as a generic type parameter, and because the syntax on the RHS is wrong.

The correct version is ArrayList<Double> list1 = new ArrayList<>(); with an empty diamond.

Surprisingly, the best (most efficient) way to code the toArray is:

Double[] list2 = list1.toArray(new Double[0]);

Apparently, the implementation is able to initialize the array faster if it allocates it itself. (Or so I have heard ...)

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